STATIC ELECTRICITY (ELECTROSTATICS)

This is the study of electricity due charges at rest/ not moving.

Electrostatic charges can be induced and easily detected in insulators (non-metals)

because these kinds of materials do not allow charges to flow through them. Metals are
generally good conductors, so it is difficult to induce electrostatic charges in them.

All materials are made out of molecules which themselves are groups of atoms. The atoms
contain electrically charged particles being protons and electrons. Normally an object is
electrically neutral since it has an equal number of positive and negative charges. The two
charges can be separated by rubbing objects together.

TYPES OF CHARGES

There are two types of charges, namely positive (+) and negative (-).

Positively charged object negatively charged object

Unit of electric charge

The SI unit of electric charge is a coulomb.

CHARGING BY FRICTION
The force of friction between two objects can cause electrons to be transferred from one
object to the other. One object will gain extra electrons and become negatively charged. And
the other one will become positively since it would have lost some electrons and remained
with excess positive charges.

A B
A. polythene strip will be negatively charged, and the cloth will be positively charged

 Explanation: when polythene is rubbed, electrons from the cloth are transferred to
the polythene making the polythene negatively charged and the cloth will be positive
because there will be a deficit of electrons.
B. cellulose acetate strip will be positively charged, and the cloth will be negatively
charged.

 Explanation: when perspex (cellulose acetate) is rubbed with the cloth it loses some
electrons to the cloth and remains short of electrons and with more unbalanced
protons and as a result the Perspex rod becomes positively charged and the cloth
negatively charged because it would have some extra electrons (negative charges).

DETERMINATION OF LAW OF ELECTROSTATICS

Experiment: positive and negative charges

- Rub a piece of polythene strip with a cloth

- Hang it up as shown in the diagram
- Rub another polythene strip and bring it near the first one.

Observation: The hanged rod moves away

This shows that the two strips became charged in the same way
- Now bring a piece of rubbed polythene close to the hanging cellulose acetate strip.

Observation: the hanged rod moves closer

This shows that the two strips became charged in different ways. The charge on the
cellulose acetate is taken to be positive and the charge on the polythene is negative.
Conclusion: “like charges repel and unlike charges attract”
INDUCED CHARGES(CHARGING BY INDUCTION)
A charge can be build up on an uncharged object by holding a charged object close to it as
shown below. These charges that would appear on an uncharged object due to a charged
object nearby are called induced charges.

A metal sphere is being charged by induction and ends up with an opposite charge to
that on the rod. Note the two never actually touched.
a) Using a negatively charge rod

- Charges separated by bringing a charged rod close to the sphere.

- While the rod is still kept at its position, the sphere is earthed by touching with hand
and the electrons flow out to earth/ground through the finger.
 - Charges are evenly distributed around the sphere when the rod and the earth (hand)
are removed.
-

ELECTROSTATIC INDUCTION
The process of electrically charging an insulated conductor using the force due to a charged
object to separate the charges in the conductor.

b) Separation of conductors

i) Using a negatively charged rod with 2 neutral spheres in contact

- A negatively charged rod is brought near one of the neutral spheres.

- Charges inside the spheres are separated such the positive charges are
attracted to sides closer to the rod and the negative chares are repelled to
the far sides.
- The spheres are separated, and then the rod is removed.
- The charges redistribute in the spheres. Sphere A becomes positively
charged and sphere B becomes negatively charged.
ii) Using a positively charged rod with 2 neutral spheres in contact

- A positively charged rod is brought near one of the neutral spheres.

- Charges inside the spheres are separated such the negative charges are
attracted to sides closer to the rod and the positive chares are repelled to
the far sides.
- The spheres are separated, and the rod is removed.
- The charges re distribute in the spheres. Sphere A becomes negatively
charged and sphere B becomes positively charged.

iii) Using a negatively charged rod with 2 neutral spheres in contact

- A negatively charged rod is brought near one of the neutral spheres.

- Charges inside the spheres are separated such the positive charges are
attracted to sides closer to the rod and the negative chares are repelled to
the far sides.
- The earth wire is connected to sphere Y.
- Negative charges are move from sphere Y through the earth wire to the
ground.
- The earth wire is removed followed by the rod.
 - the charges re distribute in the spheres. The spheres become positively
charged and repel each other.

iv) Using a positively charged rod with 2 neutral spheres in contact

- A positively charged rod is brought near one of the neutral spheres.

- Charges inside the spheres are separated such the negative charges are
attracted to sides closer to the rod and the positive charges are repelled
to the far sides.
- The earth wire is connected to sphere Y.
- Negative charges are move from earth through the earth wire into the
sphere.
- The earth wire is removed followed by the rod.
- The charges re distribute in the spheres. The spheres become negatively
charged and repel each other.

DISCHARGING
A charge can be build up on an object through friction. The charge can be discharged to the
Earth by contact with a conductor. The charge stored can also be released to the nearest
object with a neutral charge or by bringing discharging object with opposite charge.
e.g. when sliding out of a car, friction between the seat and clothes causes a charge on the
person. When the person touches the car body the charge passes from his body to the car,
giving a slight shock.
*NB: an isolated charged insulator will slowly become discharged. The charge on the
insulator is neutralized by ions (charged particles) in the air.
LIGHTNING

Friction between particles rubbing against each other in a large cloud can build up a large
charge on the cloud. When the charge becomes very large it may discharge through the air
to the earth or to the neighbouring clouds and this would be in a form of flash of lightning,
therefore lightning is an electric discharge between the Earth and highly charged clouds.

Lightning conductors

A lightning conductor is a thick copper strip fixed to the outer wall of a building or a tall pole
near the building. The top of the rod ends are sharp spikes. At the bottom of the strip there is
a copper plate buried in the ground.

Thunderclouds contain a large quantity of charge. When pass over a building it induces a
build-up of opposite charge on the roof. If the electric field (voltage) between the opposite
charges is strong enough, there may be a spark of lightning as the charges flow through the
air towards each other.

With a lightning conductor, the sharp spikes at the top reduce the chance of a lightning
strike. By effect of action at points, the conductor let charges on the building leak away
before a spark can occur and some of the charges flow even up to the clouds and cancel out
some of the negative charge on the clouds, making it less likely that the lightning will strike.
However, if a flash does occur it is less violent and the conductor gives it (negative charge)
an easy path to the ground.
ELECTRIC FIELD
A region around an isolated electric charge where the electric force can be felt .
It is represented using electric field lines. The field lines have both the magnitude and
direction. They always move away from the positive charges and move towards negative
charges.
PATTERNS OF FIELD LINES
a) A field around an isolated electric charge

b) Field lines around unlike charges

c) Around like charges

INSULATORS AND CONDUCTORS

Insulators or bad conductors- they can hold charge on their surfaces. The charge does
not move through insulators.
Examples: - plastics (e.g., PVC, polythene, Perspex, etc.), glass, rubber, dry air,
sulphur and oil.
Conductors – metals are good conductors of electricity since they have free electrons in
their outermost shells. A conductor cannot be charged as the charge will flow easily through
it.
Examples: - most metals (e.g., silver, copper, aluminium), carbon, graphite, acid
solutions, salt solutions
Semi-conductors: - are in-between materials. They are poor conductors when cold but
much better conductors when warm, e.g., silicon, germanium

APPLICATIONS AND DANGERS OF ELECTROSTATICS

1. Usually, electric charges build up on the surface of the car as it moves through air
along the road that is why a passenger may get an electric shock when getting into or
out of the car. Therefore, if charges are allowed to build up on trucks carrying
flammable goods (e.g. petrol) a very small spark can cause a fire or explosion. It is
then important that such trucks are earthed by attaching a conducting strip that will
be dragged behind the truck or run on conductive rubber tyres.
2. Paint spraying – the paint becomes charged due to friction as it is forced out of the
nozzle of the spray gun. If the object to be painted is given the opposite charge the
paint will stick to it very well. This technique is used by farmers when crop spraying
and also used to coat cars with paint.
3. Dust and ash precipitator - ash in factory chimneys and power stations can be
removed by electrostatic precipitation. Wires inside the chimneys are negatively
charged and give a similar charge to the ash particles. The negatively charged ash
particles are attracted to positively charged metal plates inside the chimney walls.
The ash particles are then removed by washing
4. In electrostatic photocopying machines – inside the photocopier, there is a light-
sensitive plate that would be given a negative. The image of the document to be
copied is projected onto the drum. The bright areas on the drum lose their charge
because of reflected light from the corresponding white parts on the document paper
but the dark areas on the plate keep their charge. The powdered ink (toner) is
attracted to the charged (dark) areas. A blank sheet of paper is pressed against the
plate and picks up the toner. The paper is heated so that the powered ink melts and
sticks to it. The result is a copy of the original document.