Page 195 - NCERT Science Class 10 English Medium

Q U E S T I O N S

1. Why does the cord of an electric heater not glow while the heating element
does?
2. Compute the heat generated while transferring 96000 coulomb of charge ?
in one hour through a potential difference of 50 V.

3. An electric iron of resistance 20 Ω takes a current of 5 A. Calculate the
heat developed in 30 s.

11.7.1 Practical Applications of Heating Effect of

Electric Current
The generation of heat in a conductor is an inevitable consequence of
electric current. In many cases, it is undesirable as it converts useful
electrical energy into heat. In electric circuits, the unavoidable heating
can increase the temperature of the components and alter their properties.
However, heating effect of electric current has many useful applications.
The electric laundry iron, electric toaster, electric oven, electric kettle
and electric heater are some of the familiar devices based on Joule’s
heating.
The electric heating is also used to produce light, as in an electric
bulb. Here, the filament must retain as much of the heat generated as is
possible, so that it gets very hot and emits light. It must not melt at such
high temperature. A strong metal with high melting point such as
tungsten (melting point 3380°C) is used for making bulb filaments. The
filament should be thermally isolated as much as possible, using
insulating support, etc. The bulbs are usually filled with chemically
inactive nitrogen and argon gases to prolong the life of filament. Most of
the power consumed by the filament appears as heat, but a small part
of it is in the form of light radiated.
Another common application of Joule’s heating is the fuse used in
electric circuits. It protects circuits and appliances by stopping the flow
of any unduly high electric current. The fuse is placed in series with
the device. It consists of a piece of wire made of a metal or an alloy of
appropriate melting point, for example aluminium, copper, iron, lead
etc. If a current larger than the specified value flows through the circuit,
the temperature of the fuse wire increases. This melts the fuse wire and
breaks the circuit. The fuse wire is usually encased in a cartridge of
porcelain or similar material with metal ends. The fuses used for domestic
purposes are rated as 1 A, 2 A, 3 A, 5 A, 10 A, etc. For an electric iron
which consumes 1 kW electric power when operated at 220 V, a current
of (1000/220) A, that is, 4.54 A will flow in the circuit. In this case, a 5 A
fuse must be used.

190 Science

2024-25