Static Electricity

Definitions

• Conductor: Allows current to pass• Insulator: Does not allow current to pass• Point discharge: Charge concentrates at a point and if the

electric field is high enough it can ionise air causing movement of charge to and from the point.

• Coulombs law :The force between 2 point charges is proportional to the charges and inversely proportional to the square of t he distance between them.

• Electric field strength: The force per unit charge at that point.

• Potential difference: The work done transferring a charge from one point to another.

• Volt: The pd between 2 points is 1 volt if 1 Joule of energy is used to transfer 1Coulomb of charge between the points.

What is Static Electricity?

Static electricity is a stationary electric charge that is built up on a material creating an imbalance between the positive and negative charges of the material.

A common example of static electricity is the slight electric shock we can get when we touch a car door during dry weather.

The Atom

When an atom

gains electrons it

become negatively

charged and when

it loses electrons it

becomes positively

charged.

To Demonstrate Forces Between Charges

Charge a polythene rod by friction and hang from a stand as shown. Charge another polythene rod and bring it near the suspended rod. They repel.Repeat using two perspex rods. They too repel.If one rod in charged positive and the other negative they will attract.

Like charges repel, unlike attract.

How do objects get charged?

1.By Friction: When two materials are rubbed together (like a balloon and your hair), one will lose electrons and one will gain them. Polythene tends to gain electrons and perspex tends to lose them.

It is always the ELECTRONS that leave.

2. Charging by Induction

It is always the ELECTRONS that leave.

The Electroscope

Uses of the electroscope

•Detecting Charge

•Determining whether

a charge is positive or

negative

•Determining whether

an object is an

insulator or conductor.

Insulator

To charge positively by induction.

A negatively charged object is

brought near the electroscope.

The negative charges are repelled

to the leaves.

The electroscope is earthed and

negative charges move to ground

leaving an excess of positive charge

on the electroscope.

The charged object is removed and

the electroscope is positively

charged.

Charging By Induction

The Van De Graaf GeneratorThis device can be used to build up a high charge.

Charge Distribution• Total charge resides on the outside of a metal object.

To demonstrate this, an insulated hollow metal can is charged by induction. A proof plane is touched to the inside of the can and then brought to the cap of an electroscope. The gold leaf does not move showing there is no charge on the inside of the can The proof plane is touched to the outside on the metal can it and then brought to the cap of an electroscope. The gold leaf moves showing there is charge on the outside of the can. Coaxial cables, ESB worksuit

Charge Distribution

• Charge concentrates at a point.

To demonstrate this attach a pointed object to the dome of a Van De Graaf generator. Hold a lit candle near the point and switch on the generator. The flame bends away from the point as it is affected by the charge being repelled from the point due the build up of charge here. Holding the candle next to the dome does not have the came affect as the charge is more spread here.

Charge Distribution

The point effect can cause charges to escape from a point and cause sparks which are dangerous in refuelling planes and in flour mills.

A lightening conductor uses the point effect to protect a building.

Uses of Static ElectricityThe Electrostatic Precipitator

This is used to clean the emissions from a chimney.

The Photocopier

Uses of Static Electricity

Coulombs Law

The force between 2 point charges is proportional to the product of the charges and inversely proportional to the square of the distance between them.

d

qqF 2

21

4

1

Electric Field

• This is the area around a charge where a force is experienced by charges.

• The direction of the field is the direction that a positive charge would move.

Electric Field Intensity (Strength)

• This is the force per unit charge in an electric field. Unit : Newton per Coulomb N/C

• Formula

d

qE

24

1

q

FE

To Demonstrate Electric Field Intensity

EHTConnect 2 plates to a high voltage supply and place in a dish of oil. Sprinkle semolina between the plates and the grains align with the field.