2. HEATING EFFECT OF ELECTRIC CURRENT
Heating effect of electric current
• When an electric current is passed through the wire it gets
heated
• This effect due to flow of electric current through the
conductor is called heating effect of electric current.
• Amount of heat produced in the conductor or resistor is
obtained by Joule’s law of heating
3. JOULE’S LAW HEATING
• Heat produces in a resistor is directly proportional to
(i) square of the current flowing through the resistor
(ii) resistance for the given current
(iii) time
2
H = I R t (Joules)
2
H I
H R
H t
4. DERIVATION
JOULES LAW HEATING EFFECT
Work done (W) = Heat energy dessipated (H)
H = W = V Q
= V I t
= IR I t
2
H = I R t
• Let ‘ I ’ is a current flowing through resistor R
• Let V be the potential difference across the resistor
• Let t is the time taken to by Q charges to flow across it
• Work done to move electrical charges, W or heat energy
dissipated or produced during time t
5. PROBLEMS
Q.1 100 joules of heat is produced each second in a 4 ohms of
resistance. Find the potential difference across each resistor?
Ans: Given:
H = 100 J
R = 4 Ω
t = 1 s
To find :
Potential difference (V) = ?
Solution :
We from joules law of heating ,
2
H = I R t
2 H
I =
R t
2
H
=
R t
V
R
2
2
V H
=
R R t
2 H R
V =
t
2 100 4
V =
1
V = 400 20 volts
V
I = , ohms law
R
6. PROBLEMS
Q.2 An electric iron of resistance 20 ohms takes a current of 5 A.
Calculate the heat developed in 30s.
Ans: Given:
I = 5 A
R = 20 Ω
t = 30 s
To find :
Heat produced (H) = ?
Solution :
We from joules law of heating ,
2
H = I R t
2
H = (50) 20 30
H = (2500) 600
H =1500000 J
H =1500 kJ
7. POWER
Power :
• Rate of doing work is called power
• Work done per unit time
work done
Power =
time
W
P =
t
8. Electric power
Electric power :
• Rate at which electric work is produced or consumed in an
electric circuit is called electric power
Electric work done
Electric power =
time
W
P =
t
V Q
=
t
V I t
=
t
P = V I
P = V I
P = (IR) I
2
P = I R
P = V I
V
= V
R
2
V
P =
R
( V= I R)
V
( V= I R , I = )
R
