[Physics-Class X]
Describe with the aid of a diagram some form of a fuse which is used in the electric lighting circuit of
a house. Give two reasons why a fuse must not be replaced by an ordinary copper wire.
Answers
So let's first understand the arrangement of the fuse.
Construction :
Fig. 9.13 (attachment) shows one of the most common fuse arrangement in which the fuse wîre F is stretched between the two metallic terminals T1 and T2 in a porcelain holder (since porcelain is an insulator of electricity). This holder fits into porcelain socket having two metallic terminals to each of which the live wire of the circuit is connected. Thus the fuse wire gets connected to the live wire. Fuse wire is connected in series connection with the live wire.
Working :
When current in the circuit exceeds the specified value (due to any reason such as high voltage, short-circuiting, overloading, etc.) the temperature of the fuse wire rises to the extent that it melts. As a result, a gap is produced and the circuit breaks. Now current does not flow through the live wire and the appliance (or the circuit) is saved. After removing the fault in the circuit, a new fuse wire of the same current rating is connected in the holder to complete the circuit again.
A copper wire is unsuitable for use as the fuse wire because it has high melting point (≈ 1080°C) and low resistance. Moreover, the use of an ordinary copper wire as a fuse must be avoided as it is very thick so it will not melt even if the current exceeds its safe limit.
Therefore, an alloy of lead and tin is used because its melting point is low and its specific resistance is high. It permits current upto its safe limit to pàss through it, but an excessive current melts it so that it blows off.
Answer:
The figure above shows the most common fuse arrangement in which the fuse wire is stretched between the two metallic terminals T1 and T2 in a porcelain holder. This holder fits into a porcelain socket having two metallic terminals to each of which the live wire of the circuit is connected. A fuse must not be replaced with a copper wire because copper has very low resistivity and high melting point.