1. Electrostatics Lesson 3
Forces and Fields
Electrostatics
Lesson 3

2. Electrostatic Series Sulphur Brass Copper Ebonite
Paraffin wax
Silk
Lead
Fur
wool
glass
Increasing tendency to hold on to electrons
Increasing tendency to lose electrons
Also called triboelectric series.
Whichever material is closer to the top holds electrons more tightly so will gain a negative charge.
Whichever is closer to the bottom has a tendency to lose electrons so will gain a positive charge.

3. Two Static Examples
Rubbing ebonite with fur produces a negative charge on the ebonite and a positive charge on the fur.
Rubbing glass with silk produces a negative charge on the silk and a positive charge on the glass because the glass gives up electrons more easily.

4. Grounding
The process of sharing charge with the earth. The earth can gain or donate electrons as needed to transfer a charge. The overall effect of this process is to make the object that comes into contact with the earth neutral.
The symbol for ground is
Earth
Conductive wire
electrons

5. Three Ways Charge Can Be Transferred
1. Friction (also called abduction)
2. Conduction – touching a charged object to an neutral object
3. Induction – bringing an charged object near a neutral object to cause temporary charge separation which polarizes the neutral object

6. Result – paper is attracted to the comb
Electrons in the paper are repelled to the most distant point leaving a localized positive charge near the negative comb.
Result – paper is attracted to the comb
When the comb is removed, electrons are redistributed in the paper.
Charge Migration

7. Induction Steps to charge by induction:
Bring a charged object close to (but not touching) a neutral object.
Ground the neutral object.
Remove the ground.
Remove the charged object.
(The previously neutral object will retain a charge because of the grounding step.)

8. Diagram of Induction

9. The Electroscope A simple device used to detect a charge on an object
It does not identify the quantity of charge, only whether a charge is present. It can be used to detect the sign of the charged object with some additional steps, but not directly.

10. Two Types 1. Pith Ball Electroscope
When pith balls are touched with a charged object, the two balls acquire the same charge and therefore repel each other.
After (repel)
before

11. 2. Gold Leaf/Foil Electroscope
When the metal knob is touched with a charged object, the leaves/foil will repel each other.
Metal knob
Insulator
Glass container
Metal rod
Gold leaves (touching)
Gold leaves (repelling)

12. There are two ways to charge an electroscope.
Charging by CONDUCTION
Touch the ball with ebonite, (excess e’) and remove the rod. Some e’ are transferred from the ebonite which makes the ball negative. The leaves are now both negative and will repel. The electroscope’s leaves will have the same charge as the object.
Charging by INDUCTION
Connect a wire to the electroscope. Bring the ebonite rod close to but do not touch the electroscope. The negative charges in the electroscope are repelled, and move through the wire into the ground, (grounding). This causes the remaining positive charges in the leaves to repel each other, therefore spreading the leaves. Keep the ebonite in place and remove the grounding wire. The electroscope stays positively charged.
It will have an opposite charge to the object.