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.
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.
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.