Charging an
Electroscope by Induction Using a Negatively-Charged
Balloon
A GIF Animation
An electroscope is a common demonstration apparatus used by
physics teachers to illustrate electrostatic principles of charging
and charge interactions. The electroscope is most commonly used as a
charge-detecting device. The electroscope shown in the animation
below consists of a plate (near the top), a support stand (which
connects to the plate and extends through the center of the scope),
and a needle which rests upon the support stand and is free to rotate
about its pivot. The plate, support stand, and needle are all made of
a conducting material which allows for both the free flow of
electrons and a uniform distribution of any excess charge in the
electroscope. By observing any deflection of the needle, the presence
of charge in either the electroscope or any nearby object can be
determined.
One common demonstration performed with the electroscope involves
the induction process of charging. In the induction process of
charging, a charged object is brought near to but not touching the
electroscope. The presence of the charged object above the plate of
the electroscope, induces electrons within the electroscope to move
accordingly. With the charged object still held above the plate, the
electroscope is touched. At this point electrons will flow between
the electroscope and the ground, giving the electroscope an overall
charge. When the charged object is pulled away, the needle of the
electroscope deflects, thus indicating an overall charge on the
electroscope. The process of charging an electroscope by induction
using a negatively-charged balloon is depicted in the animation
below.
As shown in the animation above, the presence of the
negatively-charged balloon above the plate of the electroscope will
induce the movement of electrons from the plate of the electroscope
to the support and needle of the electroscope. This is explained by
the "like charges repel" principle. The negatively-charged balloon
repels the negatively-charged electrons, thus forcing them to move
downwards. Once the electrons leave the plate and enter the needle,
both plate and support/needle acquire an imbalance of charge. The
plate acquires an excess of positive charge (since electrons have
left this once neutral region) and the support/needle acquires an
excess of negative charge (since electrons have entered this once
neutral region).
Once charge within the electroscope has been "polarized"
(i.e., separated into opposite types), the bottom of the electroscope
is touched by a finger. Being positively-charged, electrons from the
electroscope exit and enter into the ground. Once more, this process
is driven by the principle that "like charges repel"; the electrons,
having a mutual repulsion for one another, choose to exit the
electroscope and enter into the larger region. By doing so, the
electrons are able to distance themselves and so minimize the
repulsive interactions. It is at this point in the induction process
that the electroscope acquires an overall charge. Since electrons
have left the electroscope, the overall charge on it is positive. In
general, the induction process will always place a charge on the
object which is the opposite type of charge possessed by the object
used to charge it.
It might be noted in the animation above that while the
departure of electrons may leave the electroscope with an overall
charge, the needle is still not deflected. The excess of positive
charge remains localized in the plate of the electroscope, being
strongly attracted to the negative charge of the balloon. Once the
balloon is withdrawn, there is a movement of any remaining electrons
in such a way that the positive excess charge becomes uniformly
distributed about the electroscope. At this point, the needle of the
electroscope becomes deflected and shows the presence of charge.
Additional information on physical descriptions of
electrostatic phenomenon will soon be available at
The
Physics Classroom.
This page was created by
Tom
Henderson of
Glenbrook South
High School.
Comments and suggestions can be sent by e-mail to
Tom
Henderson.
This page last updated on 2/9/98.