Page 193 - NCERT Science Class 10 English Medium

Q U E S T I O N S

1. Judge the equivalent resistance when the following are connected in
6
parallel – (a) 1 Ω and 10 Ω, (b) 1 Ω and 10 Ω, and 10 Ω.
6
3
2. An electric lamp of 100 Ω, a toaster of resistance 50 Ω, and a water
filter of resistance 500 Ω are connected in parallel to a 220 V source.
What is the resistance of an electric iron connected to the same source
that takes as much current as all three appliances, and what is the
current through it? ?
3. What are the advantages of connecting electrical devices in parallel
with the battery instead of connecting them in series?

4. How can three resistors of resistances 2 Ω, 3 Ω, and 6 Ω be connected
to give a total resistance of (a) 4 Ω, (b) 1 Ω?
5. What is (a) the highest, (b) the lowest total resistance that can be secured
by combinations of four coils of resistance 4 Ω, 8 Ω, 12 Ω, 24 Ω?

HEATING EFFECT OF ELECTRIC CURRENT
11.7
11.7 HEA11.7 HEATING EFFECT OF ELECTRIC CURRENTTING EFFECT OF ELECTRIC CURRENT
11.7 HEATING EFFECT OF ELECTRIC CURRENTTING EFFECT OF ELECTRIC CURRENT
11.7 HEA
We know that a battery or a cell is a source of electrical energy. The
chemical reaction within the cell generates the potential difference between
its two terminals that sets the electrons in motion to flow the current
through a resistor or a system of resistors connected to the battery. We
have also seen, in Section 11.2, that to maintain the current, the source
has to keep expending its energy. Where does this energy go? A part of
the source energy in maintaining the current may be consumed into
useful work (like in rotating the blades of an electric fan). Rest of the
source energy may be expended in heat to raise the temperature of
gadget. We often observe this in our everyday life. For example, an electric
fan becomes warm if used continuously for longer time etc. On the other
hand, if the electric circuit is purely resistive, that is, a configuration of
resistors only connected to a battery; the source energy continually gets
dissipated entirely in the form of heat. This is known as the heating
effect of electric current. This effect is utilised in devices such as electric
heater, electric iron etc.
Consider a current I flowing through a resistor of resistance R. Let
the potential difference across it be V (Fig. 11.13). Let t be the time during
which a charge Q flows across. The work done in moving the charge Q
through a potential difference V is VQ. Therefore, the source must supply
energy equal to VQ in time t. Hence the power input to the circuit by the
source is
Q
P V = VI (11.19)
=
t
Or the energy supplied to the circuit by the source in time t is P × t,
that is, VIt. What happens to this energy expended by the source? This
energy gets dissipated in the resistor as heat. Thus for a steady
current I, the amount of heat H produced in time t is
H = VIt (11.20)

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