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 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.

C.
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.

D.
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.

Answer 1

C.

Reason: the magnet strength of the coil on the nail is proportional to the product N·I (turns × current). Increasing the turns increases N but also increases the wire length and thus the resistance, so the current I from the 9 V battery falls roughly in inverse proportion. For the same wire gauge and battery, N and I change in opposite ways so N·I stays about the same, and the lifting ability remains essentially unchanged. (In a real circuit battery internal resistance and wire heating can cause small differences.)