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We need to understand the pattern of field lines around a straight conductor carrying current and also the direction of these field lines. Here is an experiment to map the magnetic field lines.
| Arrange a copper wire, key, battery and a thick sheet of cardboard with a hole at its centre as shown here
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The iron filings arrange themselves in concentric circles around the conductor
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This is because the electric current produces a magnetic field around the conductor and the iron filings being magnetic in nature align themselves in concentric circles
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| Sprinkle some iron filings on the cardboard
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The direction of magnetic field lines can be found out using a compass needle
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| Gently tap the cardboard
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| The iron filings do not show any change in their arrangement because no current is flowing through the conductor
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| Switch on the current and observe the magnetic field lines
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| Repeat the experiment by reversing the direction of flow of current
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The iron filings arrange themselves in concentric circles around the conductor but in the opposite direction
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This is because the direction of magnetic field changes when the direction of flow of current is reversed
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