Values Reflected in Energy-Related Physics Concepts Open Access

In physics, efficiency is defined as the ratio of the “useful energy output” to the “total energy input.” This canonical definition of efficiency reflects social values in that it includes (and excludes) certain value-laden quantities in this ratio. This definition of efficiency also limits the kinds of questions that students can explore and excludes many questions that relate to their everyday life. By recognizing the relationship between our traditional physics quantities and the everyday world of our students, we can encourage our students to find new calculations of efficiency that are relevant to their everyday life and concerns.

Deciding what is considered “useful” energy requires a value judgment. Incandescent light bulbs are notoriously inefficient sources of light: some bulbs are only 2% efficient,⁶ with the vast majority of the input energy “lost” as thermal energy. However, if thermal energy is considered useful in a given context, an incandescent bulb may be very efficient. For example, a popular child’s toy of the late twentieth century allowed children to bake small cakes and cookies in a small oven containing an incandescent light bulb; the light bulb produced enough thermal energy to cook the small snacks. In this case, the thermal energy is the useful energy, rather than the waste, and the incandescent light bulb may be considered very efficient.

Another value judgment in efficiency relates to the decision about what to consider relevant energy inputs. James Thompson, who in 1840 was working to improve British steamships, considered the labor of sailors to be essentially free compared with coal, which was expensive. Therefore, when he was comparing the efficiency of two different methods of distillation (to make the fresh water that steamships require for operation), he preferred having sailors pressurize water with pumps to create steam at a lower boiling point, rather than burning more coal. After all, he explained, “the labor of sailors is of not very much consequence, as they have but little to do except in stormy weather, and when going into and out of ports.”⁷ 

Thompson’s calculations were in line with his Protestant values, which did not consider human effort to be relevant to efficiency calculations. The Protestant work ethic was a theological commitment that hard work and discipline were outward evidence of one’s Christian faith; in this context, there was no reason to reduce or even track energy transformed through human effort, as it was spiritually beneficial for humans to exert unlimited effort.^7,8 Although modern physics classes do not explicitly teach the Protestant work ethic, they do tend to continue the practice of ignoring or idealizing the energy put into a system by humans. Most physics calculations also ignore the energy required to extract natural resources, build and operate manufacturing machines and facilities, ship materials to manufacturing facilities, or ship products to consumers.^9,10 Evidently, physicists’ decisions about what to include as “input energy” are shaped by historical values.

The canonical definition of efficiency produces a unitless quantity, a multiplier allowing calculation of the percentage of the available energy that has been put to use. However, thinking of efficiency more broadly as “how much we get for what we put in” creates additional opportunities to connect to everyday situations. For instance, a canonical calculation of the efficiency of a car would take the ratio of the output of kinetic energy (of the car) to the input chemical energy (of the gas). Yet, in everyday life we represent kinetic energy output as miles traveled, and chemical energy input as gallons of fuel: gas mileage, or miles per gallon, is a familiar representation of the efficiency of cars.

One way to invite students to interrogate the values reflected in physics concepts is to have them investigate the values reflected in the definition of fuel efficiency described above as it applies to finding the most efficient method of commuting to school. Using the gas mileage representation of efficiency discussed above, small gasoline-powered cars have relatively high gas mileage (e.g., 25 mpg), while school buses have very low gas mileage (e.g., 7 mpg), suggesting that the bus is the less efficient method of transportation. However, the relevant calculation is not how far the automobile travels using a certain amount of gas, but how far a person can travel using a certain amount of gas. Therefore, the definition of efficiency being used should be expanded. A typical small car holds four people, resulting in 100 person-miles per gallon. A school bus that holds 48 people would result in 336 person-miles per gallon. Expanding the definition of efficiency expresses the sense in which the school bus is the more efficient commuting option. This type of argument isn’t supported by the canonical definition of efficiency. Students can then extend this calculation by considering the route the bus takes vs. the car, the effects of dropping off and picking up passengers, hybrid or electric cars, electric or biodiesel buses, vehicle occupancy, alternative commuting options, etc. Finally, students can extend this analysis to consider the inequities of urban transit.¹¹ When we acknowledge the connections between traditional physics concepts and students’ everyday lives, we support students to create new calculations of efficiency that matter for their everyday activities or that more closely align to policy calculations.

Introductory physics often introduces energy by defining it as “the ability to do work,” where work corresponds to the transfer of energy that occurs when a force acts on an object over a displacement. This transfer of energy tends to be associated with changes in the kinetic energy of a system (the “work–kinetic energy theorem”). In the context of teaching energy, there are pragmatic pedagogical reasons to emphasize work: it relates to forces, which are usually a key topic in an introductory physics course, and it is associated with a dot product, providing an opportunity to teach students one form of vector multiplication. However, energy has many manifestations that have nothing to do with pushing an object through a displacement (such as melting snow and lighting lamps), and the work–kinetic energy theorem is an unnecessarily limited statement of conservation of energy.^12,13 In our view, work is not the only mechanism of energy transfer, and energy should not be defined in terms of work. With Hecht, we define energy as a measure of the capacity of interacting objects to make (physical) change happen.¹⁴ 

The special status that scientists and teachers have conferred on work is likely attributable to its role in ancient engineering. The idea of work preceded the technical idea of energy by several centuries: ancient engineers knew that an inclined plane lets you push a load up to some height with less force than lifting it directly, but the distance over which that force must be exerted increases proportionally.¹⁴ The value of lifting and shoving things translated well to the British Industrial Revolution, in which scientists were concerned with lifting and shoving things in the context of factories and steam engines.¹⁴ These coal-powered factories and steamships were essential for transporting and processing goods from Britain’s global trading empire.^7,15 The concepts of work and heat as mechanisms of mechanical energy transfer were developed for the purpose of improving those factories and steamships, e.g., predicting and preventing the overheating of factory machines.

In the modern world, many different kinds of energy transfer are relevant, and students are interested in them. For example, the heating and cooling of our living spaces represents a majority of home energy use in the US¹⁶; expanding access to these energy technologies is a matter of life and death during extreme weather events associated with global warming, and is also a major driver of greenhouse gas emissions.¹⁷ Grappling with the physics of these critical social concerns motivates fluency with many forms of energy and mechanisms of energy transfer. Moving away from a definition of energy that foregrounds work supports teaching about energy in a greater variety of phenomena relevant to students’ lives, including the operation of incandescent bulbs, heat pumps, and electric power plants.^12,18–20

Physics concepts, such as efficiency and work, reflect values that represent particular social needs and empower certain people. Far from being abstract, pure descriptions of nature, concepts of efficiency and work are mathematical models³ laden with their historical origins, carrying forward the values of capitalism and colonialism that they were developed to serve. For example, the physics concept of work currently represents the interests of the factories and steamships that were important social concerns at the time the concepts were developed.

Teaching about the values reflected in physics concepts allows our students to investigate how their values intersect with those assumed in textbooks or used in physics calculations. For example, students might reimagine efficiency calculations to include cost, pounds of carbon dioxide produced, groundwater pollution, or labor exploitation. As additional examples, students might consider how science could center electrical energy rather than mechanical energy; include ways to characterize the renewability or cleanliness of energy resources; or situate energy production in its social and material context, including impacts on land, water, air, human communities, and plant or animal communities.^19–22 Considering the values reflected in physics concepts can help make physics more applicable to students’ everyday lives and decisions and inspire them to address energy problems and opportunities they see around them.

The authors are grateful to the Energy and Equity Team for contributions that influenced this work, including W. Tali Hairston, Adrian Madsen, Trà Huynh, Lane Seeley, Jessica B. Hernandez, Sarah B. McKagan, and Lauren Bauman. The authors would also like to express deep appreciation to all the teachers who have participated in Energy and Equity activities for their insightful ideas and curriculum development. This material is based on work supported by National Science Foundation Grant No. 1936601.

Kara E. Gray is an associate professor of physics at Seattle Pacific University and co-leads the Energy and Equity Project. Her current work focuses on supporting preservice and in-service teachers. [email protected]

Rachel E. Scherr is an associate professor of physics at the University of Washington Bothell and leads the Energy and Equity Project, working with high school physics teachers to integrate equity into the teaching and learning of physics.