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 refers to the opposition a material offers to the flow of electric current. According to Ohm’s Law, resistance is directly related to the voltage and current in a conductor through which electricity flows.
Every material has some level of resistance to the flow of electric current. Based on the amount of resistance they offer, materials are electrically classified as conductors or insulators. In simple terms,
- a conductor offers very low resistance, while
- an insulator provides very high resistance.
However, it is important to note that all materials exhibit some degree 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 proportionality constant is called Resistivity of the material. It is also called Specific resistance.
The equation relating all these is:
Resistance and Length
The longer the conductor, the higher its electrical resistance.
This concept can be understood through real-life analogies:
- Water takes more time to flow out of a long pipe than a short one.
- We take longer to reach our destination on a long path than on a short one.
Resistance and Cross Sectional Area
The thicker the conductor, the greater its cross-sectional area, and the lower its electrical resistance. In other words, a thin conductor offers higher resistance than a thick one.
The thicker the conductor, the greater its cross-sectional area, the lower is its electrical resistance. In other words, a thin conductor offers higher resistance than a thick one.
Real-life analogies make this concept easier to understand:
- It is easier to move quickly in a wide corridor than in a narrow one.
- Traffic flows more smoothly on a wide road than on a narrow road.
- Water pressure is lower in a wide pipe compared to a narrow 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.
Specific resistance, also known as resistivity, is defined as the resistance offered by a material with a length of 1 meter and a cross-sectional area of 1 square meter. In other words, it represents the resistance of a material measuring 1 cubic meter. Its unit is ohm-meter (Ω·m).
Conductors and Insulators
Conductors have very low resistivity while insulators have very high resistivity.
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
Resistance increases with temperature. When temperature rises, moving charges collide more with the surrounding particles, hindering their movement and increasing resistance.
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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