Evan's Space

Wonders of Physics

Electromagnetism

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A U-shaped magnets sits on top of a electronic beam balance. A wire is placed horizontally between the poles of the magnets as shown in the diagram below.

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Initially, when there is no current flowing through the wire, the balance reads 170.05 g. When a steady current of 1.50 A (flowing out of the paper) is passed through the wire, the balance reads 180.25 g as shown above.

Solutions: As a direct current is provided through the wire, Fleming’s Left Hand Rule (FLHR) is applied here. As magnetic field is from N to S (left to right) and current is out of the paper, the force on the wire will be acting upwards (in a direction from strong magnetic field to weaker magnetic field). We are all very familiar to this type of question.

But one has to remember that the forces always come in a pair (action is equal to reaction – Newton’s 3rd Law).

Hence due to the combined magnetic field between the magnets and wire, a force is acting on the wire upwards, hence there must be an equal and opposite force acting downwards on the magnets.

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That explains why the balance reads a higher value. On the other hand, if current is into the paper, the principle applies here too. The force acting on wire will be downwards, and hence there is a equal and opposite force acting on the magnet upwards.

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