O-Level Physics Notes

1. STATIC ELECTRICITY!
Introduction:
Hey everyone! How are you all? Today we are going to start static electricity. First of all, have you girls ever
wondered why your hair get attracted to the sweaters you wear in winters? It’s because when your hair get rubbed
with the sweater, they acquire static electric charges and become charged. This shows that friction from rubbing can
cause an object to be charged or electrified.
Electrostatic Charging by Friction:
Electrostatics is the study of static charges, just like the case of hair we just discussed. Some objects like duster and
polythene will acquire electric charges when they are rubbed together. How do they acquire the charges? This
happens because the electrons are transferred from one object to another.
For example, take a look at the diagram below. The duster is rubbed with the polythene as shown. Initially, before
rubbing, both have equal negative and positive charges, that is, they are neutral. After rubbing, the duster becomes
positively charged while the polythene becomes negatively charged i.e. the duster has lost electrons and of course,
polythene has gained it.
How are these charges transferred by friction?
Let’s get into more detail of what’s happening. We all know that all matter is made up of tiny particles called atoms.
And we know the composition of an atom: it consists of negatively charged electrons, positively charged protons and
neutral (uncharged) particles called neutrons. The number of electrons and protons is equal which causes the atom
to be neutral.
What will happen if, somehow, the charges are unbalanced? As in the number of electrons and protons is not equal.
The atom will carry some charge, right? If you are thinking of how the number of electrons and protons can be
different, it can occur when electrons are added or removed from the atom (protons are never added or removed,
they lie within the nucleus) such that when electrons are added, the atom will carry negative charge and viceversa.
Such an atom which is charged is called an ion.
As we saw, in case when the duster was rubbed with the polythene, what happens is the atoms at the surface are
disturbed and some electrons from the surface atoms of one object (duster in this case) are transferred to the other
object (that is polythene).
Notice that the electric charge is not created or destroyed: it is only transferred!
Electric charge is measured in Coulombs ( C), no matter if its negative or positive. The amount of charge on an
electron is 1.6 x 10-19
C, indeed a very small charge.
Insulators and Conductors:

2. We have seen that objects like silk, duster and polythene get charged when they are rubbed together. The electrons
that are transferred are unable to move within the new material, and they remain at the surface where the material is
rubbed. Now coming to the point, such materials where electrons are not free to move are known as electrical
insulators. They do not conduct electricity and are charged by friction.
We all know, however, that metals like copper allow electrons to move freely within them. Such materials are called
electrical conductors. As they are able to conduct electricity, they are charged by induction.
Discharging:
Before coming towards induction, let us first take a look at how objects can be neutralized after they are charged i.e.
discharging. Any ideas?
Okay, insulators and conductors are discharged differently.
● Heating the insulators discharges them, e.g. if a charged glass rod is heated, it will get neutralized. Now to
understand how this happens, the air surrounding the glass rod gets ionized due to intense heat. The ions in
the surrounding air then neutralize the excess charges in the glass rod. Humid conditions can also
neutralize a charged insulator as the water vapour in the air helps to remove the excess charges on the
insulator.
● A charged conductor can be neutralized by earthing it. As shown below, the electrons from the earth flow
through the body to the positively charged metal sphere to neutralize it. Note that the positive charge
indicates the absence of electrons, so it can’t flow. Only negative charge i.e. electrons will flow.
Induction:
Okay, now coming towards induction, it is the process of charging a conductor without any contact with the charging
body.
Lets understand it in steps (do refer to the diagram above):

3. 1. Firstly, the two conductors on insulating stands are placed touching each other.
2. A positively charged rod is bought near object A. This causes the electrons in the metal to be attracted to
the rod and they stay at the near end. Object A can be seen to have excess negative charges while object B
has excess positive charge.
3. Now, without removing the rod, both objects are separated and they are seem to have equal amounts of
opposite charges.
They are charged by induction. Easy, right?
Okay this was for two objects, how to induct charge in a single metal object? Let’s study that!
1. Okay firstly, the bring a negatively charged glass rod near the neutral metal sphere hanged with an
insulating wire. The free electrons in the metal will be repelled to the far end as shown in the diagram above.
2. Now, without removing the glass rod, earth the negative side of the metal conductor by touching it with hand
or earthing it otherwise. The electrons will flow to the earth. This will neutralize the negative charges.
3. Remove earthing and remove the glass rod. The positive charges will be redistributed on the surface of the
conductor. The conductor is now positively charged.
Any confusion? Lets move on!
Electric Field:
We all know that unlike charges attract while like charges repel.
If two charges are placed in vacuum, it will experience a force, either attraction or repulsion, right? This force is called
electric force. Note that the charges are not in contact with each other. So what is electric field then? It is basically a
region where an electric charge experiences an electric force.
The diagram shows the
(a) field lines of a positive charge
(b) field lines of a negative charge

4. ( c) and (d) are the field lines of like and unlike charges placed near each other.
The direction of the field is the direction of the force on a small positive charge and the strength of the field is
indicated by how close the filed lines are to each other.
Note that the direction of filed lines is always away from the positive charge and towards the negative charge.