Introduction to Electricity

What is a ‘CHARGE”? A charge can occur when materials are

rubbed This can cause these materials to be

attracted to other materials Examples:

1. Rubbing your feet on carpet to create STATIC ELECTRICITY

2. Water and ice droplets can rub together in the clouds to create LIGHTNING

Types of Charges There are TWO kinds of charge:

POSITIVE CHARGE NEGATIVE CHARGE

Neutral Objects: DO NOT CARRY A CHARGE BUT ARE ATTRACTED TO BOTH TYPES OF CHARGES

(POSTIVE AND NEGATIVE)

LAW OF ATTRACTION AND REPULSION This Law states that:

‘LIKE CHARGES REPEL AND UNLIKE CHARGES ATTRACT’

Ever heard of the old saying: ‘Opposites Attract?’

How do Charges Move?Two Types Insulator:

Does not carry the charges If you rub an object and the charges stay only on

the spot where you have rubbed. Examples: Plastic, rubber, rock, dry air and glass

How do Charges Move?Two Types Conductor:

Carries the charges Allows charges to move through that material

freely. Examples: gold (Au), copper (Cu), YOU!

Atomic Model The nucleus cannot move from the atom,

therefore the protons (POSTIVE) cannot move from the atom.

The nucleus has an overall POSTIVE charge The electrons (NEGATIVE) can move from the

atom.

Particle Theory of Electricity

1. Two types of subatomic particle are important in the theory: electrons (-ve) and protons (+ve)

2. Charge cannot be created or destroyed- charge is CONSERVED

Particle Theory of Electricity

3. Positive charges are fixed and negative charges are free to move.

4. A neutral object has equal numbers of positive and negative charge.

Part 3 of Particle Theory of Electricity: Negatives can move

Part 4 of Particle Theory of Electricity: Neutral Object

EQUAL AMOUNTS OF BOTH POSTIVE AND NEGATIVE CHARGES MAKES THIS OBJECT NEUTRAL

Particle Theory of Electricity 5. A negative object has an excess of

negative charge. A positive object has a deficit of negative charge.

6. Charge is shared by contact. Materials which allow charge to move easily are called conductors, and materials that do no allow charge to move easily are called insulators.

7. Like charges repel; unlike charges attract.

Attraction of Neutral Objects to Charged Objects Neutral objects have a balance of both

positive and negative charges. If they have that balance, why would they be

attracted to charged objects?

The difference lies in the difference between INSULATORS and CONDUCTORS

Attraction of a Neutral Object (Insulator)

Materials like paper, which do not allow the free movement of electrons, are called insulators.

The charges on insulators do not separate readily but polarize (align themselves). Since opposite charges are always closer to each other, there is a net force of attraction.

Explanation If you bring a POSTIVELY charged ruler

near bits of paper (INSULATOR), the paper will be attracted to the ruler.

If you bring a NEGATIVELY charged ruler near bits of paper (INSULATOR), the paper will be attracted to the ruler.

This is because the charges in the paper are aligning opposite to what ever charge the ruler has.

The charges stay together, but align depending on what charge they are attracted to.

Attraction of a Neutral Object (Conductor)

Metals, allow the free movement of negatives, and are called conductors.

The charges on conductors separate readily, and are some distance apart from each other.

EXPLANATION The pith ball (CONDUCTOR) is neutral and

if you bring a charged rod to it the pith ball will be attracted to it.

The negatives (electrons) are free to move, so they move towards the positive charge of the rod.

If the pith ball and rod touch, the negatives on the pith ball are attracted to the rod making the rod less positive and the pith ball more negative.

Meter Stick Demo What happened? Why do you think this happened? Which was the conductor? Which was the insulator? How would you draw the charges in the

diagram? Why would you draw them this way?

Pop Can Races What happened? Why do you think this happened? Which was the conductor? Which was the insulator? How would you draw the charges in the

diagram? Why would you draw them this way?

Early Models of Electricity Going back to the past again… How is

scientists in the past explain ELECTRIC CHARGE?

Three main ideas to study: 1) One-fluid model 2) Two-fluid model 3) Particle model

1) One- Fluid Model A Neutral object has a

“natural” amount of electric ‘fluid’.

A charged object either has too much OR too little fluid.

Ben Franklin’s Model As a result, we credit

Ben Franklin with coining the terms ‘positive’ and ‘negative’

2) Two-Fluid Model A neutral object has

equal amounts of a positive and negative fluid.

A charged object has more of one than the other.

Proposed by Dufay

3) Particle Model There are 2 kinds of

particles positive or negative

A charged object has more of one particle or another

We accept this model today because of experiments done by Rutherford (gold foil) and Millikan’s Oil Drop Experiment.