Name:
Experiment 6A: Electrostatics
Introduction: This will be a very simple hands-on investigation that you will perform in your groups and individually complete a small laboratory
report for. Believe it or not, if it is a day with high humidity, this won’t work well; hopefully the air conditioning is working.
Materials:
Pith ball (Cork)
Electroscope
Glass Rod
Silk Thread
Fur/Wool
Rubber Rod/Vinyl
Holder/Lab Stand
Silk
Van de Graf Generator
NOTE: Depending upon the availability of some materials, the first part of the investigation may be performed as a demonstration. You will still be
required to answer all of the questions in the lab report concerning this section.
Background
Things can gain a static electric charge (either negative or positive) by either gaining or losing electrons. An object that gains electrons has
a net negative charge; an object that loses electrons has a net positive charge.
Any object can acquire a static electric charge. Only objects that are separated from the ground by nonconductors will retain their charge
for any length of time. (Such as you acquiring a charge from walking across the floor and then shocking the cat.)
In this experiment you will use the materials provided (or observe) to experiment with objects having static electric charges.
As you perform the experiment, keep in mind these three important laws/rules:
1.) Electrons are highly mobile: they can easily move around within a body or can move from one body to another.
2.) Rubber rods (plastic strips or rods) when rubbed with fur/wool they acquire electrons from the fur and become negatively charged.
3.) Glass rods when rubbed with silk lose electrons to the silk and become positively charged.
Figure 1: (a) Charges are redistributed on the neutral pith ball so that it is attracted to the rod. (b) After the ball acquires a charge, it is repelled.
Negatively charged pith ball
Neutral pith
ball
-+
_ _ ++
_ + +
- - - - - -
- - - - - -
- - - - - -
Negatively charged
plastic
Negatively charged
plastic
(a)
(b)
If a negative charge is brought close to a suspended pith ball, electrons in the side of the pith ball facing the charge will be repelled to the
other side of the ball. This will leave the side of the pith ball facing the negative charge positive and the ball will be attracted to the body bearing
the negative charge. (Remember: unlike charges attract one another.) As the bodies make contact, the negative plastic will feed electrons into the
positive side of the pith ball. This results in an excess of electrons on the pith ball. The pith ball has gained a net negative charge; the negatively
charged plastic repels it. (Remember: like charges repel one another.)
Procedure
Follow the procedure below; the lab report will be a little different this time. Instead of a formal report, you’ll be filling in a form. EACH
OF YOU will fill in the form and hand it in at a specified time in the future. (Probably less than a week.) The form should be attached at the end of
this document. If it is attached, remove it and fill in the answers as you complete the lab. There will be an analysis section at the end that you will
complete after the ‘experiment.’
Part A – Charging a Pith Ball
[Remember, if we do not have the needed equipment, this will be done as a demonstration, with you answering the questions based on
what you observe.]
YOU MAY HAVE TO USE A VAN DE GRAFF TO CHARGE THE PITH BALLS
For this part you’ll need the rubber rod, glass rod, silk, fur, and suspended pith ball.
1. Rub the rubber rod with the fur or wool. Do this for a minute or so.
2. Bring the rubber rod close to one of the suspended pith balls.
3. On your lab report form answer QUESTION 1 and QUESTION 2.
4. Ground the pith ball (allow the charge to go to ground through you) by touching it.
5. Rub the glass rod with the silk. Do this for a minute or so.
6. Bring the glass rod close to the neutral pith ball.
7. On your lab report form answer QUESTION 3 and QUESTION 4.
Part B – The Electroscope
For this part you’ll need the rubber rod, glass rod, silk, fur, and an electroscope.
THE ELECTROSCOPE
The electroscope is a relatively primitive device that allows you to determine if there is a charge on an object and qualitatively how much
of a charge there is. Make sure you use a small piece of sandpaper or steel wool to clean off the corrosion on the top plate. If you don’t this
experiment won’t work very well.
Top plate – you will touch the glass and rubber rods
here to transfer charge.
Foil armature – this movable piece will indicate
whether or not a charge is present.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Touch the VDG with your hand.
Move your finger very near (not touching) the top plate. (Make sure you’ve used steel wool to clean off the plate.)
On your lab report form answer QUESTION 5.
Move your finger away from the vicinity of the top plate.
On your lab report form answer QUESTION 6.
Charge your hand again.
Touch the top plate of the electroscope with the charged finger.
On your lab report form answer QUESTION 7 and QUESTION 8.
Touch the top of the electroscope with your finger.
On your lab report from answer QUESTION 9 and QUESTION 10.
Charge the GLASS ROD by rubbing it with silk.
Bring the glass rod to the top plate and touch it.
On your lab report form answer QUESTION 11 and QUESTION 12.
When the electroscope was charged with the VDG and had a bunch of extra electrons and somebody not charged touched it, what
happened?
15. On your lab report form answer QUESTION 13 and QUESTION 14.
A. Return all materials to where you found them.
B. Complete the analysis section of the lab report form.