Electrostatics is a branch of
physics that deals with charges in the state of rest and its applications. We
know from very long days that a glass rod rubbed with silk cloth can attract is
able to attract small pieces of paper.
Basing on this kind of understanding, charges are divided into two types
called positive and negative charges. It is also concluded that the similar
charges repel each other and opposite charges attract each other. All bodies by
default are neutral and they have equal number of positive and negative
charges.
According to modern concepts of
physics, negative charge is due to electrons and positive charge is due to
protons and they are inside the nuclei. The actual charge that is mobile is due
to electrons and protons won’t move and transfer. All charge and current is due
to electrons only. The body who gain electrons is said to be negatively charged
and the body that lost electrons is said to be positively charges. Thus charge
is neither created nor destroyed and it just transfers from one body to other.
This is called the concept of conservation of energy and it is always true and
it is one of the basic concepts of conservation.
Charge is also available as
integral multiples of electrons. It is always available as electron transfer
and always it is the complete electron that transfers from one body to other.
It won’t happen as integral electrons but not as part of electron.
The body gain electrons will get
negative charge and it also gains a little mass of that electrons itself. The
body that loose electrons, will lose negative charge and hence it will have
more positive charges and its mass also decreases slightly. Charges are
conserved and quantized. This charge is treated as scalar and it has only
magnitude but not any direction.
The force of attraction or
repulsion between two charges depends on the magnitude of charges and is
inversely proportional to the square of the distance between the bodies. This
is called Coulomb’s inverse square rule. This force also depends on the medium
between the two charges. Here we are going to understand the impact of the
medium on force between two charges. If the medium is good, it conducts charge
is better and vice versa. The impact is studied with parameter called
permittivity of medium or free space.
The force of attraction between
two charges is independent of presence of third charge. It means even after the
third charge, the force between two charges remains same. But on each charge,
now there is more force due to extra charge. To find the effective force on any
charge, we need to add all the existing forces using the vector laws of
addition.
Between two charges, we can find the
effective force on any charged particle placed somewhere in between them. We
can also find a null point where the effective electric force on it is zero.
This can be found when the two charges are same and even when the two charges
are different.
Neutral Point
Let us consider two similar charges separated by
certain distance. Each charge repels the other one with a certain force. Now
let us assume that we have kept a third charge between them. On the third
charge there will be force of repulsion if it is similar to the two existing
charges. We need to keep the third charge at a certain place so that the two
forces are acting on it in opposite directions and they are equal in magnitude
so that they can cancel each other.
Thus the resultant force on the third
charge will be zero and hence at that particular location, the new charge is
not going to experience any force. That point is called neutral point. If the
third charge is opposite to the two existing charges, it will experience
attractive force and they can cancel each other.
If two charges are opposite in
nature and they are separated by a certain distance, then we cannot find a
neutral point in between them as the two force acting on a third charge are in
the same direction and hence there will be some resultant force and they cannot
cancel each other. We can get null point only beyond them but not in between
them.
We can find the neutral point not in between the two charges and outside of the system as shown.
If suppose two similar charges of
equal magnitude are separated by some distance, we can keep either a positive
or negative charge between them and it can be in neutral mode. It means when we
place it properly, it won’t experience any resultant force. But to keep the
entire system in equilibrium mode, we shall keep negative charge only when the
initial two charges are positive and vice versa.
Between two positive charges, if
we keep a positive charge that third charge can be in equilibrium position, but
these two charges do experience a resultant force and hence we cannot get
equilibrium. If we keep a negative charge, that charge as well as the other two
charges also experience two forces and these two forces are in opposite
direction. Hence they can cancel each other and can get a equilibrium.
Problem
Let us consider two charges separated by a certain distance and we need to know the location of third charge so that the system is in equilibrium.
Solution
We need to identify either positive or negative charge which can keep the system in equilibrium.
Only negative charge can create the required equilibrium.
Resultant Gravitational Force and Neutral Point
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