PHYSICS EXPERIMENT: ELECTROSTATIC CHARGES
rev. 04/17
This laboratory is designed to provide an overview of the force effects due to stationary electric charges on bodies.
It is designed to be a qualitative rather than a quantitative experience, meaning that observation rather than
measurement is the key aspect. Remember to record (describe) everything you observe in as clear terms as possible as
if you were the first person in history ever doing the observations. Your eventual conclusions and explanations will be
based on your recorded observations.
INTRODUCTION
Electric charge is a basic property of all matter. It has the following properties:
1.
There are two kinds of charge, positive and negative. [In normal situations, only electrons are mobile,
thus it is their movement that creates a charged object.] Like charges repel and unlike charges attract;
uncharged bodies have equal amounts of each type of charge.
2.
The force between point charges varies as the inverse square of their separation and is directly
proportional to the product of the magnitudes of each charge. Force effects due to non-point charge
distributions will show behavior similar to point charge effects.
3.
Charge is conserved; hence, it can both neither be created nor destroyed, only transferred.
4.
Charge is quantized, occurring only in integer multiples of the basic amount of charge which is that
amount on a proton or an electron.
The charge of a body can be one of three states: positive, negative, or neutral. A positively charged body has a
deficiency of electrons, a negatively charged body has an excess of electrons, and a neutral body has the same
number of electrons as protons.
Conductors transfer charge easily via free electrons; excess charge moves to the outside surface due to repulsion
of like charges. Insulators have very little free charge movement through the material but charge can be added or
removed from the surface in several ways.
Charging an object is usually accomplished in one of three ways: friction (release of electrons by rubbing them
off), induction (redistribution without touching), and conduction (exchange of electrons due to contact between
bodies). In addition, polarization may be used to temporarily rearrange the internal charge distribution of nonconducting materials that have a dipole structure.
This lab will employ a variety of techniques and procedures to observe the aforementioned properties.
APPARATUS/MATERIALS (more or less…)
Electrostatic generator
1-2 electroscopes (with shields)
metal spoon, balloon
Styrofoam/vermiculite pieces
compass needle (and stand)
pith ball chamber/equivalent
matches, incense, candle
other stuff...
glass rod, pvc pipe
non-glass rods (plastic/rubber)
nail/tack with streamers
thin metal hoop (pop can)
smoke chamber/flask
double foil ball electroscope
cloths: fur, silk, wool, rayon cloth, cotton,..
fluorescent light bulb
*Think about which materials provided are conductors and which are insulators.
NOTES: Different rod and cloth combinations can generate different types of charge when rubbed together. Only
three or four rubs should be necessary to create a static charge. A certain degree of caution is advised when
using the glass rod since it will shatter if dropped. The Van De Graf generator cannot generate enough
charge to hurt you, but some shocks can still sting. Please exercise normal caution when using it and turn it
off when it is not in use. Use the outlet covers (rather than lab partners) as grounds to neutralize excessive
change if needed.
Procedure
1.
Disperse the supplies over the lab table. Be sure that the generator is not too close to the electroscopes so
that their effects are isolated. Identify which rods and cloths you have for testing purposes.
2.
Test each cloth with each rod to determine the ability to generate an amount of static charge. Use the
electroscopes to determine the magnitude of the charge and the fact that oppositely charged bodies will
attract (and like charges repel) each other. Keep track of results in a chart similar to the one given below.
Note your best cloth-rod pairings for creating oppositely charged rods. From this point on you will use
only those pairings throughout the rest of the experiment. Also record the method you used to be sure
that the two rods were actually oppositely charged. [You may find the triboelectric sequence on the last
page of this handout to be useful for this step.]
Glass Rod
Pvc pipe
Other Rod (specify)
Rabbit Fur
Wool
Silk
Polyester
Latex Balloon
etc…
Key: S=strong effect
3.
W=Weak effect N=No effect
Now that you have determined your best pairings that produce opposite charges, charge either rod (by
friction) and bring near enough (without touching!) the small sphere at the top of an electroscope to see the
effect on the leaves below. Record your observations under the heading Procedure 3.
QUESTION #1: Make a sketch of an electroscope and indicate the distribution of positive and negative charges
when a positively charged rod is brought near. What causes this distribution?
4.
Charge an electroscope with one type of charge by conduction (touching the rod to the sphere) so that a
deflection is observed in the leaves even when the rod is removed.
Now, bring each charged rod (recharge as needed) near the electroscope and observe how the leaves react
in the presence of either a like or oppositely charged rod. Record your observations under the heading
Procedure 4.
QUESTION #2: An experimenter noticed that when a positively charged rod was brought near a negatively
charged electroscope, the leaves at first moved back together but then moved apart again as the
rod got even closer. What could have caused this effect?
5.
Set up the compass needle on its stand, ground it out, and then bring each charged rod individually near one
pointed end of the compass. Record (under procedure 5) what you observe.
QUESTION #3: Why do both rods produce roughly the same effect on the compass needle?
6.
Theoretically, it should be possible to bring both rods (aligned vertically) near opposite sides (left/right) of
one pointed end of the needle without causing any motion of the compass needle. Try this. Record
observations.
QUESTION #4: Why might it be difficult to achieve/maintain a balance using both rods?
7.
Turn on a faucet so that a very light stream of water is flowing. Bring each charged rod near the stream and
observe (and record) what happens. Vary the flow rate and see if it changes the effect of the charged rods
on the water. Record all observations.
QUESTION #5: What can you conclude about the net charge of tap water? Explain. Was the electric force on
the water decreased as the flow rate increased? (Or was the acceleration effect due to the force
less?) Explain.
8.
Set the metal hoop on the table as if it were a wheel (but not rolling). Bring a charged rod into the hoop
along the axis of the hoop (through the center). Then, move the rod off-axis (towards the inside wall of the
hoop) and observe what happens. Record your observations and sketch the probable charge distribution
causing the latter effect.
9.
Turn on the generator and use it to charge the two hanging foil balls via contact with the top surface. Once
the balls are charged they should repel each other but maintain equilibrium.
QUESTION #6: What three forces act to create this equilibrium? Draw a free body diagram for one of the
balls. Ask your instructor for help if you are unclear on free-body diagrams.
Bring each charged rod near the charged balls and observe their behavior. Do your observations verify that
each rod carries a different charge? Explain. Bring a neutrally charged conductor (the spoon) near the
charged balls and record what you see in this case.
10. Place several pieces of Styrofoam on the generator and turn it on. Record your observations. Repeat with
the vermiculite. Try to place any pieces that fly off back on the surface. Turn off the generator and keep
trying to put the Styrofoam/vermiculite back on the surface. Record all observations.
11. Take a piece of notebook paper and tear it into small pieces. Try picking the pieces up with your charged
rods. Then, place the paper pieces on the generator and turn it on. Record all observations.
QUESTION #7: Why does some Styrofoam/vermiculite fly off the generator? Why might it sometimes fly off
even after the generator has been turned off? What about the paper?
12. Hold the pith ball chamber at one end and place the other end near the generator while it is on. Record your
observations.
QUESTION #8: What causes the movement of the pith ball? How does the behavior of the pith ball change
after the generator is turned off?
13. Put the nail/tack with streamers at the top of the generator and turn on the device. [You may have to tape
the tack to the generator to keep in on the hemisphere.] Record your observations and explain what causes
what you observe. Bring each charged rod and the spoon near the streamers and record what happens.
QUESTION #9: If the generator hemisphere is positively charged, which rod carries which type of excess
charge? If one of the rods is positively charged, where did the excess electrons go?
14. You may need to wash out and dry the smoke chamber before initiating this step. Use a match and some
incense to put smoke into the smoke chamber (or test tube). Bring the chamber near the generator and
observe what happens. Try it with the chamber open or closed to see it there’s a difference. Record all
observations.
QUESTION #10:
Do some research and describe how an electrostatic precipitator works.
15. Hold a lighted candle near the generator, repeat with a lighted match. Record the results and discuss your
observations.
16. Dim the house lights and hold the fluorescent light bulb (at one end) and hold the bulb near the generator so
that the discharge hits the bulb in the middle of the bulb. Also try the same effect with the metal end of the
bulb near the generator. Record all observations.
QUESTION #11:
How does a discharge of static electricity create light in a fluorescent tube? (provide a
scientific answer)
17. With the room still dark, determine and record the maximum distance that charge will jump from the Van de
Graaff sphere to the rounded edge of the spoon. Determine the same distance replacing the spoon with your
finger. Also try the back of your hand. Put some hand lotion on your hands and repeat the finger & hand
procedure. Pull on a pair of vinyl gloves and repeat. Put your hand inside a metal can and repeat.
QUESTION #12:
18.
Should excess charge jump more readily towards a conductor or towards an insulator?
Why?
Using any available materials, create and perform another procedure which demonstrates the basic
nature of electrostatic charge. Record both your procedure and observations.
REPORT
The write-up for this lab should consist of a short introduction (on the nature of electric charge) followed by all of
your observations (and sketches), neatly recorded. They should be written in complete sentences and well labeled as to
which procedure they describe. The answers to the questions should appear among your observations at the point at
which they were asked. Finally, a brief discussion should summarize what you have observed and how it relates to the
given properties of charge and methods of charge generation.