When a source of emf e is connected to an external resistance R, the
voltage Vext across R is given by
Vext = IR = 
R
R
r
ε
+
where r is the internal resistance of the source Rationalised 2023-24
Physics
104
Physical Quantity
Symbol
Dimensions
Unit
Remark
Electric current
I
[A]
A
SI base unit
Charge
Q, q
[T A]
C
Voltage, Electric
V
[M L
2 T
–3 A
–1]
V
Work/charge
potential difference
Electromotive force
e
[M L
2 T
–3 A
–1]
V
Work/charge
Resistance
R
[M L
2 T
–3 A
–2]
W
R = V/I
Resistivity
r
[M L
3 T
–3 A
–2]
W m
R = rl/A
Electrical
s
[M
–1 L
–3 T
3 A
2]
S
s = 1/r
conductivity
Electric field
E
[M L T
–3 A
–1]
V m
–1
Electric force
charge
Drift speed
vd
[L T
–1]
m s
–1
vd
e E
m
=
τ
Relaxation time
t
[T]
s
Current density
j
[L
–2 A]
A m
–2
current/area
Mobility
m
[M L
3 T
–4 A
–1]
m
2 V
–1s
–1
vd/
E
12 Kirchhoff’s Rules –
(a) Junction Rule: At any junction of circuit elements, the sum of
currents entering the junction must equal the sum of currents
leaving it (b) Loop Rule: The algebraic sum of changes in potential around any
closed loop must be zero