Electric motors

Imagine a wire carrying an electric current, placed between two magnets. The magnetic fields from the magnet interact with the electrical magnetic field from the wire. The force that results moves the wire to a new position. This is called "catapult effect". Electric motors use this idea.

Make an electric motor

It may help you to understand electric motors better if you build one yourself. You will need:

  • 2 permanent magnets

  • 1 large cork

  • 6 pins

  • 1 knitting needle

  • some thin, insulated copper wire

  • plasticine

  • soft board (fibreboard)

  • 4.5 V battery

  • 2 pieces of thicker, insulated copper wire

  • a sharp knife

  • 2 drawing pins

 

  1. Cut a narrow channel on either side of the cork. Push the knitting needle through the center of the cork and push two pins into one end of the cork.

  2. Strip about 2 cm of the insulation off one end of the thin wire. Wrap it round one of the pins. Wind the wire round the cork about 30 times. Strip the insulation off the other end and wind it round the other pin.

  3. Push two pairs of pins into the board so the knitting needle can rest in them like a cradle. Strip the insulation from the ends of the thicker copper wires. Use drawing pins to hold them so they just touch the pins in the cork. Use plasticine to support the magnets on each side of the coil, with opposite poles facing. Connect the wires to a 4.5V battery and give the cork a flick to start it going round.

         (The Usborne Book of Science,1993)

 

What is happening?

There are two separate fields of force working together in the motor. Imagine the wire carrying the current is sticking straight out of the page towards you.

The two magnets, with their opposite poles together, set up fields of force that cross in the space between them (look at diagram 1). 

Each wire makes its own field of force. (diagram 2).

The combined force looks like this (diagram 3). It has a "catapult effect" on the wire, pushing it to one side. In the motor this has the effect of pushing one side of the coil up and the other side down, which means the coil will rotate. 

Electric motors are used to do many useful jobs of work; in vacuum cleaners, drills, trains, lifts and washing machines for instance. The motor is using electrical energy to do "work" (i.e. drive a machine).

 

 

 

Click here to see animation of electric motor,

and here for Lorentz force.

 


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