Saturday, October 1, 2011

The heating effect of an electric current:

When a charge moves in a conductor, work is done by that charge. Devices can be made which convert this work into heat ( electric heaters), light (light bulbs and neon tubes), or motion, i.e. kinetic energy (power tools).

From the definition of potential difference, V, we have V = W/Q, where W is the work done by charge Q. Hence, W = VQ.

Current is the flow of charge, so that in time t, the amount of charge moving through the conductor will be Q = It.

Therefore, W = VIt gives the work done in time t, by a current I, flowing through a conductor across which the potential difference is V. This may be written in two other ways by substituting from Ohm's Law:

where R is the resistance of the conductor.

Electric power:

Remember that power is defined as the rate at which work is done:

By substituting from W = VIt, we obtain the formula for the power dissipated in an electric circuit, as follows:


This formula gives the power which is dissipated when a current I moves through a conductor across which there is a potential difference V.

From Ohm's law we may also write


The unit of power is the WATT, W which is equivalent to one joule per second, J.s-1.



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