RESISTANCE

Definition – Resistance is the friction in an electrical circuit that controls the flow of current or Resistance is a material’s behavior to resist the flow of charge (current).

As we know that voltage is a driving force which causes current to flow in the circuit. The value of current in a circuit depends on the resistance. It is measured in Ohms (Ω)–

Voltage,     V = IR    (Current X Resistance)

Or Current I    = V/R  which means current is inversely proportional to resistance. If resistance is more, current will be less & vice versa.

In other words we can say –
1. For a fixed value of Resistance, more Voltage causes more current.
2. For a fixed Voltage, more Resistance causes less current.

TYPES OF RESISTANCE

Broadly there are two types of resistances – 1) Natural resistance, 2) Artificial resistance.

  • Natural resistance used in electrical system which is offered by the material used for current carrying purpose in electrical system such as cables, wires, bus-bars, motor windings, transformer’s windings etc. This resistance is very important in transmission & distribution of electrical power so that power can be transferred from one place to another with suitable size of wires/ cables. The value of resistance offered by material depends on the resistivity of the material. Let’s understand resistivity.

Resistivity – Every material has natural tendency to offer resistance against flow of current & this value of resistance is called resistivity. Some material offers small resistance against current such as Gold, silver, copper, aluminum etc.  And some offers high resistance glass, rubber, wood etc. In other words we can say that material which offers low resistivity is a good conductor & vice versa.

Factors affecting the value of resistance – The value of Resistance depends on following factors given below in the formula –

Resistance, R = ƿ L/A

Where – ƿ = Resistivity of material in Oms.Meter

L = Length of material (say wire) in Meters,

A = Cross sectional area of wire in Meter2.

From above equation, it is clear that the resistance of a material is directly proportional to the length of material it means if length is more, the resistance of that wire will be more.

And resistance is inversely proportional to the cross sectional area of wire. If cross-sectional area is more, the wire will offer less resistance & vice versa.

Note – Resistivity of a material depends on temperature also. Mostly material offers more resistance on higher temperature.

Resistivity of various types of material at 20DegC is given in the table below –

Material Resistivity Material Resistivity
(ohm•meter) (ohm•meter)
Silver 1.59 x 10-8 Nichrome 150 x 10-8
Copper 1.7 x 10-8 Carbon 3.5 x 10-5
Gold 2.2 x 10-8 Polystyrene 107 – 1011
Aluminum 2.8 x 10-8 Polyethylene 108 – 109
Tungsten 5.6 x 10-8 Glass 1010 – 1014
Iron 10 x 10-8 Hard Rubber 1013
Platinum 11 x 10-8 Wood (oven dry) 1014 -1016
Lead 22 x 10-8 Air 1.3×1016 – 3.3 x1016
  • Artificial resistance which are used in electronic circuits. These are called resistors. Most resistors have stripes or bands of colors painted on them. The colors reveal information about the resistance value. Some photos of resistors are given below –

(Note- The function of resistors is to limit the current in the circuit as it takes/consumes energy which can’t be returned. So, all devices which consume energy must have resistance in their circuit models.)

This can be understood from following equation –

V= IR     if voltage is constant then current is inversely proportional to resistance. The power consumption by a resistor can be explained by equation – P = I2R

Series & parallel connections of resistance

In electronic circuits, these resistors are connected in series, parallel & its combinations. When these resistors are connected with each other in the forms as mentioned above, they make a different value of resistors (equivalent resistance) which can be understood from following diagrams –