Pre- AP Physics
Static Electricity
Name__________________
January 22, 2016 Period: ___
Production of Static Charge
Purpose: To learn how to create and transfer static charge and determine the sign of the
charge.
Materials: Wool felt, Silk cloth, glass rod, black rubber rod, pith ball suspended from silk
thread, aluminum pie pan with Styrofoam cup, square of Styrofoam insulation, iron, heater, or
hair dryer.
Background: Clothes removed from a clothes dryer usually cling
to each other and spark or crackle with static electricity when they
are separated. When two dissimilar materials are rubbed together,
they can become charged. Objects can acquire static electric
charges by either gaining or losing electrons. An object that gains
electrons has a net negative charge and is said to be negatively
charged. An object that loses electrons has a net positive charge
and is said to be positively charged. Only those objects separated
from a ground, or Earth, by an insulator will retain their charge for
any length of time. Objects that are attached to a ground through a
conductor will remain uncharged, since the charge travels into the
ground and is quickly dissipated. Recall that a black rubber rod
rubbed with wool or fur will become negatively charged, while a
glass rod rubbed with silk will become positively charged. In this
experiment, you will charge objects and observe their interactions with other charged objects.
Procedure: Perform each part of the experiment several times to be sure you have proper
observations of the phenomena. You will have to rub the rods vigorously to charge them,
particularly when you use the glass rods. Warming the cloth with the iron or heater produces
better results on damp days.
A. Negatively Charging a Pith Ball
1. Rub the rubber rod with the wool felt to charge it. The rod is now ___________
charged. Bring the rod close to, but not touching, a suspended pith ball, as shown in
Figure 1. Observe the behavior of the pith ball and record your observations below.
2. Touch the pith ball with your finger to remove any charge it may have. Charge the
rubber rod with the wool felt again. Bring it close to the pith ball and allow it to touch the
ball. Then bring the charged rod near the charged ball and observe the behavior of the ball.
B. Positively Charging a Pith Ball
1. Rub the glass rod with a piece of silk to charge it. The glass rod is now __________
charged. Bring the rod close to, but not touching, a suspended pith ball. Observe the
behavior of the pith ball and record your observations.
2. Charge the glass rod again. Bring it close to the pith ball and allow it to touch the ball.
Then bring the charged rod near the charged ball and observe the behavior of the pith ball.
Record your observations.
C. Charging a pie pan positive by Induction
1. Rub the Styrofoam insulation sheet briskly with the wool felt. This makes it negatively
charged. Place the pie plate on top of the Styrofoam sheet as show in figure 3. Note how
the charges separate. Touch the top of the plate with your finger. You should hear and feel
a spark of negative charge leaving and moving to your finger. Write your observations here.
Figure 3
3. Hold the inverted Styrofoam cup and lift off the pie pan so as not to discharge the
aluminum plate which is now positive. Notice as you lift the pie pan is there an
attractive or repulsive force between the pan and insulation sheet. What does this mean
about the charge on the aluminum plate?
4. Now touch it with the finger of the other hand and see if you feel the spark. If you had
difficulty feeling or hearing the spark then charge it again and discharge it to your
earlobe. You will definitely be able to hear it this time. Write your observations.
Questions:
1. The rods where charged by induction, conduction, or friction?
2. Explain why the pith ball in part A was repelled by the charged rod after being touched with
the charged rod.
3. The pith ball was charged by induction, conduction, or friction?
4. The Styrofoam sheet is an insulator and therefore does not conduct charge well. It the plate
is placed on the Styrofoam sheet the charge does not move to the plate. Instead the plate
which is a conductor has negative charges move from the bottom of the plate to the top.
Explain why this happens.
5. Explain why the plate becomes positive after the finger is touched to the top of its surface.
6. Explain why you must hold the Styrofoam cup when transferring charge.
7. What purpose did your finger serve when you charged the plate by induction?
8. Compare the sign of the charge that an item gains when it is charged by conduction and by
induction.