Motors and Bulbs in Series

One of Paul Hewitt's “Figuring Physics” that appeared in this journal dealt with the heating of a motor. This phenomenon can be demonstrated with a miniature motor and a bulb as part of a series of activities with “batteries and bulbs.” Students examine the effect on the brightness of a single bulb when a second, identical bulb is placed in series with it (Fig. 1). The second bulb is then replaced with the motor (Fig. 2). When the motor is running, the first bulb is dimmer than it was in series with the other bulb. However, if the shaft of the motor is held so it cannot move (like Hewitt's saw motor), the bulb is noticeably brighter. If one uses the brightness of the bulb as a qualitative measure of current, then there is clearly more current when the motor is not moving. This provides an opportunity to introduce some very interesting physics.