14-11-2012, 02:22 PM
THERMAL AND CHEMICAL EFFECTS OF ELECTRIC CURRENT
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Heating Effect of Electric Current: Joule’s Law
The electric current in a conductor is due to the motion of electrons. During their motion,
electrons collide with the oscillating positive ions in the conductor and impart part of their
energy to them. Ions oscillate faster and their increased energy is manifested as heat.
The heat energy released in a conductor on passing an electric current is called the “Joule
heat” and effect is called the ‘Joule effect”.
The potential difference of V volt applied between two ends of a conductor means that V
joule of electrical energy is utilized and converted into heat when one coulomb charge passes
through the conductor.
Practical Applications of Joule Heating
Joule heat is used in domestic appliances such as electric iron, toaster, oven, kettle, room
heater, etc. It is also used in electric bulbs to produce light. The temperature of the filament
of the bulb rises considerably when current flows through it and it emits light. Hence it
should be made of a metal of high melting point such as tungsten whose melting point is
3380° C. Also it should be thermally isolated from the surrounding as far as possible. Only a
small fraction of electrical power supplied to the bulb is converted into light. Normally a bulb
emits 1 candela of light per watt of electrical power consumed.
Another common application of Joule heat is fuse wires used in circuits. It consists of a
piece of metal wire having low melting point ( such as aluminium, alloy of tin / lead, copper
etc. ) and is connected in series with an appliance. If a current larger than a specified value
flows, the fuse wire melts and breaks the circuit thus protecting the appliance.