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.

Why do the compass needles change direction when the electric current is flowing?
A.
The Earth’s magnetic field changes direction while the electricity is connected.
B.
The compasses create their own electric field, which forces their needles to rotate.
C.
The current creates a magnetic field that exerts a force on the compass needles.
D.
The electricity is flowing through the compass needles, causing them to turn toward the wire.

Answer 1

C.

A compass needle is a tiny magnet and aligns with the local magnetic field. A current-carrying wire produces a magnetic field in concentric circles around the wire (Ampère’s law/right‑hand rule). When current is on, that wire field adds to the Earth’s field, so the needle turns to point along the resulting (circular) field lines. Options A, B and D are incorrect for the reasons above.