Law of Electric Charges

Law of Electric Charges

Law of Electric Charges • Charges cannot be created or destroyed • They can only be moved around or transferred • Opposite Charges Attract • Attraction – Force bringing objects together • Like Charges Repel • Repulsion – Force causing objects to move apart

- - + + + - - + + + + + + - - - - - Bringing Charges Together Attract Repel Repel

The Test for Charge + + + + + + Build up of negative charge on this side The positive rod attracts the balloon NO So the balloon must be negative The balloon is uncharged

+ + + - - - The Test for Charge is Repulsion This balloon must be negative This balloon must be positive

+ + + The Gold Leaf Electroscope

Electrostatic Series List Acetate weak hold on electrons Glass Wool Cat’s fur, human hair Silk Cotton Ebonite Polyethylene (plastic) Rubber strong hold on electrons

When charging by friction the substance higher in the list loses electrons and becomes positively charged while the substance lower on the list gains those same electrons and becomes negatively charged

+ - - + Charging Up Rods It’s all about moving electrons Acetate positive Polythene Negative Cloth Negative Cloth Positive + + Acetate - - Polythene + + + - - - + + - + - -

Rules of Attraction and Repulsion • Students and Teacher co-develop rules based on exploration and observations of pith ball electroscope.

Rules of Attraction and Repulsion • Step 1 – Apply Law of Electric Charges.

Rules of Attraction and Repulsion • Confirm – Experiment Using Pith Ball Electroscope.

Charging By Contact • Touching objects together allows charges to spread and distribute across the objects. • Question: Why do the charges move and spread out? • Answer: Like Charges Repel and try to spread out as much as possible. • Touching a Charged Object to a Neutral Object • Some of the charge moves to the neutral object • Some charge is “left behind” when the charged object is moved away. • Touching a Charged Object to another Charged Object • Opposite charges cancel out • Remaining (leftover net charge) spreads out over both objects

Grounding by Contact • A Charge Sink is an object that has an extremely large capacity to hold charge. • Example: The Earth is trillions of times larger than a pith ball. • Small additions have a minor effect on the Earth’s net charge • Touching a Charged Object to a Charge Sink • Charge spreads out over both objects. • Almost all charge goes into the sink. • Almost no charge remains on the object. • This is called Grounding

Rules of Attraction and Repulsion • Confirm – Experiment Using Pith Ball Electroscope.

Uses of Static Electricity Spraying a Car Positive Car Negative Spraygun The paint spreads out as each negative drop repels No paint is wasted as the positive car attracts the negative paint

Removing Smoke from Power Station Chimneys • using electrostatic chargemuch of the smoky pollution is removed from the chimneybefore it can get out into the atmosphere.

Earthing Petrol Tankers Petrol rubbing against the pipe can build up a static charge which could cause an explosion The tanker is joined to the ground with a wire to stop a charge building up

Rules of Attraction and Repulsion • Step 2 – Explore neutral objects using pith ball electroscope.

Electric Fields Ball • Forces in nature are modeled using Force Fields • Gravity is a familiar force that acts through empty space between two objects. • Lines of force extend through space surrounding the objects. • Force fields explain how objects can affect each other without touching. • The study of forces is an important topic in Physics • Electric Charges create Electric Fields • Larger charges create larger electric fields • Other charges are affected (attracted / repelled) by electric fields. Gravity Earth

Charging by Induction • Bringing a Charged Object close to a Neutral Object • Opposite charges in neutral object are attracted by the electric field • Like charges in neutral object are repelled by the electric field • Neutral object has a imbalance of charge on each side • Attracting charges (forces ) are closer (stronger) than repelling charges • Grounding the Neutral Object… • Will leave behind a permanent charge. • Without grounding, it will be temporary.

Rules of Attraction and Repulsion • Step 2 – Explore neutral objects using pith ball electroscope.

Summary – Charging Methods • Charging by Friction- rubbing two objects together – creates two opposite charges • Charging by Contact – touching an object with another object- creates two same charges • Charging by Induction-not touching! – creates two opposite charges • Also… Grounding – discharging electricity to the earth – process of making an object neutral by placing it in contact with the ground

Recall • Electrically charged objects are produced by a separation of charges. All matter contains both positive and negative charges. • When these charges are equal in number, the object is electrically neutral. • When certain materials are rubbed together, friction causes the negative charges to be transferred from one surface to the other. • The object that gains the negative charge becomes negatively charged • The object that loses the negative charges becomes positively charged • The direction of transfer is determined by the differences in affinity for negative charges by different materials

Law of Electric Charges