Physics Lab Activity Answers
Electrostatics - 3
Induction & Polarization
Part 1 - Charging by Induction
- You can tell that the electroscope has an electric charge by
observing that the vane is repelled from the support inside the
- The charge on the electroscope is opposite the charge on the
charged strip, since when the strip is brought close to the top of
the electroscope (but not touching), the vane collapses toward the
- (1) When the electroscope is initially grounded, any excess
electrons on the electroscope can escape to ground, or any
electrons that would like to get onto the electroscope can do so.
Therefore, the electroscope is neutral after grounding.
(2) Bringing a charged strip near the electroscope polarizes it.
If the strip has a negative charge, the electrons in the
electroscope are repelled toward the bottom of the instrument.
This creates a region of positive charge near the top of the
electroscope and a region of negative charge near the bottom -
although the electroscope, as a whole, is still neutral. The vane
separates from the support since they both have a net negative
charge (they are at the bottom end of the electroscope), and like
charges repel. If the charged strip is positive, the argument is
(3) When the electroscope is grounded, Any electrons that want to
get away from a negatively charged strip. or any electrons that
would like to get closer to a positively charged strip, can travel
to and from the electroscope.
(4) When you remove your finger, any excess electrons that were
attracted to a postive strip are now marooned on the electroscope
- giving it an net negative charge. If electrons fled the
electroscope to excape a negatively charged strip, they can't get
back - giving the electroscope a net positive charge.
Part 2 - Electrical Polarization
- The (neutral) pop can is attracted to the charged strip.
Suppose the strip has a negative charge. Electrons in the pop can
will be repelled to the opposite side of the can from the charged
strip. This will leave the side of the can nearest the strip with
fewer electrons than normal, and the side farthest from the can
with more electrons than normal. The pop can is still neutral, but
it is polarized. Since the electric force depends on the distance
between charges, even though there are the same number of positive
and negative charges in the pop can the positive charges are
closer. This means that the attractive force due to the positive
charges in the pop can will be greater than the repulsive forces
due to the negative charges in the can. There will be a net force
on the (neutral) can which will pull it toward the charged strip.
- Just as with the pop can, the neutral pith ball is attracted
to the charged strip. The charged strip polarizes the pith ball,
attracting its unlike charges and repelling its like charges..
Since the unlike charges end up on the near side of the pith ball,
their attraction force toward the charged strip is greater than
the repulsion force away from the charged strip due to the like
charges. There are the same number of positive and negative
charges in the pith ball, but the unlike charges are closer.
- Charged styrofoam "peanuts" or a charged balloon stick to a
wall (or you!) becauss the charged object (peanut or balloon)
polarizes the wall (or you) so that the attractive force between
the unlike, nearby charges is greater than the repulsive force
between the farther away, like charges. Even though there are the
same number of positive and negative charges in the wall (or you),
there is a net attractive force toward the charged object!
last update February 21, 2001 by JL