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Specific resistance or resistivity is the resistance of a wire of unit length and unit area of cross-section.
It depends on two factors.
1. Nature of the substance.
2. Temperature of the substance
It is not effected by
1. Length of the wire.
2. Area of cross-section of the wire.
So., when you stretch a wire its RESISTIVITY remains constant.
The electric power is generated at 11,000V at a power station. It is then transformed to 1,32,000V using a step up transformer.
For a given electric power, when voltage increases, the electric current decreases as P=VI or I=P/V.
Hence , at high voltage low current is involved.
As current decreases, the power loss(I in the transmission lines decreases in the form of heat .
To reduce this loss, high voltage is preferred
ALTERNATING CURRENT
1. Its value changes continuously.
2. Its polarity changes periodically.
3. It can be transformed by a transformer.
DIRECT CURRENT
1. Its value doesn't change, it has a constant magnitude.
2. Its polarity doesn't change.
3. It can not be transformed by transformer.
Earthing is required in a circuit when
1. the appliance draws high current. High power devices draw large current.(Current =power/voltage).
2. the appliance has outer metallic case.
It is longer so that when we put the plug into the socket, earth connection is made first for the safety.
It is thicker so that it is not inserted to the holes of live and neutral.
It has split ends so that the plug is fitted firmly into the socket to avoid sparking.
Fuse wire is a safety device to limit the current in an electric circuit. It gets melted when excess current flows in a circuit. It works with the principle of heating effect of electric current. It is made with an alloy of lead and tin (solder wire).
Its material has two important properties
1. It has low melting point (around 250 degree celcius).
2. It has high resistivity.