Copyright 20 13 Dorling Kindersley (India) Pvt. Ltd

1.1 
ElEctromotIvE ForcE, PotEntIal and voltagE
Charge is the attribute of matter that is responsible for a force of interaction between two pieces of 
matter under certain conditions. Such an attribute was seen to be necessary as a result of experiments 
in the past which revealed the existence of a certain kind of interaction force between particles that 
could not be explained by other known sources of interaction forces.
Charge is bipolar. Two positively charged particles or two negatively charged particles repel each 
other. Two particles with charges of opposite polarity attract each other. Further, charge comes in 
integral multiples of a basic unit – the basic unit of charge is the charge of an electron. The value of 
electronic charge is –1.602 
× 
10
-
19
coulombs. The SI unit of charge – i.e., Coulomb – represents the 
magnitude of charge possessed by 6.242 
× 
10
18
electrons.
1.1.1 
Force between two moving Point charges and retardation Effect
The force experienced by a point charge of value q
2
moving 
with a velocity of 
v
2
due to another point charge of value q
1
moving with a velocity of 
v
1
at a distance r from it contains 
three components in general. If the charges are moving 
slowly, the force components are given by approximate 
expressions as below.
The first component is directly proportional to the 
product q
1
q
2
and is inversely proportional to the square of 
distance between them. This component of force is directed 
along the line connecting the charges and is oriented away 
from q
1
. This component is governed by Coulomb’s law; is 
termed ‘the electrostatic force’ and is given by
F
q q
r
u
12
1 2
0
2
12
4
es
=
pe
N
where 
ε
0
is the dielectric permittivity of free space (
=
8.854 
× 
10
-
12
F/m) and 
u
12
is the unit vector 
directed from q
1
to q
2
.
q
1
q
2
r
u
12
→
v
2
→
v
1
→
Fig. 1.1-1 

Forcebetweentwo
pointchargesin
motion
www.TechnicalBooksPDF.com

---

ElectromotiveForce,PotentialandVoltage