A student places a magnetic compass near an insulated copper wire with electric current moving through it. He notices that the compass needle no longer points north.

Question

22.A student places a magnetic compass near an insulated copper wire with electric current moving through it. He notices that the compass needle no longer points north.

Which of the following best explains this observation?

A. The magnetic poles that determine north and south reverse periodically.

B. The insulation prevents the compass from detecting Earth's magnetic field.

C. The compass detects the magnetic field produced by the electric current.

D. The copper wire exerts an electrostatic force on the compass needle.

I think the answer is either C or D?

Answer 1

Ampere's law states that if there is a current I in a long straight wire, then magnetic fields form in the shape of concentric circles with the wire passing through the center of these circles. For a distance r from the long straight wire, Ampare's law states that the magnetic field B felt there by a test compass is given by

B = μ_oI/(2πr)

where μ_o is the magnetic permeability of free space, which is a Universal constant given by 4π X 10^-7 Teslas⋅meters squares per Amperes squared.

Essentially, Ampere's law is a one of the five fundamental principles of Electromagnetic Theory and it states that a current causes a magnetic field at a distance r. Therefore, c is your answer.