Question

In science class, students built the device pictured below using cardboard, wires, pipes, compasses, and a battery.



When there is no electricity running through the wire, all the compass needles point toward Earth’s magnetic north pole. When electric current is flowing through the wire, the compass needles change direction and point around the wire in a circle.

As part of their investigation, the students moved each compass farther from the wire and wrote down their observations. What were the students most likely testing in this part of the investigation?
A.
the size of the compasses
B.
the force of gravity between the compasses and the wire
C.
the direction that the current is flowing
D.
the strength of the magnetic field

Answer 1

D. the strength of the magnetic field

Moving the compasses farther from the current-carrying wire changes the magnetic field strength at their location (the field of a long straight wire drops off with distance, B = μ0I/2πr). As distance increases the wire's field weakens and the needles are less deflected from Earth's magnetic north. The other choices are not tested by changing distance.