Resistance

 1. Resistance:

Resistance is the tendency of material to oppose the flow of electric charges or current through it. The material Which opposes is called resistor. It is the basic element of any circuit. Metal such as, Silver, Copper and Aluminum are good conductors of electricity and have less resistance . Semiconductors Silicon and Germanium have semiconducting property and have a more resistance than metals. Insulators Such as , wood , rubber polythene etc. Have even less conducting property than semiconductors and have high resistance. Generally in Normal conditions insulators have zero conductivity and having extremely high resistance. Resistance of a conductor is directly proportional to the length (l) of the conductor and it inversely proportional to the cross-sectional area (A). Hence, resistance of q conductor is given by ;

                              R=p(l/A)

• R=resistance of conductor. Ohm

• P=resistivity of conductor, ohm-meter

• L= length of conductor in meter

• A=Cross-sectional area of conductor in meter²

     Resistance of a conductor gets increased in temperature while the insulators and semiconductors acts in opposite directions,Means its resistance gets decrease by increasing temperature of it.

2. Characteristics of Resistance in an electric circuit

         Resistor are used for many reasons in an electric circuit.some of them are given below;

• Limiting current: By introducing a resistance in an electric circuit, resistor can limit the amount of current flows through the circuit. In accordance with ohm’s law . If voltage in a circuit remains the same, the current will decrease if resistance increases. Rheostat (a variable resistor) can used for this purpose.

• Dividing voltage : Resistor can be used to reduce voltage to a level that’s appropriate for specific part of any circuit.

• Heating Element: Resistor blocks the flow of current and hence the electrical power gets loss and its converted to heat. This principal is employed to make an electrical heating equipment’s such as , electric heaters, Irons,etc. The principle of electric heating is also employed in filament lamps an fuse wires.