Electrostatics Lesson 3 Forces and Fields Electrostatics Lesson 3

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.

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.

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

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

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

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.)

Diagram of Induction

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.

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

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)

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.