Day 131: Loose Ends and a New Perspective on Electric Motors

Today was the day before spring break. I had originally planned for the AP Physics 2 students to complete the quantitative photoelectric effect lab. However, I realized that a bunch of loose ends had piled up and it would probably take the entire class period to work through them. So, the lab was postponed.

After gathering feedback on yesterday’s field trip, providing feedback on recent exams, and scoring a quiz, I wanted to revisit the electric motors that we built. Students wrote paragraph-length responses explaining how their electric motor worked. I had a “bonus” prompt that asked why the battery connected to the motor would die much faster if the motor was unable to rotate than if it was rotating. Very few students could explain this phenomenon.

Today, I approached the phenomenon from a different perspective. I asked what torques are applied to the armature. Students readily identified the torques to the magnetic force and friction. I then asked if there was an unbalanced torque. Students readily answered that there was. I then ask why their motor didn’t continue to accelerate but, instead, reached a constant rotational velocity. Students didn’t know. At this point, I drew a diagram, reviewed Faraday’s Law of Induction and Lenz’ Law, and demonstrated how the electric motor reacher rotational equilibrium due to the reduced current from the back emf. Next year, I think I’ll try this approach first instead of why the battery dies so fast with a stalled motor.


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