Page 9 - Electrostatics 11

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Electric field intensity:

Electrical intensity at any point in the electric field is defined as the force
experienced by unit positive charge (+1) placed there.

P

+Q
r (+1)

let us consider a point P in the electric field of +Q charge at a distance r. We

want to find electric intensity at point P. Put +1 charge (test charge) at point P.

The electric intensity at a point P between +Q and +1 according to Coulomb's

1 2
formula is = Where is permittivity of the free space.
0
4 2
0
(+ )(+1)
=
4 2
0

=
4 2
0

By definition, this force is electric intensity (E).

∴ =
4 2
0

This is the required expression for electric intensity. It is a vector quantity. Its
direction is radially away from the +Q charge and radially towards the -Q charge.

Force between Multiple electric charges:
Electric force at a point due to the number of charges is vector sum of the
forces due to individual charges.

P
Fn
F1

F3 Qn
F2

Q1
Q3

Q2

E = E1 + E2+ E3+ ... ... ... + En ;Where E stands for force.
9 Electrostatics