Small Motors

Seat Experiment

This simple electric motor (based on Faraday's ideas) is constructed from magnet (enamel-covered) wire, a steel-clad D cell, two paper clips and a magnet as shown. First wind a coil of 10 turns around the D cell, then wrap it so that it stays together. 

Remove the enamel insulation from the two coil ends and hang the armature (as the moving coil is called) from the paper clips as shown. We added a small kink of the armature to better show rotation of the armature in this picture. Start the motor by placing the magnet on the battery directly above the coil and giving the coil a small spin.

Note: If all works well, it should immediately start turning. If not, you may have to adjust the magnet position, or remove more enamel insulation, or adjust the paper clips.

Q1: My motor armature had a resistance of 0.015 ohms. Assuming the 1.5V battery could drive a constant current through this coil at rest, what would the current be?

Q2: What would the electric power driven through the (stationary) coil be in watts?

Q3: Where would this energy go? 

Q4: An actual measured current flow through the coil is about 5 A. What magnetic field does this  produce in an ideal coil? Draw the direction of this field in a diagram showing the coil.

Q5: Why is the permanent magnet stuck to the D cell needed in our motor? 

Q6: Sketch and describe the magnetic fields in this motor, including how they change direction.

Q7: View the motor in the dark. What do you see at the paper clip/armature connections? Why?

Q8: An engineer could claim that this motor should simply lock itself into a single position, and the motor does have a tendency to do so. What is this (electrically powered) position? Why does it not continuously lock in this position?

Q9: A second magnet can be brought beneath the coil and can either slow or accelerate the armature's rotation (see below). Explain why, using a diagram.


This device is described in Johnson, (1997). The Johnson DC Motor, Journal of College Science Teaching, NSTA.
Seat Activities