Static Electricity

Chapter 19

To Show the Forces Between Electric Charges

Rub two polythene rods with a woollen cloth.

Place one of them in a paper stirrup hanging from a retort stand. Bring the second rod near the first one.

The rods will repel each other, showing the Force between Charged Bodies.

To Show the Forces Between Electric Charges

Rub a polythene rod and a cellulose acetate rod with a woollen cloth.

Place one of them in a paper stirrup hanging from a retort stand. Bring the second rod near the first one.

The rods will attract each other, showing the Force between charged bodies.

Name the two types of Electric Charge.

Positive Charge and Negative Charge.

Do electric charges exert Attractive or Repulsive forces on each other?

Both. Like Charges Repel and Unlike Charges Attract.

Electric Charge

An Electrical Conductor is a substance through which electric charge can flow.

Silver, Copper and Aluminium are metals commonly used as conductors. Water, the Human Body and the Ground are also conductors.

What is an Electrical Conductor?

Name four electrical conductors.

An Electrical Insulator is a substance through which electric charge cannot flow.

Glass, any Plastic, Rubber and Air are electrical insulators.

What is an Electrical Insulator?

Name four electrical insulators.

All matter is made up of atoms.

Every atom has a central part, called the Nucleus, which contains particles called Protons.

Particles called Electrons orbit in the space around the nucleus.

Electrons are attracted to the protons and are repelled by other electrons. Protons repel protons.

The Structure of the Atom.

The forces of attraction and repulsion between protons and electrons are called Electrostatic Forces.

Particles that exert electrostatic forces on each other are said to be Electrically Charged.

Electrons are said to be Negatively Charged.

Protons are said to be Positively Charged.

The amount of positive charge on a proton is the same size as the amount of negative charge on an electron.

It is only the electrons that actually move when objects become charged.

Atomic Nature of Electric ChargeIf a object is Negatively Charged it has Gained Electrons.

If a object is Positively Charged it has Lost Electrons.

Bring a negatively charged rod near a metal conductor. Free electrons in the metal are repelled by the negative charge on the rod and head to the opposite side of the metal.

Thus a negative charge appears on one side of the metal and a positive charge on the other.

Remove the charged rod and the electrons move back again.

The charges produced on the metal are called Induced Charges.

The induced negative charge is the same size as the induced positive charge.

Metal Conductor

Charged Rod

Charging by Induction

Bring a negatively charged rod near, but not touching, the conductor.

Keeping the charged rod in place, earth the conductor, by touching it with your finger.

Remove your finger and then remove the rod. The conductor will be positively charged.

To Charge a Conductor positively by Induction

Detect electric charge.

Indicate the approximate size of an electric charge.

Test if a charge is positive (+) or negative or (–).

Test if an object is a conductor or an insulator.

Indicate roughly the size of a potential difference.

A Gold-Leaf Electroscope can be used to:

Why does the gold leaf on an electroscope diverge when a negatively charged object is brought close to the metal cap?

The negative charge on the object repels electrons from the cap down to the leaf and the end of the metal rod.

The negative charge on the end of the rod repels the negative charge on the leaf.

Thus the leaf diverges.

To Charge an Electroscope by Induction

Bring a charged rod near – but not touching – the cap.

Keeping the charged rod in place, earth the cap of the electroscope, by touching it briefly with your finger.

Remove the rod.

The electroscope will now have a charge opposite to that on the rod.

Where on a conductor does Static Charge reside?

All Static Charge resides on the Outside Surface of a conductor.

Static charge on a spherical conductor is distributed uniformly over the sphere.

Static charge on a pear-shaped conductor accumulates most at the pointed end.

Distribution of Charge on Conductors

Static Charge on a conductor tends to accumulate where the conductor is most pointed.

Point Discharge

There is a very large charge density (charge per unit area) at the point.

This causes a very strong electric field in the region around the point.

Ions in the air are either attracted to or repelled from the point.

Ions with opposite charge to that on the point head towards the point and neutralise the charge on it.

Ions with the same charge head away from it creating an "electric wind".

It is as if the charge was removed from the point. The loss of charge from a point by this manner is called Point Discharge or the Point Effect.

Point Discharge

An “Electric Wind” produced by ions streaming away from a highly charged point.

Point Discharge

State Coulomb’s Law of Force between electric charges.

Coulomb’s Law states that the force of attraction or repulsion between two point charges is:

Directly Proportional the product of the charges and Inversely Proportional to the square of the distance between them.

F is the force Q1 and Q2 are the charges

d is the distance between the charges

ε is the permittivity of the medium

221

4

1

d

QQF

This means that:

The force is inversely proportional to the square of the distance between the charges.

i.e. If the distance between the charges is doubled the size of the force is 4 times smaller.

If the distance between the charges is made 3 times bigger the size of the force is 9 times smaller, etc.

Coulomb's Law is an Inverse Square Law

What is an Electric Field?

An Electric Field is any region of space where a static electric charge experiences a force other than the force of gravity.

An Electric Field is always caused by other static charges in the vicinity.

What is an Electric Field Line?(also called a line of force).

Where the electric field is strong the field lines are close together.

Where the electric field is weak the lines are far apart.

An Electric Field Line is a line drawn in an electric field such that the tangent to it at any point is along the direction of the force on a positive (+) charge placed at that point.

The Electric Field Lines around an isolated Positive Charge

The Electric Field Lines around an isolated negative charge

The Electric Field Lines around a positive charge near a negative charge

The Electric Field Lines around two Positive Charges

The almost uniform Electric Field between two Oppositely Charged Parallel Plates

Experiment to demonstrate an Electric Field Pattern

Use the equipment shown.

Connect a High Voltage Source to the metal plates which are in the oil.

The semolina particles line up in the direction of the field, showing the electric field pattern.

What is Electric Field Strength?

The Electric Field Strength (E) at a point is the force per unit charge at that point.

Electric Field Strength is a Vector Quantity.

What is the SI Unit of Electric Field Strength?

The SI Unit of Electric Field Strength is the newton per coulomb ( N C-1).

A charge of Q coulombs

experiences a force F newtons

when placed in an electric field of strength E newtons per coulomb.

Write down a formula relating Q, F and E.

Q

FE