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Magnetic Circles

Abstract

To conduct a simple experiment to demonstrate how a current carrying conductor generates a magnetic field.

Materials

A one foot long coat hanger wire (With any insulation scraped off).
A wooden stand with a flat surface at around halfway height.
5 or 6 small compasses.
A 6 volt battery.
Two electrical lead wires with alligator clips

Planning the Procedure

Make a hole in the centre of the flat surface of the stand and pass the coat hanger wire through it so that it is suspended vertically.

Arrange the 6 compasses in a circle around the wire on the flat surface.

Attach the lead wires to the battery terminals. Leave the other ends of the wires free for now.

Observe the compasses now. You will see that all compasses point in the same direction, towards magnetic north.

Now connect the battery terminals to the coat hanger wire using the lead wires. Observe the orientation of the compass needles now. Each compass will point in a direction tangential to a circle centred on the coat hanger wire.

Rotate the support stand and observe again. The compasses will continue to point in tangential directions.

Connect the battery to the coat hanger wire with opposite polarity by switching the lead wire clips and observe the compasses again. You will see that the compass needles now reverse the direction they are pointing in, but continue to point in a direction tangential to a circle centred on the wire.

Scientific explanation:

Usually a compass lines up with the earth’s magnetic field. The north pole of the compass needle, which is itself a magnet points towards the south pole of the earth’s magnetic field as opposite poles attract each other. Now, when a current passes through the wire, it generates a magnetic field which is much stronger than the earth’s magnetic field. Hence the compass needle is now affected by both the earth’s magnetic field and the current carrying wire’s magnetic field. Since the latter is much stronger, the compass needle gets aligned with the wire’s magnetic field. This magnetic field can be visualized as a set of concentric circles with the wire as the centre. When the direction of current in the wire is reversed, the compass needle also points in the opposite direction as the magnetic field is reversed.