How do magnetic forces cause an electric motor to spin?
An electric motor uses electric current to create rotational motion, which can then be used to spin a fan or propel an electric car. In this investigation you will create a simple electric motor using a current-carrying coil and a magnet. The magnetic field from the permanent magnet exerts a torque on the coil, causing it to spin. The principle behind the electric motor is the same as for a galvanometer.
How it works
When current flows through the coil shown on the left, the magnetic field exerts a clockwise torque that causes it to rotate. This rotation brings it to the position shown on the right, where the torque acts counterclockwise. Under these conditions, the coil can never spin continuously. To ensure that the torque acts in only one direction, the current (and therefore the torque) must shut off during half of each rotation.
Build the motor
Wrap approximately 80 cm of magnet wire tightly around a “D” cell battery, leaving a few centimeters free at each end.
Remove the coil. Create axles by wrapping each free end once around the coil to hold it tightly together.
Create a stand from two large paperclips and make a loop in each to hold the axles. Tape one support to the tabletop.
Test that your coil spins freely by inserting an axle into the support, setting the second support in place, and flicking the coil.
Remove the coil and use a knife to remove the insulation from the upward-facing sides of both axles. Do not remove insulation from the back side of the axles.
Place the coil into the supports. Connect the supports to the battery so that current will flow when the coil is in the face-down position.
Bring the magnet close to the coil. You may need to start it spinning but the magnet will keep it spinning.
Predict the effect that changing the number of batteries or magnets will have on the operation of the motor. Explain your prediction using the equation F = ILB.
Test your predictions. Were you correct?
Propose additional investigative questions: What variables could you change that might affect the operation of your motor?
Create the coil
Magnet wire is coated with a thin insulating layer. There is a difference in color between the insulating layer and the underlying metal that will help you distinguish between the two. This illustration shows where to remove the insulation so that your coil will spin. The insulation must be thoroughly removed over a full half turn of each axle. To access the top and bottom edge of each axle it is helpful to hold the coil off the edge of the tabletop. This allows you to twist the coil slightly so that these edges face upward.