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An simple example of a working model for a science exhibition is a model of a DC electric motor. See the figure below.
Take a cork and fix two needles pins at its opposite ends. With a 0.1-0.3 mm copper wire make about 20-50 turns around the cork fixing the ends of the wire on the needles. Support the two needles on two horizontal thin screw-drivers (or anything metallic you can imagine to use as support rails, like for example two horizontal long and thin nails). Wire the two metallic rails to a DC battery (any voltage between 1.5-4.5 V will do the trick). The internal resistance of battery will limit the current in the circuit. Place under the cork a small permanent magnet of the size of half of your thumb taken from a toy or a home appliance (a small fragment of magnet taken from a loudspeaker will be just fine). The distance between the cork and the magnet should be about 2-5 cm.
When the voltage from the DC battery is turned on the cork will begin to rotate above the permanent magnet. This happens because the permanent field from the magnet interacts with the field created by the current and a rotating torque appears. (I have done myself this experiment and I was very much enjoying when the cork begun to rotate like a real motor).
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