Lab 3– PC226 1.1 Charging by Friction 1.2 Conductors and Insulators

Lab 3– PC226
The following lab has two parts: In the first you will get familiar with electrostatics. In
the second as a test of Coulumb’s law you will find the number of electrons on a balloon.
For the first part you may cut and paste the worksheet and mark x (see below) and fill in
the needed questions. For the second you will write a lab report.
Part I
1.1 Charging by Friction
a. Inspect the construction of the electroscope. Then use a wire to connect the
base of the electroscope to “ground”. Is this done _____
(This connection prevents the build-up of charge on the outer ring of the
electroscope; erratic results could result from such charge.)
b. Discharge the electroscope by momentarily touching the disk (at the top) with
your hand. The movable vane of the electroscope should now be at (or near) a
vertical orientation. Is it _______
c. Charge the white rod! While holding the dry rod in one hand, rub it vigorously
with the piece of fur and/or clothing. You should hear faint “cracking” sounds,
and perhaps feel it getting warm.
d. Hold the charged rod about a foot above the electroscope, and then lower it
toward the disk of the electroscope, touching the disk with the rod if you like.
Does the vane rotate away from the vertical position as the rod approaches
the disk ______
Does it start to rotate even before the white rod touches the disk _____
(Repeat steps b to e as necessary, until you have confidence in your
observations and answers.)
e. Repeat these activities, using now the solid clear rod (acrylic). (Discharge the
electroscope, charge the rod, and bring it near the disk, etc. - steps b, c, d, and
e above.)
Does the vane rotate away from the vertical position as the clear rod
approaches the disk ______
Does it start to rotate even before the clear rod touches the disk _____
Question Q1: When you rubbed the white PVC plastic rod with tissue, it
gained a net negative charge. When you rubbed the clear acrylic plastic rod
with tissue, it gained a net positive charge. Did the behavior of the
electroscope depend noticeably on the sign of the charge of the rod _______
Explain this result if you can.
1.2 Conductors and Insulators
1. Metals: Do they appear to be conductors or insulators
a. Place a net charge on the electroscope. To do this, charge the white rod,
and then drag the length of the rod several times across the flat portion of
the electroscope disk. Repeat this until the electroscope vane remains
deflected even when the charged rod is removed from the vicinity of the

electroscope. Is the electroscope now charged ______
How can you tell ____________________________________________
b. Holding an aluminum rod in your hand, touch the rod momentarily to the
disk at the top of the “charged” electroscope.
Describe what happened. ______________________________________
c. Repeat steps a and b if necessary until you have confidence in your
observations and statements above.
Your conclusions: Does the aluminum rod appear to be a conductor of
electric charge, or an insulator __________________ Justify your
conclusion:
d. An ordinary lab wire consists of a core of copper strands with an outer
covering of plastic. Each end of the copper core is connected to metal
plugs.
Repeat the process above (steps a, b, and c), using now an ordinary lab
wire instead of the aluminum rod. With one hand, hold the bare metal plug
at one end of the wire. Then with your other hand holding the plastic cover
of the wire, bring the other bare metal plug into contact with the disk of the
charged electroscope.
Repeat these steps until you have confidence in your observations.
Does the length of lab wire act as a conductor of electric charge, or as an
insulator __________________ Justify your conclusion:
2. Plastics: Do they appear to be conductors or insulators
a. Again, place a net charge on the electroscope (as you did in part 1-a
above).
b. Holding any available plastic rod in your hand, touch the rod momentarily to
the disk at the top of the “charged” electroscope, and then remove it.
c. Did the electroscope discharge (as it did when you touched the
electroscope with the aluminum rod) _____
Repeat these steps until you have confidence in your observations.
Your conclusions: Does the plastic rod appear to be a conductor of electric
charge, or an insulator __________________ Justify your conclusion:
_
In part 1-a above, you were directed to use a charged rod to charge the
electroscope as follows, “drag the length of the rod several times across the
flat portion of the electroscope disk”.
Q1. Why can’t one simply just touch the disk with the rod Why should one want
to “drag the length of the rod several times across the ....disk”
Comments: Materials are not purely insulators or conductors. Different materials
have values of electrical conductivity (or resistivity) that are distributed over a
range covering many orders of magnitude. When we say a material is an
insulator, we mean that the electric current through it is likely to be negligible for
typical applications.
3. The plastic insulation on an ordinary lab wire: Is it perfectly insulating
pressing the insulation of a lab wire against the disk of a charged electroscope
a. Place a net charge on the electroscope.
b. Hold the two metallic plugs of a short lab wire (say 12” long) tightly in one

fist. Then with the other hand, press the plastic insulation (only!) near the
middle of the wire against the disk atop the charged electroscope.
Does the electroscope discharge (perhaps slowly but noticeably) through the
plastic insulation ____
Estimate roughly the observed discharge rate (e.g., 30% in 10 s): ________
Comments: Typically a noticeable amount of charge flows from the electroscope
through the plastic insulation. This flow of charge, or current, is of the order of 10-
9 Amperes ( 1 nA, or 1 nanoampere).
4. Teflon insulation: Is current leakage through Teflon noticeable
a. Locate a length of wire insulated with (red) Teflon. (There should be one
at your table; it is red, and should have a label near one end.)
b. Using the Teflon wire, repeat steps a.) and b.) in section 3 above.
Does the electroscope discharge (slowly but noticeably) through the Teflon
covering _____
Does the Teflon appear to a better insulator than that of the PVC plastic used on
ordinary lab wires ______
Comments Because Teflon is an excellent electrical insulator, it is used for key
components of the apparatus for this experiment. The white insulating material
directly below the disk of the electroscope is Teflon, as is the white handle of the
electrophorus which you will use shortly.
Part II
you will use your knowledge of Coulomb’s law and Newton’s laws to find out the
number of extra electrons on every gram of balloon material. One practical reason for
such calculation is that if we have too many charges the balloon may pop. Also, extra
number of electrons on an insulator may result in physical and chemical changes.
This experiment will be explained in the lab. Basically: 2 balloons are hung from a single
point and charged up by rubbing them with fur. Because the fur charges both with
negative charges, they will repel.
Here is what you have to do before the experiment:
1. Write down Coulomb’s law. The distance between the balloon centers will be the
distance in the denominator of Coulomb’s law.
2. Draw a free-body diagram for one of the two balloons.
3. Write down Newton’s 2 nd law for the x and y directions (remember: the balloons
are not moving at equilibrium!)
4. Solve the two equations to find a relationship between the charge of the balloons
and the other measurable quantities (balloon mass, distance between balloon
centers, g, angle of the string).
5. Describe in details the procedures that you are going to do in order to obtain the
Electric force between the two balloons. Remember to include ways to measure:
5.1 The mass of the balloon with AND WITHOUT the air (see #9).

5.2 The location of the center of mass of the balloon
5.3 The angle the string makes with the vertical (no protractor required, or
recommended. A meter stick and some trigonometry are more accurate!)
6. Call the instructor to check and see that your plan is good
7. Perform the experiment: Charge the balloons, measure the mass of the balloons,
measure the angle. Measure the distance between the charged balloons.
8. If you have questions about HOW to measure a certain quantity be sure to ask!
For example, you will find out that measuring the distance between the balloons
and measuring the angles is tricky. What is the problem How can you do that
9. Find the number of electrons on the balloon. Remember that you can find that
number from the relationship between the total charge and the charge of one
electron.
10. Perform the analysis in class, and find n+/- n , where n is the number of
electrons per gram of balloon material (should you use the mass with or without
air) .
Note: It is interesting to find how many extra electrons there are per atom of balloon
material. Since balloons are made from chains of Carbon and Hydrogen one could
assume a 7 grams per mole ratio and find the actual number of electrons per atom. This
will be done in class.
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