4a)

The magnetic flux density at a point in a magnetic field is the force per unit length per unit current

acting on a straight current-carrying conductor placed perpendicular to the field.

4bi)

The rod is experiencing a magnetic force because it is carrying a current and placed perpendicular to the magnetic field.

The balance reading decreased because a magnetic force now helping to support the magnet’s weight.

This means that the magnetic force acting on the magnet must be upward.

By Newton’s 3^rd Law, the magnetic force acting on the wire must be downward.

With the magnetic field directed from N to S, the current in the rod must be from X to Y. (FLHR)

4bii)

Change in mass reading

The change is due to the magnetic force:           

4biii)

No change.

Since the change in magnetic force is same as before, the change in mass reading is also same as before.

COMMENT: The mass of the rod is irrelevant. It only plays the part of exerting the magnetic force on the magnet.

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