Resistance to electric current is present in all materials. The amount of resistance depends on certain physical factors. What are they? We will see now.
Resistance means the opposition offered by a material to the flow of electric current through it. Ohm’s law gives the relationship between voltage, current and resistance of the material through which the current is flowing.
Any material will have some amount of resistance to current through it. Depending the quantity of resistance offered, materials can be electrically classified as Conductors and Insulators. In simple words, a conductor offers very low resistance, an insulator offers very high resistance. But any material will have some amount of electrical resistance.
Factors Affecting Resistance
A conductor offers some amount of resistance to current. The amount of resistance offered depends on these factors:
a. Length of the conductor
b. Thickness of the conductor
c. The material the conductor is made of
d. The temperature of the conductor
Resistance is –
a. directly proportional to the length (l) of the conductor
b. inversely proportional to the cross sectional area (A) of the conductor
Therefore,
The equation relating all these is:
Resistance and Length
More the length of the conductor, higher is the resistance.
Analogy in real life:
* Water takes longer time to come out of a long pipe than in a short pipe.
* We take longer time to reach in a long path than a short one.
Resistance and Cross Sectional Area
* It is easier to move quickly in a wide corridor than in a narrow corridor.
* Traffic is smooth in a wide road than a narrow road.
* Water pressure is low in a wide pipe.
Resistivity or Specific Resistance
Specific resistance of a material is the inherent property of a material to oppose current through it. It is constant for a particular material.
Its unit is ohm-metre.
Conductors and Insulators
For example,
Silver is a very good conductor, with resistivity of 1.6 x 10-8 ohm-m.
An insulator like wood has a resistivity between 1014 to 1016 ohm-m.
Resistance and Temperature
The reverse effect of this is that high resistance increases the temperature of the material. This effect is utilised in tungsten filament incandescent bulbs, iron boxes, heaters and radiators.
Specific Resistance of Some Materials
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