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The simplest magnetic field is that produced by a current flowing through a straight wire. Imagine a straight vertical wire carrying a current and passing through a small hole in a horizontal card. Iron filings sprinkled on the card set themselves in concentric circles around the wire. The lines produced by the filings correspond to lines of magnetic flux.
A compass needle tends to set itself tangentially to a line of magnetic flux so by definition the positive direction of a line of magnetic flux is taken as the direction in which the north pole of a compass needle tends to point. The positive direction of the lines of magnetic flux can be found from the 'Corkscrew rule': if a (right-handed) corkscrew is turned in the direction of the current, the thumb rotates in the positive direction of the lines of flux. |
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Click on the
wire to see the magnetic field effects of current flow through a straight wire.
