Question

Jen is conducting an experiment involving a device known as a pith-ball electroscope, shown below.



The plastic ball of the electroscope hangs in midair and is uncharged. Charges cannot move along the silk thread that holds the ball in the air.

Jen completes the following steps in her experiment:
She collects three identical, uncharged rubber tubes.
She holds a magnet to the first tube for 5 minutes.
She submerges the second tube in a tank of water for 5 minutes.
She rubs the third tube with animal fur for 5 minutes.
Jen's teacher tells her that she can use the pith-ball electroscope to see if any of the rubber tubes are surrounded by an electric field. What should Jen do to test for an electric field surrounding each tube?
A.
bring the magnet close to the plastic ball and note any motion of the ball toward the magnet
B.
connect the three tubes to each other and see if they attract the plastic ball
C.
hold the magnet in direct contact with the tube that was submerged in water
D.
bring each tube close to the plastic ball and note any movement of the ball

Answer 1

Answer: D

Bring each tube close to the pith ball and watch for motion. The pith‑ball electroscope detects electric fields/charges: a charged tube will induce charge separation in the neutral ball and cause it to move (attract or, after contact, possibly repel). A magnet will not move a nonmagnetic plastic ball, and tying the tubes together or touching them with a magnet doesn’t test whether each tube has an electric field.