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Energy exists in various forms such as mechanical energy, heat energy, chemical energy, electrical energy, light energy and nuclear energy. According to the law of conservation of energy, energy can be transformed from one form to another.
In our daily life we use many devices where electrical energy is converted into heat energy, light energy, chemical energy or mechanical energy. When an electric current is passed through a metallic wire like the filament of an electric heater, oven or geyser, the filament gets heated up and here the electrical energy is converted into heat energy. This is
known as the 'heating effect of current'.
Why is heat produced when current is passed through a wire? A metallic conductor has a large number of free electrons in it. When a potential difference is applied across the ends of a metallic wire, the free electrons begin to drift from
a region of low potential to a region of high potential. These electrons collide with the positive ions (the atoms which have lost their electrons). In these collisions, the energy of the electron is transferred to the positive ions and they begin to vibrate more violently. As a result, heat is produced. The greater the number of electrons flowing per second, the greater will be the rate of collisions and so greater is the heat produced.
A potential difference is a measure of work done in moving a unit of charge across a circuit. Current in a circuit is equal to the amount of charge flowing in one second.
Therefore, the work done in moving 'Q' charges through a potential difference 'V' in a time 't' is given by
Work done = potential difference x current x time
 W = VIt
The same can be expressed differently using ohm's law.
According to ohm's law V = IR
Therefore work can be expressed as
W = VIt
or W = (IR) It = I2Rt
The work done 'W' gets dissipated in the resistors as heat energy H
Thus, heat produced is directly proportional to the resistance, to the time and to the square of the current.
The heating effect of current is utilised in the electrical heating appliances such as electric iron, room heaters, water heaters, etc. All these heating appliances contain coils of high resistance wire made of nichrome alloy. When these appliances are connected to power supply by insulated copper wires then a large amount of heat is produced in the heating coils because they have high resistance, but a negligible heat is produced in the connecting wires because the wires have low resistance.
The heating effects of electric current is utilized in electric bulbs for producing light. When electric current passes through a thin high resistance tungsten filament of an electric bulb, the filament becomes white hot and emits light.
An 'electric fuse' is an important application of the heating effect of current. When the current drawn in a domestic electric circuit increases beyond a certain value, the fuse wire gets over heated, melts and breaks the circuit. This prevents fire and damage to various electrical appliances.
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