Question

A student wraps insulated copper wire 12 times around a nail, and connects the ends to a fresh 9-volt battery. The student finds that the nail can now be used to lift small iron weights. The student disconnects the circuit and wraps a longer copper wire 30 times around another nail of the same type. The student then connects the wire to a fresh 9-volt battery. What should the student observe when attempting to use the new nail to lift small iron weights?

A.
The nail will be able to lift heavier weights, because the greater number of turns of the wire yields a more powerful magnetic field.

B.
The nail will only be able to lift the same amount of weight, because the physical properties of the nail determine magnetic force, and nails of the same type were used in both trials.

C.
The nail will be able to lift heavier weights, because the greater mass of conductive matter in the system will balance a greater mass of iron in the weights.

D.
The nail will only be able to lift the same amount of weight, because the magnetic force of the system depends on the current from the battery, which has not changed.

Answer 1

A.

Reason: The magnetic field of an electromagnet is roughly proportional to the product of the number of turns (N) and the current (I) through the coil (B ∝ N·I). Increasing the turns from 12 to 30 increases N and so (in typical setups) produces a stronger magnet able to lift heavier iron weights. (Options B and C are incorrect; D is incorrect because adding turns changes the coil resistance and can change the current — and even if current changed somewhat, the larger N usually yields a stronger field.)