Page 131 - Science Class 07

Working of a Fuse
Electrical Appliances
Working of An Electric Bulb
An electric bulb consists of a coiled filament
made up of tungsten. An inert gas (Argon) is
also filled in the glass to avoid any chemical
reac on with heat. We use the filament made
of tungsten because it offers a great resistance
to electric current and it also has high mel ng
point. That is why, when electric current is
passed through the filament, it starts glowing,
but does not melt. You should not touch a
glowing electric bulb because it is very hot.
An Electric Bulb
Why do we use a coiled filament in a bulb? We
can shorten the length of wire by making its
coil. Similarly, to shorten the length of the
filament, we make a coil out of it. The more the
length of the filament the more resistance will
be offered by it. However, when an electric
current of large amount flows through the
filament, which is beyond the capacity of it, the
filament melts and we say that the bulb has
been fused.
Science-7
In every house or office, electricity is an
important requirement. It is distributed
through copper wire coated with insulator like
rubber or plas c. In a house or office it is
distributed in different places in a circuit.
But some mes, when a wire breaks in a circuit
or a connec on is faulty, a leak of electric
current is likely to occur. Since an electrical
current produces heat, such a leak may be very
dangerous and may cause a fire. A break of this
kind is known as a short-circuit.
To avoid such kind of accidents, we insert a fuse
in the circuit. Some mes, due to sudden
increase of voltage supply electric appliances
get damaged. A fuse inserted in a circuit also
saves electric appliances during such situa on.
The fuse is a piece of wire made of a metal wire
having very low mel ng point. It is generally
encased in a porcelain cover the screws into
the fuse box.
Fuse
Whenever, a short circuit occurs in the circuit
the wires including the wire of the fuse also get
heated. Since the mel ng point of this wire is
low, suddenly melts and electric supply is cut.
Similarly, when there is a sudden increase in
voltage supply the fuse wire gets heated up and
melts. Thus, the electric supply is cut.
Fuse are generally rated according to the
amount of electric current supplied through the
circuit. A fuse rated 5A means, it allows only a
131