Materials in general have a characteristic behaviour of resisting the flow of electric charge. This physical property, or ability to resist the flow of current, is known as resistance and is represented by the symbol R. The Resistance is measured in ohms ( ). The circuit element used to model the current- resisting behaviour of a material is called the resistor.

Fig. G (a) Typical Resistor, (b) Circuit Symbol for Resistor

The resistance of a resistor depends on the material of which the conductor is made and geometrical shape of the conductor. The resistance of a conductor is proportional to the its length ( and inversely proportional to its cross sectional area (A). Therefore the resistance of a conductor can be written as,

The proportionality constant is called the specific resistance o resistivity of the conductor and its  value depends on the material of which the conductor is made.

The inverse of the resistance is called the conductance and inverse of resistivity is called specific conductance or conductivity. The symbol used to represent the conductance is G and conductivity is . Thus conductivity and its units are Siemens per meter

By using Ohm’s Law, The power dissipated in a resistor can be expressed in terms of R as below

The power dissipated by a resistor may also be expressed in terms of G as

The energy lost in the resistor from time 0 to t is expressed as

where V is in volts,

I is in amperes,

R is in ohms, and

energy W is in joules