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Materials needed per group:

  • Bar magnet

  • Bag labeled “A” with assorted objects

Name(s): __________ Date: __________ Period: __________

  • Bar magnet

  • Bag labeled “A” with assorted objects

You will be given a bag labeled “A” with an assortment of objects. Write the name or description of each object in the chart below:

Name of object Attracted Not Attracted 

1.

 
  

2.

 
  

3.

 
  

4.

 
  

5.

 
  

6.

 
  

7.

 
  

8.

 
  

9,

 
  

10.

 
  

11.

 
  

12.

 
  
Name of object Attracted Not Attracted 

1.

 
  

2.

 
  

3.

 
  

4.

 
  

5.

 
  

6.

 
  

7.

 
  

8.

 
  

9,

 
  

10.

 
  

11.

 
  

12.

 
  

Use a bar magnet to test each of the objects in Bag A. Place a check in the table above to show whether the object is attracted to the magnet or not attracted.

How are all of the objects that are attracted to the magnet alike?

Show answer Hide answer

__________

What can you infer about the kind of objects that are attracted to a magnet?

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Objects that are attracted to a magnet are made of metal.

Observing

Classification

Inferring

none needed

10 minutes

Make sure the bar magnets are magnetized. If not, you can magnetize them with a magnetizer. (See Sargent-Welch WLS44385 High-Strength Magnetizer or google, “How to magnetize a screw driver.”)

Bag labeled “A” with assorted objects (e.g.):

  • Bubble level

  • Cotter pin

  • Nail

  • Plastic/porcelain knob

  • Clear plastic plate

  • Red plastic anchor

  • Rubber cap

  • Screw

  • Screw eye

  • Washer

  • Wing nut

  • Wood button

Items listed above are only suggestions. Any objects that are available may be used.

However, make sure that all metal objects included for testing are attracted to the magnet.

Also, if possible, make sure that all “silver” looking objects used are attracted to the magnet.

Be sure to include some nonmetal objects.

Do not talk to students as they conduct the activity. It is best to allow them to perform their own tests and draw their own conclusions.

All of the objects that are attracted to the magnet are made of—or containmetal.

It is expected that students may infer (erroneously) from this activity that all metals are attracted to a magnet; however, some experienced students may know otherwise. Activity 2 addresses this misconception.

Allow students to share their findings, but otherwise limit follow-up discussion. It is imperative to immediately follow this activity with Activity 2 which shows that only some metals are attracted to a magnet.

For a follow-up activity, students should do Activity 2: What Kinds of Objects Are Attracted to a Magnet - II?

You could ask students to find several additional objects in the classroom. Predict and then classify them as attracted or not attracted.

Name(s): __________ Date: __________ Period: __________

  • Bar magnet

  • Bag labeled “B” with assorted objects

  • Neodymium magnet (optional)

  • U.S. paper money (optional)

You will be given a bag labeled “B” with an assortment of objects. Write the name or description of each object in the chart below:

Name of object Attracted Not Attracted 

1.

 
  

2.

 
  

3.

 
  

4.

 
  

5.

 
  

6.

 
  

7.

 
  

8.

 
  

9,

 
  

10.

 
  

11.

 
  

12.

 
  
Name of object Attracted Not Attracted 

1.

 
  

2.

 
  

3.

 
  

4.

 
  

5.

 
  

6.

 
  

7.

 
  

8.

 
  

9,

 
  

10.

 
  

11.

 
  

12.

 
  

Use a bar magnet to test each of the objects in Bag B. Place a check to show whether the object is attracted to the magnet or not attracted.

How are all of the objects that are attracted to the magnet alike?

Show answer Hide answer

__________

Now what can you infer about the kind of objects that are attracted to a magnet? Compare your answer to your answer in the previous activity.

Show answer Hide answer

__________

OPTIONAL: If you have a strong magnet (e.g., Neodymium), loosely hang a $1 bill and test it with the magnet. Describe what you observe and what you can infer.

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Objects that are attracted to a magnet are made of ferromagnetic metal.

Observing

Classifying

Inferring

10 minutes

Students should do Activity 1 before doing this activity.

Make sure the bar magnets are magnetized. If not, you can magnetize them with a magnetizer. (See Sargent-Welch WLS44385 High-Strength Magnetizer or google, “How to magnetize a screw driver.”)

Bag labeled “B” with assorted objects (e.g.,):

  • Aluminum nail

  • Copper tube

  • Dime coin

  • Iron nail

  • Iron washer

  • Mossy zinc piece

  • Nickel coin1

  • Nickel metal

  • Penny coin

  • Safety pin

  • Small piece of aluminum foil

Items listed above are only suggestions. Any available objects may be used. However, make sure that all objects included for testing are metal and that a few ferromagnetic materials such as iron, cobalt, and nickel are included. Be sure not to include any nonmetal objects. NOTE: If nickel or cobalt objects are included, be sure to test that they are attracted to the magnet your students are using. Some magnets are not strong enough to attract nickel and cobalt objects. If available use a neodymium magnet.

Do not talk to students as they conduct the activity. It is best to allow them to perform their own tests and draw their own conclusions.

All of the objects that are attracted to the magnet are made of—or contain—ferromagnetic metals.

It is expected all students will infer (correctly) from this activity that only certain metals are attracted to a magnet; some experienced students may have known this.

If the magnetic is strong, the student can observe that the bill is attracted to the magnetic. From this the student can infer that bill has some magnetic component. It is the ink.

Allow students to share their findings, but otherwise limit follow-up discussion. Share with the students the name given to materials that are magnetic. This is different from magnets. Discuss the fact that only ferromagnetic materials (materials containing iron, cobalt, nickel and some rare earth elements) exhibit magnetic attraction. Most metals (aluminum, copper, gold, lead, silver, zinc, etc.) are NOT attracted to a magnet. The prefix “ferro” comes from the Latin word for iron. Thus, a ferromagnetic material is a material that is magnetic, as iron is. Iron is the most “commonly” found magnetic element due to its abundance and five unpaired electrons in the d orbitals of the iron atom. If your students have studied high school chemistry, this may interest them. Otherwise, it is not necessary to bring it up. You may choose to point out that materials that are attracted to a magnet (ferromagnetic materials) can themselves be made into a magnet (i.e., be magnetized).

There are several families of stainless steels with different physical properties. Some stainless steel is magnetic. This form is alloyed by the addition of chromium and can be hardened through the addition of carbon and is often used in cutlery. However, the most common stainless steel alloys have nickel as well as higher chromium content. It is the nickel that modifies the physical structure of the steel and makes it nonmagnetic. So the answer depends—the magnetic properties of stainless steel are very dependent on the elements added into the alloy, and specifically the addition of nickel can change the structure from magnetic to nonmagnetic. Type 304 stainless steel is austenitic. It has a minimum of 18% chromium and 8% nickel, combined with a maximum of 0.08% carbon. It is defined as a chromium-nickel austenitic alloy.

Martensitic stainless steel contains chromium (12-14%), molybdenum (0.2-1%), nickel (less than 2%), and carbon (about 0.1-1%). This composition increases the hardness but makes the material a bit more brittle. It is quenched and magnetic.

A nonmagnetic stainless alloy can be changed to magnetic by shearing it. The sheared edges have a changed crystal structure that makes them magnetic.

The change in magnetic response is due to atomic lattice straining and formation of martensite.

You could ask students to find several additional objects in the classroom and predict if they will be attracted or not attracted to a magnet.

It is interesting to test paper money. U.S. paper money is printed with magnetic ink. This helps mechanical bill changers detect counterfeit bills, and the patterns of magnetic ink are used to designate if a bill is $1, $5, $10, etc. Since photocopy machines do not print with magnetic ink, it prevents people from “making money” with a standard photocopy machine.

Composition of Canadian Nickels

Date

 

Composition

 

1964-1981

 

99.9% nickel

 

1982-1999

 

75% copper and 25% nickel

 

2000-present

 

94.5% steel, 3.5% copper, and 2% nickel-plated

 

Date

 

Composition

 

1964-1981

 

99.9% nickel

 

1982-1999

 

75% copper and 25% nickel

 

2000-present

 

94.5% steel, 3.5% copper, and 2% nickel-plated

 

Name(s): __________ Date: __________ Period: __________

  • Small steel paperclips (No. 1 size about 1-3/8 in)

  • Bar magnet

  • Horseshoe magnet

  • Small rubber band to contain magnetic marble

  • Small neodymium magnet marble

  • Graph paper or finely ruled line paper

  • String (e.g., kite)

Consider the three magnets given to your group. Which one do you think is strongest? Why do you think that?

Which?

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Why?

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Stick one end of each magnet into a pile of paperclips on the table. See how many paperclips you can pick up with each of your magnets. Describe your results. Construct a bar graph of your results.

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Do you think that this is a good scientific way to test the strength of magnets? Why or why not.

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Try testing the strength of the magnets by placing a paperclip on the end of the magnet and making a chain by touching another paperclip on the bottom of the first. Construct a bar graph to show the number of paperclips supported by each magnet. Describe your results.

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Another way to test the strength of a magnet is to test the distance through which a magnet will attract a paperclip. Place the magnet on a piece of graph paper. Place a paperclip one or more lines away from the end of the magnet. Determine the maximum number of lines on the graph paper across which the magnet can attract the paperclip. Record and graph the distance for each magnet.

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Rank your magnets from weakest to strongest.

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What unit of measure did you use to decide the strength of a magnet?

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How do your results for the method in #2, compare with results in #3 and #4?

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After discussing the results with other groups, discuss this question, “Did everyone get the same answers?” Explain why or why not?

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What would happen to the results if you had used heavier paperclips?

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Devise your own method for measuring the strength of a magnet. Describe your procedure. If there is sufficient time and supplies, carry out the experiment and record your results.

Show answer Hide answer

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If you have a neodymium spherical magnet, roll it across this printed page, carry out the experiment and record your results.

Show answer Hide answer

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Which of the three magnets is strongest? How do you know?

Show answer Hide answer

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Magnets come in various shapes, sizes, and strengths.

Observing

Experimenting

Inferring

Measuring

Hypothesizing

Designing

30 minutes

Students must have good measurement skills.

Provide many sizes and shapes of magnets for comparison.

During this activity the students seek a reliable method for measuring the strength of two different magnets. Try to strengthen the concept of finding a consistent method of measuring. Units may include paperclips, page lines and others. The SI unit for magnetic strength is the tesla, but this is better left for later. Allow students time to share promising ideas and encourage positive evaluation.

Neodymium magnets are very strong; you should caution students to keep fingers et cetera from between two magnets.

Part 2 & 3: Number of paperclips

Part 4: Number of lines.

It is not likely that different groups will get the same results, because magnets that look alike may not be the same strength and points of attachment vary.

Strength the same.— Strength doesn’t change even though the unit of measurement is changed. Number of clips and distance may be less, but the relate result should be the same.

If the page is photocopied, a neodymium sphere will come to rest on a line of print. This is due to the fact that most photocopy ink has some iron in it. Watch for print on the back of the page.

Which of the three magnets is strongest? How do you know? There are several methods that students might use to compare the strength of magnets. A few of the methods are listed below:

  • Method 1: Count the number of paperclips a magnet can lift from a pile of paperclips.

  • Method 2: Make a chain of paperclips by hanging them from the poles of a magnet. Count the number of paperclips in the chain.

  • Method 3: Measure the minimum distance that a magnet can attract a paperclip lying on a piece of graph paper.

  • Method 4: Measure compass needle frequency for magnets held the same distance from the compass. Greater frequency means a stronger magnet.

  • Method 51: Make a hacksaw blade balance as show in Fig. 2. Since a hacksaw blade is steel it will be attracted to a magnet. Attach magnet at the end of the blade and see how far down the magnet can be pulled before it releases the blade. See NOTE on next page for diagram of apparatus and suggestion for calibrating scale in newtons.

  • Method 6: Suspend a force sensor from a support rod. Hang a paperclip from a string attached to the sensor hook. Zero the sensor. Test each magnet by letting it attract the paperclip, then slowly pull them apart while recording the force vs. time. By showing each data set or run on the graph you can compare strength of magnets.

  1. The strength of a magnet can be measured using a standard unit of measure. For example using the lifting capacity or the maximum distance at which an object is moved can be used to compare the strength of magnets.

  2. Discuss why students were not concerned with the size or weight of the magnets when they were comparing the strength of the magnets. See response 8 above.

  3. Figure 1 shows a method for estimating the strength of a magnet. (See Method #2 on preceding page.)

    • Figure 2 shows a hacksaw blade balance that can be used to compare the strength of magnets. Attach a magnet to the right-hand end of hacksaw blade and see how far down the magnet can be pulled before it releases the blade.

The balance scale can be calibrated in newtons by hanging weights of known value from the end of the hacksaw blade. Record values measured on the graph paper.

This activity is the type of activity that includes “Engineering Design.” The student is expected to design a method to compare the strength of magnets.

Name(s): __________ Date: __________ Period: __________

  • Bar magnet

  • No. 1 size small steel paperclips (1-3/8 inch)

  • Horseshoe magnet

  • Small neodymium magnet

Bring each of the magnets close to a pile of paperclips. What happens?

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Try various spots on the magnets. Where does the magnet pick up the most paperclips?

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Where, then, is the magnet the strongest?

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Support a chain of several paperclips at various spots on the magnets. Find where on the magnet the most paperclips can be suspended. Continue to move away from the ends until a point is reached where there is no attraction. On the picture below, record the number of clips that can be held at each spot by drawing on the appropriate number of paperclips.

Place an X on the picture of the magnets showing the places where the magnet is strongest.

The places where a magnet is strongest are called the poles. Every magnet has at least two poles. How many X’s did you place on each magnet?

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Are there places on the magnet where no paperclips are attracted? __________ Where?

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If there is time, experiment with one of the other magnet shapes to find where its poles are located. Report your findings.

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The places at which a magnet is strongest are called the poles. Every magnet has at least two magnetic poles.

15-20 minutes

None required.

Other than distribution of laboratory materials, no special advance preparation is needed.

Make sure students are determining the maximum number of paperclips that can be suspended from any one point on the magnet. Be sure to use steel paperclips, not the plastic type.

Both the bar magnet and the horseshoe magnet are strongest at the ends.

Student drawings should show the most paperclips suspended from the ends. No paperclips should be shown hanging from the middle of either magnet.

Students should draw an X at each end of the bar magnet and the horseshoe magnet. These X’s show the location of the poles of the magnet.

Yes

No paperclips are attracted to the middle of the magnet.

The poles are at the flat sides of the disc magnets.

  1. A magnet is stronger at some places than at others.

  2. The places where a magnet is strongest are called the poles.

  3. Every magnet has at least two poles.

  4. The poles of a bar magnet and a horseshoe magnet are located at the ends of the magnet.

  5. The point midway between the poles of a magnet does not attract any paperclips.

Are the poles always located at the ends of the magnet? Try experimenting with disk, donut-shaped or cylindrical magnets to find where the poles are located. Also, try a piece of magnetic stripping (similar to that often used for refrigerator magnets). Try finding the poles of a magnetic marble. These will be similar to the Earth Magnetic Poles.

Most magnets have distinct north and south poles; however, flat refrigerator magnets are made with alternating north and south poles on the same surface. You can test this by sliding two refrigerator magnets past other each other with the magnetized sides facing each other. The magnets will alternately repel and attract as they are moved a few millimeters. This arrangement is responsible for making the front of the magnet (the picture side) nearly nonmagnetic. The arrangement is called a Halbach array.

Suspend a bar magnet as shown above and ask students,

Why are there more paperclips in some places?

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(Answer: Magnet is stronger at the poles.)

Why do the middle paperclips lean toward each other?

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(Answer: The ends of the middle paperclips have opposite poles and thus attract each other.

Hot gluing a small transparent compass2 on the end of a dowel makes a nice magnetic probe.3 Moving the magnet probe slowly while approaching a bar magnet or horseshoe magnet will allow students to find the strongest part of the magnet. The compass needle points toward the magnetic pole, which is the strongest part of the magnet. This compass probe will also allow you to determine north or south pole location on an unmarked magnet.

Name(s): __________ Date: __________ Period: __________

  • 2 bar magnets

  • String or thread

  • 8-1/2 by 11 white paper

  • String (e.g., kite)

  • Large paperclip (Jumbo)

  • Directional compass

  • Neodymium magnet

  • Horseshoe magnet

  • Ring stand with cross bar

  • Masking tape

  • Ruled line paper

Tape a sheet of paper to the table. On the paper indicate the directions of north, south, east and west.

By bending a large paperclip (Jumbo - 2 in), make a holder for the bar magnet as shown on the right.

Tape or tie a piece of fishing line or thread to the paperclip holder.

Hang the fishing line so that the magnet hangs directly over the center of the sheet of paper. (The magnet should be horizontally balanced and free to swing horizontally).

Twist the magnet a couple of turns. Release it, and when it stops swinging, note the direction in which the north pole of the magnet points. In what direction does the north pole of the magnet point? __________ the south pole? __________

Compare your magnet’s orientation to the others hanging in the room. How do they compare?

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Now remove the magnet and place a directional compass in the center of the sheet of paper. Make sure that the bar magnet is not near the compass. In which direction does the needle of the compass point?

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Try turning the compass a few times. Does the needle always point in the same direction?

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How is the compass needle like a freely suspended bar magnet?

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Suppose you had a bar magnet and the N and S markings at the ends had been rubbed off. How could you tell which end was the north pole and which was the south pole?

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Why do you think the poles of a magnet are named north and south?

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Test both ends of the hanging magnet by bringing a neodymium magnet close. Describe and explain your observations.

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The end of a magnet that points toward the north is called a north seeking pole of the magnet, and is generally labeled with “N.”

10 minutes

Students should be familiar with the terms north pole and south pole and with the N and S markings on a bar magnet.

Help the students tape the sheets of paper indicating direction to their worktables. Use a compass to make sure that the paper is correctly aligned.

Students may need to use tape to make sure the magnet is secured to the paperclip holder. While the activity is going on, walk around to make sure that the students have correctly suspended their magnets so that they rest horizontally. If your students have a break-time coming, it is good to set this up and then have them do another activity while they wait.

Often compass needles have the poles reversed and thus the “N” end of compass points toward the south geographic pole. To fix this:

  1. Bring the compass slowly up to the pole of a bar magnet.

  2. Stop momentarily and let the compass needle settle down.

  3. Move compass very fast to other end of magnet. If you do this quickly, the needle will not have time to rotate and you will reverse the poles of the compass needle. The smaller and stronger the magnet, the easier it is to do this.

The north pole of the magnet should always point toward the north when it comes to rest. The south pole, of course, will face south.

Both are free to swing and have north poles that always point toward the north.

Suspend the magnet and turn it. When it comes to rest, the end that points toward the north is the north pole.

A neodymium magnet is so strong that it attracts both the north and south side of a typical bar magnet. Thus attraction is not a test for a material to be a magnet. Repulsion is the test that a material is a magnet.

  1. The end of a freely suspended magnet that points toward the north is labeled the north pole. This pole is called the “North Seeking” pole. The end that points toward the south is labeled “South Seeking” pole.

  2. A compass needle is a freely suspended magnet.

  3. A very strong magnet (e.g., neodymium) will attract both sides of a typical bar magnetic.

If an unmarked bar magnet is available, have students repeat the experiment to determine and label the N and S poles. If no unmarked bar magnet is available, turn over or tape over one of your marked bars.

Name(s): __________ Date: __________ Period: __________

  • 2 bar magnets

  • String (e.g., kite string)

  • Large paperclip (Jumbo, 2 in) bent to hold magnet

  • #2 pencil with donut magnets for demonstration

Suspended a bar magnet horizontally similar to Activity 5. Allow magnet to reach equilibrium.

Bring the north pole of a second bar magnet near the north pole of the hanging bar magnet. Describe what you observe.

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Predict what you think will happen if you bring the south pole of the second magnet near the north pole of the hanging magnet.

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Try it. What did you observe? Was your prediction correct?

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Predict what you think will happen if you bring the north pole of the second magnet near the south pole of the hanging magnet?

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__________

Try it. What did you observe? Was your prediction correct?

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__________

Now bring the south pole of the second bar magnet near the south pole of the hanging magnet. Describe what you observe.

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From this experiment, what did you discover happens when like poles of two magnets are brought near each other?

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What happens when unlike poles of two magnets are brought together?

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What you have discovered is sometimes called the Law of Magnetic Poles. Try to write a short statement of this law below.

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Suppose that you had a bar magnet that did not have the N and S poles labeled. How could you use another bar magnet that did have its N and S poles marked to find and label the poles of the unmarked magnet?

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__________

Unlike poles of magnets attract each other and like poles repel each other.

15 minutes

Students should know that magnets have both a north and a south pole.

Be sure the N and S poles on your bar magnets are labeled correctly.

Keep the hanging magnets from the previous activity.

The magnets move apart. The north poles repel each other.

The magnets come together. The south pole of one magnet attracts the north pole of the other magnet.

The poles will attract each other.

The poles will repel each other.

Like poles repel. Unlike poles attract.

Answers will vary, but should in essence state that like poles of magnets repel and unlike poles attract.

Answers will vary. Possible correct answer: Bring the north pole of the marked bar magnet near one end of the unmarked magnet. If it is attracted, the unmarked end is a south pole because opposite poles attract each other. If it is repelled, then the unmarked end is a north pole because like poles repel each other. Do the same thing at the other unmarked end. Actually since magnetic materials are attracted to magnets, attraction is not a true test. Only magnets can repel. Look for repulsion to be sure.

Attract proves not the fact, but repel doeth compel.” - Bill Reitz, PTRA, Ohio

  1. Like poles repel and unlike poles attract. This is a statement of the Law of Magnetic Poles.

  2. Unlike poles are often called opposite poles.

When your students are finished with the laboratory activity and have shared their results, it will be time to sing “The Magnetism Song” on the next page.

To be sung as a round to the tune of “Are you sleeping?”

Magnetism, Magnetism North and South, North and South Opposites Attracting, Opposites Attracting Likes repel, Likes repel

Arrange three donut magnets on a pencil or dowel so that they are separated from one another as illustrated. Show that when pushed together, the three magnets spring back apart.

Ask the following questions:

What can you infer about the location of the poles of the three magnets? (Like poles must be facing one another.)

Is the original spacing between the magnets the same?

(The bottom two magnets will be pushed together more due to a magnetic push from the from the top magnet due to a magnetic force which is equal to the weight of the top magnetic.)

Predict what will happen if the middle magnet is turned upside down.

(The three magnets will be attracted to one another and come together).

This apparatus was taken into space on the space shuttle, and then the magnets were evenly spaced. You can approximate this condition by holding the pencil (dowel) horizontal.

As a review of free-body diagrams, high school students can be assigned the task of doing free-body diagrams for the top magnet in a stack of two donut magnets.

Consider a stack of two donut magnets and the forces on top magnet as shown in Fig. 1:

Fig. 1.
Free-body diagrams on top magnet for a stack of two donut magnets.
Fig. 1.
Free-body diagrams on top magnet for a stack of two donut magnets.
Close modal

Because the donut magnets are the same thickness, then

(Force of NorthBottom on NorthTop) > (Force of NorthBottom on SouthTop)

= (Force of NorthTop on SouthBottom) > (Force of SouthBottom on SouthTop)

  • Assume that (NorthBottom - NorthTop) = 8 units up1

  • (NorthBottom - SouthTop) = 4 units down

  • (NorthTop - SouthBottom) = 4 units down

  • (SouthBottom - SouthTop) = 2 units up

  • Thus the gravitational force on the top magnet must = 2 units down

Make a dancing doll exhibit by hanging a cut-out, cardboard paper doll from a lightweight string as shown in the diagram. Attach a horizontal “floor” to her feet and “hide” a donut magnet in the floor. Under the doll’s hidden magnet, on a stand other than iron, hide another donut magnet so that the poles of the donut magnets repel. Make sure that neither the stand nor the hanger from which the doll is suspended can move relative to each other and the distance between the magnet hidden in the floor attached to the doll and the magnet hidden in the stand are close enough to interact, but not so close that they touch when the doll swings by.

Let the students see what happens when they gently swing the doll past her stand. The students can write their explanation of how the exhibit works. If they understand how like poles repel and where the poles are on a donut magnet, they should be able to create a valid explanation.

Name(s): __________ Date: __________ Period: __________

  • Directional compass

  • Horseshoe magnet (optional)

  • Iron nail 4-in long (#6, 20 penny)1

  • Bar magnet

You have already learned that a compass needle is a magnet. Usually the colored or pointed end of the compass needle is the north pole of the compass. The other end of the compass needle is the south pole. Your teacher will tell you which side of the classroom is north so you can check to make sure that your compass has not had its magnet poles reversed. Your teacher will tell you what to do if your compass is reversed. (Pole reversals will be discussed later.)

Bring the north pole of the compass needle near the head of the nail and observe what happens. Then bring the south pole of the compass needle near the head of the nail. Record all of your observations below.

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__________

Based on your observations, do you think that the iron nail is acting like a magnet? Why or why not?

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What do you think will happen when the ends of the compass needle are brought near the pointed end of the nail?

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Try it and describe what happens.

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Do you think the compass needle will behave the same way when it is brought near the ends of a bar magnet? Why?

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Predict what will happen when the north pole of the compass needle is placed near the south pole of the bar magnet.

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Try it and describe what actually happens.

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Predict what will happen when the north pole of the compass needle is brought near the north pole of the bar magnet

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Try it and record what actually happens.

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In what way does the compass behave differently toward the nail and the bar magnet?

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Explain a simple way to use a compass to tell whether or not an object is a magnet.

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The diagram to the right shows a compass placed near one end of an unmarked bar magnet. Label the ends of the magnet.

Is the end of the magnet nearest the compass a north or a south pole?

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__________

How do you know?

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__________

On the picture above try showing the position of the compass needle if it is placed near the other end of the same unmarked bar magnet.

Try placing the compass directly above the middle of your bar magnet. Draw the position of your compass needle in the diagram at right. Be sure to show which end of the compass needle is the north pole and which end is the south pole.

Try to explain why the compass needle is positioned this way.

Show answer Hide answer

__________

OPTIONAL: Try bringing the compass needle near the ends of a horseshoe magnet. Does the needle behave the same way near each end? Why or why not?

Show answer Hide answer

__________

Like poles repel and unlike poles attract.

Observing

Predicting

Inferring

20 minutes

Students should have already been introduced to the Law of Magnetic Poles. Students should also know that a compass needle is a magnet, and be able to identify the north and south poles of the compass needle.

Make sure the nails used are not magnetized. Make sure that there are no magnets or magnetic materials near by that could affect the results of the experiment. Make sure that the colored ends of the compass needles are, indeed, north poles. (Sometimes the poles of compass needles reverse, especially if they are stored near strong permanent magnets. If this has happened, stroke the compass with one end of a strong bar magnet so that the compass needle reverses its direction.) Have your students recheck the compass from time to time during the activity. They can reverse polarity easily.

If the compass becomes magnetized backwards, see direction in teacher s notes of Activity 5 for reversing the poles of a bar magnet.

When the students are exploring the behavior of the compass near the nail in the first part of the activity, make sure that the bar magnet is nowhere nearby or it could affect the compass needle. In fact, it is a good idea to wait to distribute the bar magnets until after students have completed the first part of the activity.

Both ends of the compass needle are attracted (or at least not repelled) by the head of the nail.

No, if the nail were a magnet, the end of the nail would repel one end of the compass needle.

Again, both ends of the compass needle are attracted to the tip of the nail.

No, the compass needle will be repelled or attracted by one pole of the magnet.

The north pole of the compass needle will be attracted to the south pole of the magnet. This is because like poles repel and unlike poles attract.

The north pole of the compass needle is repelled by the north pole of the magnet.

Both ends of the nail attract the north (and south) poles of the compass needle. Only one end of the bar magnet (the south pole) attracts the north pole of the compass needle; the other end of the bar magnet (the north pole) repels the north pole of the compass needle.

If one end of the object repels the north pole of a compass needle, then the object is a magnet.

The end is a south pole because it attracts the north pole of the compass needle. Unlike poles attract.

a.

The north pole of the magnet will attract the south pole of the compass.

The compass needle will be aligned horizontally (parallel to the magnet). The north pole of the compass needle will point in the direction of the south pole of the bar magnet. The south pole of the needle will point in the direction of the north pole of the bar magnet. Because the compass is equidistant from the two poles of the bar magnet, both poles of the magnet equally attract the opposite poles of the compass needle.

OPTIONAL: One end of the horseshoe magnet (south pole) will attract the north pole of the compass needle and the other end of the horseshoe magnet (north pole) will repel the north pole of the compass needle.

  1. A compass can be used to tell whether an object is a magnet or not. If either end of the object repels the north pole of the compass, then the object is a magnet. Repulsion is definitive. Magnetic materials can be attracted without being magnets.

  2. A compass can be used to locate the poles of a magnet. The pole that repels the south end of a compass needle is a south pole. The pole that repels the north pole of a compass needle is a north pole.

Students could look up and report on the history of the compass.

Name(s): __________ Date: __________ Period: __________

  • Diagram of Earth on page 33

  • Bar magnet

  • Globe - about 12 inches in diameter

  • Small directional compass (diameter about 16 mm)

  • Dip compass

This teacher-led activity, when effectively conducted, allows students to “discover” that the magnetic pole of the Earth located in the Northern Hemisphere is actually a magnetic south pole, and the magnetic pole of the Earth in the Southern Hemisphere is a magnetic north pole.

Before conducting the activity, check each compass to make sure that its poles have not become reversed. The colored end of the compass needle should point toward the south pole of a bar magnet.

Supply each group of students with a diagram of the Earth, a bar magnet, and a small directional compass (diameter about 16 mm). Make sure the students recognize that the colored end of the compass needle points toward the north. Discuss the idea that the Earth behaves like a giant magnet. Point out that the magnetic poles of the “Earth magnet” are located close to, but not in the same place, as the Earth’s geographical poles, and that’s why the outline of the magnet on their Earth diagram is skewed.

Instruct each group to place a bar magnet and its compass on the places respectively outlined for them on the Earth diagram. Have each group check the compass needle to see if the colored end points generally toward the magnetic pole of the Earth in the Northern Hemisphere. (Most will find that it does not; the colored needle instead points toward the Earth’s Southern Hemisphere. This is because they have oriented the bar magnet so that its north pole is near the Earth’s geographic North Pole.)

Stress to the students that we KNOW that the colored end of a compass needle points toward the north. Challenge them to find a way to make the compass needle behave as we know it should. Most students will soon discover that this can be done by reversing the position of the bar magnet, so that the north pole of the bar magnet lies in the Earth’s Southern Hemisphere!

Discuss the notion that Earths magnetic North Pole is actually located in the Southern Hemisphere (in Wilkes Land in the Antarctic) about 1400 miles from the geographic South Pole. Conversely, the Earth’s magnetic South Pole is located near Bathurst Island in northern Canada, about 1400 miles from the geographic North Pole.

This demonstration uses transparencies to show that the north pole of a compass needle points toward the magnetic, rather than geographic, pole of Earth. It can also be used to assist students in learning to correctly read a compass. (You’d be surprised at the number of people who do not know that you must rotate the base of the compass until the label N rests underneath the colored half of the needle!) Generally the north-seeking pole of a compass is marked or colored.

Navigators use the term “magnetic north” when they reference the location of the magnetic pole located in the Northern Hemisphere. Remember that the magnetic pole located in the Northern Hemisphere is actually a magnetic south pole.

The angle between magnetic north and geographic north (i.e., a) is called the angle of declination. The map on the next page provides the angle of declination for various locations in the U.S. To determine the angle of declination for your location, go to the NOAA website at http://www.ngdc.noaa.gov/geomag-web/#igrfwmm.1 Declination also varies over time: http://en.wikipedia.org/wiki/Magnetic_declination2 has an animated gif showing how it varied over centuries.

Example:

If you are in New York City, then magnetic north is 10° west of geographic north.

Reprinted/adapted with permission from The Science Teacher, a journal for high school science educators published by the National Science Teachers Association (www.nsta.org)

MAGNETIC BACTERIA

Pulling together the various fields of science

by Jane Bray Nelson and Jim Nelson

All organisms respond to their environment. We classify each response (taxis) by the type of stimuli that invokes it. For example, a chemotaxis is a response to chemicals, a phototaxis a response to light, and a thigmotaxis a response to pressure or touch.

A rare but fascinating taxis is the response that a few organisms have to magnetism, or specifically, to the Earth’s magnetic field. The ability to follow lines of a magnetic field has been discovered only relatively recently in organisms as diverse as birds, bees, dolphins, and butterflies. One magnetotaxic group that is ideal for study in secondary classrooms is magnetic bacteria.

Although bacteria are among the simplest of organisms, their genetics and ability to evolve under a variety of conditions make them ideal subjects for investigations of carcinogens and mutagens. Under controlled classroom conditions, these organisms can be a valuable addition to your classroom.

The photo below, at left, shows magnetic bacteria in an ordinary environment while the photo on the right shows them in a magnetic field.

Since ancient times, it has been known that some materials could align themselves to magnetic fields when suspended freely. The lodestone guided explorers long before magnetism was explained by William Gilbert in 1600.

Compass needles are suspended so they can rotate freely. If you look closely, you will notice that the needle is angled so that one end points lower than the other. This design feature compensates for the angle between the Earth’s magnetic field and the Earth’s surface (first measured in 1590 by Robert Norman, who invented the “dip needle compass”). Figure 1 shows the dip angle of a compass, which increases with latitude (from 0° at the Equator to 90° at the Earth’s magnetic pole).

FIGURE 1.
Dip angle Increases with latitude.
FIGURE 1.
Dip angle Increases with latitude.
Close modal

The direction of a magnetic field is arbitrarily defined as the direction that a magnetic north-seeking pole of a compass points. Thus, the Earth’s core corresponds to a magnet with a magnetic “south” pole near the North geographic pole of the planet (see Figure 2).

FIGURE 2.
The geographic poles (Ng & Sg) and geomagnetic poles {Nm & Sm) are not exactly antiparallel as indicated in this diagram. However, the difference is not important in the study of magnetic bacteria.
FIGURE 2.
The geographic poles (Ng & Sg) and geomagnetic poles {Nm & Sm) are not exactly antiparallel as indicated in this diagram. However, the difference is not important in the study of magnetic bacteria.
Close modal

In 1975, Richard Blakemore (a graduate student in microbiology at the University of Massachusetts, Amherst) was studying bacteria in the mud of brackish marshes. He noted that the bacteria seemed to migrate in one specific direction, and to accumulate along one edge of a hanging drop culture. He used darkfield illumination (a technique that makes the transparent organisms shimmer and refract light) to make live bacteria visible within the culture. At first, he thought that the phenomenon he was observing was phototaxis, and that the organisms were responding to the light of the microscope or the room. But when he covered the microscope or moved it, the same preferential migration occurred. It was apparent that the bacteria were responding to geography, not to the laboratory environment.

Eventually, Blakemore brought a bar magnet near the drop culture containing the bacteria. He was delighted to note that they always swam toward the south-seeking pole of the magnetic field (the end that attracts the “north” end of a compass). If the magnet were reversed, the bacteria migrated in the opposite direction.

Adrianus Kalmijn joined the study of magnetic bacteria by exploring the behavior of the organisms in fields that approximated the strength of Earth’s field—approximately one gauss or 10-4 tesla. Kalmijn accomplished this by using Helmholtz coils to produce a uniform magnetic field around the microscope while he observed the organisms (see Figure 3). Helmholtz coils are two electric coils separated by a distance equal to the radius of one of the coils, an arrangement that produces a nearly uniform magnetic field in the region between the coils. The strength in the coil is controlled by varying the magnitude and direction of the electric current, and the total field is determined by calculating the vector sum of the magnetic field of the coil and the Earth’s magnetic field.

FIGURE 3.
Experimental set-up with Helmholtz coils around a phase contrast microscope.
FIGURE 3.
Experimental set-up with Helmholtz coils around a phase contrast microscope.
Close modal

Kalmijn found that if the magnetic field produced by the Helmholtz coil was greater than the horizontal component of the planet’s field, the bacteria swam in the direction of the field produced by the coil. However, if the field of the coil was reversed, the migration of the bacteria also reversed. The organisms were actually able to make a U-turn and follow the external magnetic field, an exceptionally strong response considering that the motions of bacteria are controlled by flagella and are largely random. Even when the magnetotaxic bacteria were killed, their cells remained aligned with the magnetic field and their direction reversed when the field was reversed, suggesting that the response was a passive function of the structure of their cells or capsules.

Magnetotaxic bacteria gathers at the magnetic poles (indicated by the dark circles).
Magnetotaxic bacteria gathers at the magnetic poles (indicated by the dark circles).
Close modal

Since Blakemore’s discovery, more than 12 morphologically distinct types of magnetic bacteria have been discovered: cocci, bacilli, and spirillum, in both fresh and salt water. In order to determine the mechanism that caused the response, Blakemore and University of Illinois researcher Ralph Wolfe isolated and cultured a species called Aqnaspirillum magnetotacticum. This bacterium has a flagelium at each end and an opaque chain that runs parallel to the axis of the cell. Since their work, all magnetotaxic bacteria have been found to contain such a chain in their cytoplasm. The links of the chain are called magnetosomes, and seem to be encapsulated by a sheath that is adjacent to the cell membrane. The sheath seems to hold the magnetosomes in a constant orientation relative to the cell wall of the bacterium. X-ray emission studies have revealed that the magnetosomes contain iron.

To determine whether or not the iron is crucial to the organism’s response, Blakemore and Wolfe grew bacteria in a culture without iron. The response disappeared. Further experiments showed that, in order to create magnetosomes, the bacteria required a medium containing approximately 1.0 mg of iron per liter solution in a readily available form (soluble organic complex). Mossbauer spectroscopy reveals that most of the iron found in the organelles is in a form similar to magnetite, Fe3O4.

Biologists look for evolutionary advantages in the structure of living organisms. Why would magnetic bacteria evolve? One clue seems to come from the observation that all of the species found thus far have been anaerobic; they do not require oxygen, and usually thrive in its absence. If this is true, then bacteria that migrate downward would have a better chance for survival in swamps, marshes, or mud flats. This explanation seems a valid one in northern areas, but since the “dip angle” of the planet’s magnetic field diminishes as we approach the equator, the relationship between depth and oxygen availability becomes less clear. Magnetic bacteria collected in Brazil near the equator seem to be randomly (50:50) north-and south-seeking.

If the theory is true, then magnetic bacteria that have evolved in the southern hemisphere should migrate in the opposite direction to those evolving in the northern hemisphere. To test this theory, Blakemore and Kalmijn went to New Zealand and Tasmania in 1981 to collect bacteria. These locations were selected because they are equidistant from the equator as Massachusetts, and feature a similar climate. They found that the bacteria collected there did indeed migrate against magnetic field lines to achieve more anaerobic environments. It seems evident that magnetotaxis prevents anaerobic soil bacteria from “accidentally” migrating upwards toward the toxic oxygen.

Experiments have also suggested that north- and south-seeking bacteria can have their magnetic orientations reversed when subjected to strong reverse fields. As strong pulses of magnetism are applied, bacteria have been seen to continually reverse their orientation. It has also been possible to demagnetize bacteria in fields that gradually build to approximately 1000 gauss. In this environment, new bacteria become about 50 percent north-seeking and 50 percent south-seeking. These results suggest that the taxic response is not truly behavioral, but is related to the structure and orientation of the iron in the magnetosomes.

Since bacteria reproduce so quickly, the effects of natural selection on the magnetotaxic responses are easy to demonstrate. In an experiment with north-seeking magnetic bacteria, the pole was reversed. In six days, south-seeking bacteria increased in population. And in eight weeks, the population polarity was completely reversed. The ability to evolve in response to changing fields may have been an advantage in the past, since the Earth’s polarity has reversed periodically.

In biology or physics classrooms where students have been trained in the proper aseptic techniques for the study of bacteria, experiments with magnetic bacteria can provide an exciting link between two sciences that are often taught separately in secondary schools. Using a phase contrast microscope or one that produces dark field illumination, the migration of magnetic bacteria can be observed. (If your school does not have such equipment on hand, you may be able to arrange a loan from a university or local laboratory.)

Alternatively, students can collect samples of pond water, sediment, and mud from different areas in a wetland and sample for magnetic bacteria in different regions of the pond. A laboratory culture can also be sampled to determine whether bacterial migration of various species has occurred.

Magnetotaxis prevents anaerobic soil bacteria from “accidentally” migrating upwards toward the toxic oxygen.

When collecting pond water, students should take both sediment and water. To view samples collected in the northern hemisphere, students should focus a phase contrast microscope on the northern edge of the drop, since that is where the magnetic bacteria will gather. Drops of sample can be contained in an O-ring, sealed to the slide and cover slip with petroleum jelly (see Figure 4). If numbers of bacteria are too small to identify, cultures can be kept in a warm, dark area for several days. Once magnetotaxic bacteria have been identified, a small bar magnet can be placed so that the south pole (the pole that attracts north-seeking needles on compasses) is close to the southern edge of the sample. If magnetic bacteria are present, they will migrate toward the magnet.

FIGURE 4.
Mounting of a sample of magnetic bacteria for observations.
FIGURE 4.
Mounting of a sample of magnetic bacteria for observations.
Close modal

Students who have had some training in aseptic and microscope techniques, as well as population sampling, might want to try to investigate the following problems using cultures of magnetotaxic organisms:

  1. Can a culture of north-seeking bacteria become south-seeking? Orient the south pole of a magnet toward the bottom of a shallow culture (the best environment for anaerobic soil bacteria) and sample the culture every few days.

  2. Measure the speed of the bacteria’s movement in relation to the strength of the magnetic field. Is this a passive or an active migration? (Can bacteria move in a specific direction with flagella?)

  3. Can magnetotaxis be lost over time? Neutralize the Earth’s polarity around the culture by orienting a Helmholtz coil with a field of one gauss so that the sum of its vector and the planet’s is zero.

  4. Can magnetobacteria use iron in different forms (ions)? Does the minimum concentration necessary for formation of magnetosomes change with the form of the mineral?

  5. Is the response of the bacterium related to temperature?

Although science is often taught as separate disciplines (such as biology, chemistry, and physics), isolationism by professional scientists is becoming increasingly impossible. The puzzle of magnetic bacteria illustrates not only the scientific method but also the important idea that modern science is multidisciplinary. Although Blakemore was interested in microbiology, his studies led him into areas most often associated with chemistry and physics. Perhaps the science curriculum we use today should incorporate more interdisciplinary problems.

Jane Bray Nelson is a teacher at the University High School, 11501 Eastwood Dr., Orlando, FL 32817. Jim Nelson is an instructional support teacher, Orange County Public Schools, 445 W. Amelia St., Orlando, FL 32801.

NOTE

This article is based on a presentation by Richard B. Frankel of California Polytechnic State University, San Luis Obispo, CA 93407 during the Woodrow Wilson National Fellowship Foundation physics institute at Princeton University in July 1988.

Name(s): __________ Date: __________ Period: __________

  • Bar magnet

  • Bare iron wire, finishing nail or wire clothes hanger

  • Small compass (diameter: 16 mm)

  • Masking tape or correction fluid

  • Wire or bolt cutter

  • Goggles or other eye protection

  • Magnet, breakable

When cutting a wire, it is important to wear goggles or other eye protection.

Cut a length of bare iron wire about 25 cm (about 10 in) long. A straight section of a wire hanger or a two-inch finishing nail works well.

Magnetize the wire or nail by stroking it in one direction with one pole of the bar magnet. Stroke the wire or nail at least 50 times. Use masking tape or correction fluid to mark the north pole of the wire.

Move a compass along the wire or nail and observe the location of the poles. In the diagram above, draw in the needle of the compass as it appears at each position along the wire.

The end of the wire that repels the north end of the compass needle is the ____ pole of the ____ wire.

The end of the wire that attracts the north end of the compass needle is the ____ pole of ____the wire.

Label the north (N) and south (S) poles on the diagram of the wire above.

Carefully mark the north pole of the wire with a small piece of masking tape or correction fluid.

Now cut the wire in half. Check each piece of the wire with the compass. Mark any magnetic poles you find. Is each piece of wire still a magnet?

Show answer Hide answer

__________

Continue to cut the wire into smaller pieces and check each piece for magnetic poles. Each time, label the north pole of the new pieces with the tape or correction fluid.

Experiment to find out how short you can cut the pieces of wire and still be able to find the magnetic poles. Describe your results below.

Show answer Hide answer

__________

A bar magnet is cut into smaller and smaller pieces. Label the N and the S pole on each piece in the diagram on the right.

As the pieces of wire get shorter, what happens to the strength of the magnetic poles? Refer to Activity 3 to review methods you can use to determine the strength of a magnetic pole.

Show answer Hide answer

__________

What happens if only the ends of the magnet are cut off (i.e., the part of the magnet where the poles are located)? If appropriate, label the N and S poles on each piece of the diagram below:

Show answer Hide answer

__________

Magnets are composed of “tiny magnets’ These are called magnetic domains.

Observing

Experimenting

20 minutes

Students should be familiar with using a compass to locate the poles of a magnet.

Try this activity before using it with students! (You may find it difficult to sufficiently magnetize the wire to get good results). Cut the 10-inch pieces of wire in advance.

Students may need help in using the wire or bolt cutter. If necessary, you may go around and cut the wires for them when they are ready. Be sure that students cutting wire use eye protection. Also, caution the students to handle the magnetized wire gently. The wire is only temporarily magnetized and any jarring or jostling will cause it to lose its magnetism more quickly.

Another option would be to use two-inch long finishing nails. Finishing nails are easy to cut with bolt cutters. If you use finishing nails, you don’t have to tape or paint the N pole. Just describe it as the head, point, or cut end of the nail.

North, south.

Different groups of students will have varying degrees of success. Some will be able to cut the wire into 16 or more pieces and still find the poles.

  1. When a magnet is cut, each new piece is a magnet.

  2. A magnet seems to be made of many smaller magnets.

  3. The smallest piece of a magnet that still has a north and a south pole is called a magnetic domain.

  4. The north poles of all the smaller magnets point in the same direction. The south poles of all the smaller magnets point in the same direction.

  5. All of the magnetic domains in a magnet are lined up with their north and south poles pointing in the same directions.

An effective way to demonstrate the breaking of a large magnet will be to stack up a number of rectangular or circular ceramic magnets. The resulting large magnet can be easily pulled apart to separate the pieces of the magnet that can be held near a compass to determine their polarity.

One model to explain the properties of a hanger stroked by a magnet is to picture south poles all at one end and north poles all at the other. Although this works to explain the properties of a hanger stroked by a magnet, it does not explain what happens when the hanger is cut in half. This is an example of added observations leading to a revision in an existing model.

Item #7 provides an opportunity to review Activity 3, “How Can the Strength of Magnets be Compared?”

It would be great to have students draw pictures of their wires before cutting them. Most will put separate N (or +) poles at one end and S (or -) at the other.

Then ask students to predict what will happen when the wire is cut in half. Will the middle act like an unmagnetized piece of steel? Students should have done Activity 7, “What Can a Compass be Used For?” so they have this experience.

Then the wire is cut. How can students modify their picture to fit their new results? What will happen if they cut off a small end of the wire?

When an iron magnet is broken into pieces, all new pieces of the magnet will retain their polarity.

Name(s): __________ Date: __________ Period: __________

  • Compass

  • Cork to fit and seal glass test tube (or bottle)

  • Glass test tube (or bottle) with cap or cork top, 1.3 cm diameter and 10 cm tall, capacity 9 mL

  • Iron filings (50-60 mesh, do not use iron dust)

  • Sugar or salt

  • Bar magnet

Fill a test tube part way with iron filings. Cork the tube.

Bring the bottom of the test tube near one end of the compass needle. What happens?

Show answer Hide answer

__________

Now bring the test tube near the other end of the compass needle. Record your observations.

Show answer Hide answer

__________

Pick up the tube with your left hand, hold the tube in a horizontal position and shake it. With your right hand, pick up a strong bar magnet. Stroke the test tube, from one end to the other with the pole of the magnet. Without shaking the test tube, do this several times, always making sure you stroke the tube in the same direction each time. Using a compass, test both ends of the test tube to determine the presence and type of magnetic pole.

What happens now? Record your observations.

Show answer Hide answer

__________

Why does the test tube now act differently?

Show answer Hide answer

__________

Shake the test tube and again bring it near the ends of the compass needle. What happens this time? Why?

Show answer Hide answer

__________

Fill another test tube part way with sugar or salt.

Bring each end of the test tube near one end of the compass needle. What happens?

Show answer Hide answer

__________

Shake the test tube. Stroke the test tube, from one end to the other with the pole of the magnet. Without shaking the test tube, do this several times, always making sure you stroke the tube in the same direction each time. Bring the test tube near both ends of the compass needle again.

What happens now? Record your observations.

Show answer Hide answer

__________

How does this tube act compared with the tube containing the iron filings? Explain why.

Show answer Hide answer

__________

Using your results from this activity, construct a model of a magnet.

Explain why a test tube of a liquid could not be made into a strong, permanent magnet.

Show answer Hide answer

__________

Materials become magnetized if most of the tiny magnets composing a ferromagnetic material align themselves into the same direction.

Observing

Inferring

Hypothesizing

Formulating Models

20 minutes

Students should have been exposed to the concept of magnetic domains, and should be able to use a compass to tell whether or not a material is a magnet.

Fill test tubes and cap with a cork. In order to avoid students spilling fillings and salt, tape stopper onto test tube and save for next class/year. Pieces of steel wool will work as well as iron filings. Filings can be put into the test tube by using a creased piece of paper that acts like a trough or funnel.

Tell students not to remove contents from the test tube. Before the activity, demonstrate to students how the tube should be stroked in one direction with the bar magnet.

It will attract either end of the needle, as would any other unmagnetized ferromagnetic material.

The test tube acts like a magnet and thus attracts one end of the compass and repels the other. It is important to have students observe what happens to the filings. They are not dragged to one end of the tube, but as the magnet passes by them they rotate and line up.

Each small piece has been lined up with all N’s pointing in the same direction. So, the effect is the same as that of a large magnet.

Again the content of the tube will attract either end of the compass needle, as would any other unmagnetized ferromagnetic material.

Neither end of the compass will be attracted.

Again neither end of the compass will be attracted.

The tube containing iron filings will attract or repel the compass needle whereas the salt will have no effect on the needle because neither salt nor sugar is ferromagnetic.

A material becomes magnetized if the particles within the material can align themselves in the same direction. A model of bar magnet that has many small magnets aligned can be developed. This is a precursor to magnetic domains.

In liquid, the atoms are free to move around. Because of this, they will not line up to form a magnet.

  1. The test for whether an object is a magnet or not is whether one pole of a known magnet, such as a compass needle, will be repelled by some part of the object. (In the first part of this activity, the test tube of iron filings did not behave as a magnet. In the second part of the activity, the test tube of iron filings did behave as a magnet since one end of the test tube repelled the north end of the compass needle.)

  2. Magnetic materials are composed of many “tiny magnets” called magnetic domains. When the magnetic domains are randomly arranged, the material does not act as a magnet (refer to transparency). However, when the magnetic domains in the material are all lined up with their north (and south) poles pointing in the same direction, the material as a whole acts like a magnet.

    A magnetic domain consists of a region a few microns across in which all of the ferromagnetic atoms have their internal magnetic moments aligned in the same direction.

    In creating a magnet, many of the domains are lined up as the iron files do in this activity.

  3. Stroking a magnetic material with a permanent magnet causes the magnetic domains in the material to align. The material, at least temporarily, becomes a magnet. (In this activity each piece of iron filing is like a tiny magnet. At first, these tiny magnets pointed in random directions. When stroked with the bar magnet, however, these tiny magnets lined up with their north and south poles facing in the same direction.)

  4. A magnet can lose its magnetism if its magnetic domains are rearranged so that they are no longer aligned. (Shaking the tube of iron filings caused the magnetic domains to become jumbled.)

  5. Nonferromagnetic materials (materials that are not attracted to a magnet, such as plastic or glass) are not made up of magnetic domains and cannot be made into magnets.

Provide students with a dozen matchsticks. (For safety, strike and then quickly blow out each match before distributing to students.) Tell students that each match represents one of the tiny magnets or magnetic domains in a magnetic material, and the burnt head of the match is the north pole. Have them arrange the matches on their desks to represent the way magnetic domains are arranged in a:

  • Ferromagnetic material that is unmagnetized.

  • Ferromagnetic material that is magnetized.

Name(s): __________ Date: __________ Period: __________

  • Strong bar magnet

  • Steel needle (nonmagnetized)

  • Compass

  • Piece of Styrofoam or poker chip

  • Dish, small plastic or glass (e.g., Petri dish)

  • Source of water

Move the north pole of a bar magnet down the length of a steel needle from the point to eye about 50 times. Rub in one direction ONLY. Start at the point of the needle and go toward the eye of the needle. Then lift the magnet and start at the point again.

Float a piece of Styrofoam1 in a dish of water. Put the magnetized needle on the Styrofoam. Observe and record what happens. (Use a compass to find out which way the eye of the needle points).

Show answer Hide answer

__________

Do both the needle and the compass line up the same way?

Show answer Hide answer

__________

Which end of the needle points toward the north geographic pole?

Show answer Hide answer

__________

Could you use the needle as a compass?

Show answer Hide answer

__________

Magnetize another needle, but this time rub it with the south end of the magnet. Be sure to rub in only one direction, starting at the point and rubbing toward the eye.

Which end of the needle points toward the south geographic pole?

Show answer Hide answer

__________

Why did you float the Styrofoam and needle in water?

Show answer Hide answer

__________

How is a magnetic material affected by stroking it with a magnet?

Show answer Hide answer

__________

What can you conclude about how a magnetic material is affected by rubbing it with a magnet? Predict which end of the needle will become north based on the direction of rubbing and the pole of the magnet used.

Show answer Hide answer

__________

Predict what would happen if the needle is stroked by the north pole of a bar magnet, but starting at the eye of the needle and pulling toward the point of the needle. Which end of the needle becomes the north pole?

Show answer Hide answer

__________

Use the domain model to predict which end of the needle will be north.

Show answer Hide answer

__________

Below, draw a diagram showing the magnetic domains that may be in the needle.

A steel needle can be made into a magnet by stroking the steel with a permanent magnet. Stroking causes the domains to have the same magnetic orientation. The magnetized needle can be used to make a compass.

Observing

Inferring

Predicting

Identifying and Controlling Variables

30 minutes

At the beginning of the activity, be sure the needles are nonmagnetic. Warming them on a hot plate will randomize the domains. Darning needles (or any long needle) work well.

Warn students about needles. A straightened paperclip or a finishing nail can be substituted for a needle. A plastic poker chip can be used to float the needle. The ribbing along the edge tends to keep the needle from rolling off. Keep in mind that students may have preconceptions from doing a similar activity. The diameter of the container holding the water should be greater than 10 cm so the needle is less likely to hit the sides of the container. The plastic top for a coffee cup works nicely.

Needle comes to rest in one direction—lines up in a north-south direction. The eye points south.

Yes.

The point.

Yes.

The point.

So the needle could move freely.

It causes the magnetic domains in the needle to line up and point in the same direction. This makes the needle behave as a magnet.

Rubbing a needle with a strong bar magnet lines up the magnetic domains in the needle. The end of the needle where the magnet is removed becomes the opposite pole. For example, if you rub the needle from point to eye with the north pole the eye becomes the south end of the needle.

The eye. The north pole of the magnet should create a south pole in the needle at the point.

The steel needle is made up of “tiny magnets,” or magnetic domains.

Each domain has a north and south pole. But the poles are not lined up in any orderly way and thus the needle does not act like a magnet.

In this activity, by stroking the object with the north pole of the magnet, the magnet attracts the opposite poles of the magnetic domains in the needle and causes them to line up with their poles in the same direction.

If the north pole of the magnet comes off the needle at the eye of the needle, that end of the needle will be a south magnetic pole. The point of the needle will be the north magnetic pole and will point toward the geographic north pole since it is the south magnetic end of the Earth.

  1. Have the students identify an object in the direction that the needle is pointing. Try deflecting the needle and see if the needle returns to the object. Then pick up the container and slowly turn around. See if the needle continues to point in the same direction.

  2. This activity can be done with various objects such as paperclips, nails, old metal saw blades, pieces of clock spring, knitting needles, and others. Students might try these in an extension. Instead of floating the metal in water, test the magnet with a compass or by picking up iron filings or paperclips.

  3. After this activity, challenge students to make a three-pole magnet. Some industrial magnets have multiple poles. As an example, some gasoline pumps have a disk with 50 poles at the disk edge. The gasoline pump makes 20 revolutions to dispense one gallon of gasoline. Twenty revolutions with 50 poles per revolution means 1000 passes by a sensor and allows the display to have a precision of a thousandth of a gallon.

Name(s): __________ Date: __________ Period: __________

  • Four 4-inch nails (#6, 20 penny) or non-magnetized steel needles

  • Hammer

  • Hot pad holder or glove

  • Hot plate, propane torch, or Bunsen burner

  • Needle nose pliers

  • Paperclips

  • Strong bar magnet

  • Water in nonplastic bowl (i.e., 250 ml_ beaker)

Label nails 1, 2, 3, and 4. Magnetize each nail by stroking it 50 times. Test the strength of the nails by putting all four into a box of paperclips. Record how many paperclips each one attracts in the chart below.

Nail number Number of paperclips after magnetizing Number of paperclips after treatment 

1

 
  

2

 
  

3

 
  

4

 
  
Nail number Number of paperclips after magnetizing Number of paperclips after treatment 

1

 
  

2

 
  

3

 
  

4

 
  

Lay nail #1 aside.

Wearing a hot pad, grip nail #2 with a pair of needle nose pliers and place it in an east-west direction on a hot plate or in a propane flame until the nail is very hot or turns red. Then using the pliers drop the nail into a container of water. Allow the nail to cool for about one minute.

Hold nail #3 in an east-west direction and strike the end several times with a hammer.

Hold nail #4 in any manner and strike it several times with the hammer.

Test the strength of each of the four nails by putting them into a box of paperclips. Record in the chart how many paperclips each of the nails attracts.

Compare the strength of the nails.

Show answer Hide answer

__________

Why does hammering and heating change the strength of the nail magnets?

Show answer Hide answer

__________

What have you learned about the way that magnets should be handled?

Show answer Hide answer

__________

Heating or hammering will cause a magnet to lose its magnetism.

Observing

Communicating

Inferring

Hypothesizing

15 minutes

Students should be familiar with the concept of magnetic domains.

Practice procedures before demonstrating in class.

The heat source must be a very hot one. A candle will not provide enough heat. Because atoms of solids do not normally change their positions, a high temperature is required to destroy the magnetism of a permanent magnet. Some hot plates will work if they get hot enough. For an iron magnet, a temperature of over 770°C (1043°F) is necessary. When heated red-hot, iron will not be attracted to a strong magnet.

Caution: Be sure to use a hot pad to hold pliers.

Try heating the magnet at different temperatures in an oven and testing the magnet while it is hot, as well as after it has cooled. It may be possible to actually increase the magnetism (rearrangement of molecules) at these low temperatures.

For safety, this activity might be done as a teacher demonstration or student demonstration.

Jostling rearranges aligned molecules so magnetism decreases.

Do not drop, hit together, etc.

Heating causes increased movement of the magnetic domains in the nail. The poles of the domains within the nail become scattered and the nail loses its magnetism. Hammering jostles the domains in such a way that they do not line up and the nail loses its magnetism.

Try heating and/or jarring a permanent bar magnet to see if its magnetism is lessened or destroyed.

Ask students to compare this result to what they found when they shook the test tube with iron filings during step 9, Activity 10.

During World War II, a magnetic switch activated some mines placed underwater. When ships were built in dry dock, the riveting continually shook the ship’s steel hull, and the magnetic domains tended to align with the magnetic field of Earth. When a ship passed over the mine, the magnetic switch would activate the mine and damage the ship. To prevent this, ships were “degaussed.” Wrapping the ship in a wire carrying a decreasing AC current was the method used to degauss ships. The current was initially large and was gradually decreased, resulting in scrambling the magnetic domains in the ship’s steel hull. The ship was still made of magnetic material, but it was not a magnet.

This can be demonstrated by placing a magnetized nail inside an air core solenoid. Connect the solenoid to a variable AC power supply (e.g., Variac). Be sure the solenoid has enough windings to be able to take 120 V for at least a few seconds without overheating. Increase the voltage to about 20 volts or until nail in the solenoid starts to chatter and then very gradually decrease the current. The nail will be demagnetized.

  • Curie temperature for iron = 770 Celsius

  • Curie temperature for cobalt = 1,100 Celsius

  • Curie temperature for nickel = 360 Celsius

  • Methane (e.g., in Bunsen burner) burning temperature = about 1,950 Celsius

  • Propane (e.g., propane torch) burning temperature = about 2,000 Celsius

One of these two iron nails is a magnet. Using only the nails how can you prove which one is the actual magnet?

This is a challenging problem to solve using only the two nails. Many students will lift one nail with the other and decide the nail doing the lifting is the magnet. However, if you ask students to reverse the nails, they will find that both nails will lift the other nail. Almost no student can solve this problem without some coaching. But you may have a clever student who reasons as follows: Magnets have poles. The strength of the magnet is concentrated at the poles. As you move from one pole to the other, the strength will gradually diminish as you move toward the center of the nail and then gradually increase. The point in the middle of the two poles will be magnetically balanced and not have magnetic properties. By applying this theory, the magnetic nail can be found. Select either nail, let’s call it nail A. Touch its point to the end of the other nail (B) and it is lifted. Whether A or B is the magnet, B will be lifted. Gradually move nail A toward the center of nail B. When the center is reached, either A will attract B or it won’t. If A attracts B, A is the magnet. If A doesn’t attract B, then B is the magnet. See Fig. 1 on next page.

Fig. 1.
The weakest point is midway between the poles.1
Fig. 1.
The weakest point is midway between the poles.1
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If compasses are available, ask students, “Can you use a compass to determine which nail is a magnet?” Since the compass pointer is a magnet, it has poles. When the end of the magnet nail is brought near the compass, the pointer will either point toward or away from the nail. If the nail is reversed, the pointer will also reverse. The nail that is not magnetized will attract both ends of the compass needle. Thus only repulsion is a true test for a magnet. See Fig. 2 on next page.

Fig. 2.
The nail is a magnet if it repels only one end of compass needle. If the nail attracts both ends of the needle, it is not a magnet.2
Fig. 2.
The nail is a magnet if it repels only one end of compass needle. If the nail attracts both ends of the needle, it is not a magnet.2
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1.

Canadian nickels made before 1981 have more nickel metal than newer coins, thus they are more attracted to a magnet.

1.

Figure 2 and method suggested by David Taylor, PTRA, KY.

1.

This activity was reviewed by Dave McCachren, PTRA, PA

2.

See Sargent-Welch WLS1762-46 Two-Sided Clear Compass

3.

Suggested by Dale Freeland, PTRA, MI

1.

The values of these forces depend on the size and separation of the donut magnets. Values given are arbitrary.

1.

Nail must be demagnetized. For directions on demagnetizing, see Activity 12.

1.

Referenced on 7/18/2012

2.

Referenced on 7/18/2012

3.

Map from Operation Physics manual on Magnetism

1.

Note: In order to be sure which end is originally the north pole; we suggest that one student hold the wire on both ends and another student cut the wire.

1.

The bottom of a Styrofoam cup makes a great float.

1.

Diagram from Operation Physics manual on Magnetism.

2.

Diagram from Operation Physics manual on Magnetism.

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