RESISTANCE

•   The fact that a wire conducting a current can become hot is evident that the work done by the applied voltage in producing current is due to accomplishment against some form of opposition.

•   This opposition,  which limits the current , is called resistance.

•   The atoms of a copper wire have a large number of free electrons, which can be moved easily by a potential difference.

•   Therefore, the copper wire has little opposition to the flow of free electrons when voltage is applied, corresponding to low value of resistance.

•   Carbon, however, has fewer free electrons than copper.

•   When the same amount of voltage is applied to the carbon as to the copper ,fewer electrons will flow.

•   Carbon opposes the current more than copper, therefore, has a higher value of resistance.

OHM

•   The practical unit of resistance is the ohm.

•   A resistance that develops 0.24 calorie of heat when one ampere of current flows through it, for one second will have opposition of one ohm.

SYMBOL FOR RESISTANCE

•   The symbol for resistance is R.

•   The abbreviation used for the ohm is the Greek letter OMEGA (W).

•   In diagrams resistance is indicated by a zigzag line.  

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