Resistance

Resistance

The property of a substance which opposes the flow of electric current (or electricity) through it is called Resistance OR Resistance is the ability of a circuit which opposes current. It is denoted by  ‘R’. Mica, Glass, Rubber, Wood etc. are the examples of resistive materials. The unit of resistance is OHM (Ω) where 1Ω = 1V/1A.  Which is derived from the basic electrical Ohm’s law = V = IR. Other definitions      of Ohm “Ω“         are     as      follows;

If there is a potential difference of 1 volt between two ends of the conductor and the flowing current through it is 1Ampere, then the resistance of that conductor would be 1 Ohm (Ω).

OR
If 1 ampere of current is flowing through a resistance, and 1 joule per second (1Watt) energy (in the form of heat) is generated, then the measurement of that resistance is 1 Ω.

Ohm (Ω) is the measurement quantity of resistance, which produces one joule of energy (in the form of heat) in one second, when one ampere of current is flowing through it.

Substances which offer low resistance are good conductor of electricity, e.g. salt solution, acids and metals and those substances which offer relatively greater resistance are of poor conductor of electricity, e.g. all insulators like rubber, glass, plastics, etc.

Laws of Resistance:

The resistance offered by a conductor depends on the following factors:

Ø R varies directly with the length of conductor (l)

Ø R varies inversely with the cross section area of conductor (A)

Ø R depends on the nature or properties of materials

Ø R depends on the temperature of the conductor (t)

Neglecting the effect of temperature,

                 R α l/A    i.e. R = ρ l/A

Effect of rise in temperature on Resistance:

The effect of rise in temperature is,

Ø To increase the resistance of pure metal, this increase is large and regular for normal range of temperature. Their temperature/resistance graph is straight line and has a positive temperature coefficient of resistance.

Ø To increase the resistance of alloys. This increment is relatively small and irregular and even sometimes negligible.

Ø The increase in temperatures decreases the resistance of electrolytes, insulators and partial conductor. So, insulators have a negative temperature coefficient of resistance

Temperature coefficient of resistance:

Let a metallic conductor be heated from 0˚ C to T˚ C. Let R_o and R_t be its resistances at 0˚ C and t˚ C respectively. Then increase in resistance ΔR= R_t – R_o

ΔR depends on,

Ø Directly on its initial resistance R_o

Ø Directly on its rising temperature T˚ C

Ø On the nature of material of the conductor

 ΔR α R_o * T˚

R_t-R_o= α*R_o* T˚ 

R_t= R_o (1+ α T˚)

Also, α= (R_t - R_o)/ R_o *T˚   i.e. α= ΔR / R_o *T˚ 

Where α is a constant quantity and known as temperature coefficient of resistance of conductor.

Hence, the temperature coefficient of a material may be defined as the increase in resistance per original resistance per rise in temperature.