HEATING EFFECT OF ELECTRIC CURRENT
When current flows
through a resistor, some of the electrical energy delivered to the resistor is
converted into heat energy and it is dissipated. This heating effect of current
is known as Joule’s heating effect. Just as current produces thermal energy, thermal
energy may also be suitably used to produce an electromotive force. This is
known as thermoelectric effect.
If a current I
flows through a conductor kept across a potential difference V for a time t,
the work done or the electric potential energy spent is
In the absence of any
other external effect, this energy is spent in heating the conductor. The
amount of heat(H) produced is
This relation was
experimentally verified by Joule and is known as Joule’s law of heating. It
states that the heat developed in an electrical circuit due to the
flow of current varies directly as
(i) the square of the current
(ii) the resistance of the circuit and
(iii) the time of flow.
EXAMPLE 2.27
Find the heat energy
produced in a resistance of 10 Ω when 5 A current flows through it for 5
minutes.
Solution
R = 10 Ω, I = 5 A, t = 5
minutes = 5 × 60 s
H = I2 R t
= 52 × 10 × 5
× 60
=25 × 10 × 300
=25 × 3000
=75000 J (or) 75 kJ
Electric iron, electric
heater, electric toaster shown in Figure 2.30 are some of the home appliances
that utilize the heating effect of current. In these appliances, the heating
elements are made of nichrome, an alloy of nickel and chromium. Nichrome has a
high specific resistance and can be heated to very high temperatures without
oxidation.
EXAMPLE 2.28
An electric heater of
resistance 10 Ω connected to 220 V power supply is immersed in the water of 1
kg. How long the electrical heater has to be switched on to increase its
temperature from 30°C to 60°C. (The specific heat of water is s = 4200 J kg-1)
Solution
According to Joule’s
heating law H = I2 Rt
The current passed
through the electrical heater = 220V/10Ω = 22 A
The heat produced in one
second by the electrical heater H = I2 R
The heat produced in one
second H = (22)2 x 10 = 4840 J = 4.84 k J. In fact the power rating
of this electrical heater is 4.84 k W.
The amount of energy to
increase the temperature of 1kg water from 30°C to 60°C is
Q = ms ∆T (Refer XI
physics vol 2, unit 8)
Here m = 1 kg,
s = 4200 J kg-1,
∆T = 30,
so Q = 1 × 4200 x 30 =
126 kJ
The time required to
produce this heat energy t = Q/ I2R = 126 ×103 / 4840 ≈
26 .03 s
Fuses as shown in Figure
2.31, are connected in series in a circuit to protect the electric devices from
the heat developed by the passage of excessive current. It is a short length of
a wire made of a low melting point material. It melts and breaks the circuit if
current exceeds a certain value. Lead and copper wire melts and burns out when
the current increases above 5 A and 35 A respectively.
The only disadvantage
with the above fuses is that once fuse wire is burnt due to excessive current,
they need to be replaced. Nowadays in houses, circuit breakers (trippers) are
also used instead of fuses.
Whenever there is an
excessive current produced due to faulty wire connection, the circuit breaker
switch opens. After repairing the faulty connection, we can close the circuit
breaker switch. It is shown in the Figure 2.32.
Furnaces as shown in
Figure 2.33 are used to manufacture a large number of technologically important
materials such as steel, silicon carbide, quartz, gallium arsenide, etc). To
produce temperatures up to 1500°C, molybdenum-nichrome wire wound on a silica
tube is used. Carbon arc furnaces produce temperatures up to 3000 °C.
It consists of a tungsten
filament (melting point 3380 ºC) kept inside a glass bulb and heated to
incandescence by current. In incandescent electric lamps only about 5% of
electrical energy is converted into light and the rest is wasted as heat.
Electric discharge lamps, electric welding and electric arc also utilize the
heating effect of current as shown in Figure 2.34.
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