Chapter 8.2, Problem 2bT

Simple electric motor
Obtain the equipment illustrated at right assemble it as shown.

You should have:
•a magnet, a battery, a switch, some connecting, and an ammeter.
• a copper wire coil. The ends of the wire leads to the coil have been stripped of the insulating enamel coating so that half the wire is bare.
• twoconducting supports for the leads to the coil

1. Examine the leads to the wire coil closely, so that you understand which portion of the wire has been stripped of the insulating coating.

For what orientations of the coil will there be a current through it due to the battery?

Check your answer by losing the switch and observing the deflection of the ammeter as you rotate the coil manually through one complete revolution.

2. Hold one pole of the magnet near the coil. Close the switch. If the coil does not begin to spin, adjust the location of the magnet or gently rotate the coil to Stan it spinning.

Use the ideas that we have developed in this and previous tutorials to explain the motion of the wire coil. (The questions that follow may serve as a guide to help you develop an understanding of the operation of the motor.)

When the coil is in the position shown, there is a current, I, through it.

a. The coil is manually started spinning so that it rotates clockwise.

During which portions of the cycle does the coil from a complete circuit with the battery such that there is a current through the wire of the coil?

The current results in a magnetic moment that interacts with the magnetic field of the magnet. Will the interaction tend to increase or to decrease the angular speed of the coil? Explain.

b. The coil is manually started spinning so that it rotates counterclockwise:

During which portions of the cycle does the coil form a complete circuit with the battery so that there is a current through the wire of the coil?

The current results in a magnetic moment that interacts with the magnetic field of the magnet. Will the interaction tend to increase or to decrease the angular speed of the coil? Explain.

Check that the behavior of your motor is consistent with your answers.

4. Consider the following questions about the motor:
• Why was insulated wire used for the coil? Would bare wire also work? Explain.
• Would you expect the motor to work if the leads to the coil were stripped completely? Explain.

5. Predict the effect on the motor of (i) reversing the leads to the battery and (ii) reversing the orientation of the magnet

Check your predictions.