Question

A group of students conducted an activity for science class. They used a galvanometer, which is an instrument that can detect an electric current. They followed these steps:
Obtain a 100 foot long electrical extension cord.
Connect each end of the extension cord to the galvanometer.
Using duct tape, secure part of the extension cord to the ground as shown below.
Swing the top part of the extension cord as if a person were to enter to jump rope.
Have a student observe the galvanometer while the cord is rotating.


Students observed that the needle of the galvanometer moves. What does this demonstrate?
A.
Moving an electrical conductor next to an electric current produces a magnetic field.
B.
Moving an electrical conducting wire close to the ground changes Earth's magnetic field.
C.
Moving an electrical conducting wire through a magnetic field changes the wire's conductivity.
D.
Moving an electrical conductor through Earth's magnetic field induces an electric current.

Answer 1

Correct answer: D.

Explanation: Swinging the long conductor through Earth's magnetic field changes the magnetic flux linked with the circuit, which by Faraday's law produces an emf and therefore a current the galvanometer can detect. This is electromagnetic induction (a motional emf, roughly E ≈ B·l·v for a straight segment moving through a field). Earth's field is small, but a long cord and noticeable motion can create a small but measurable current.