Unit of Resistance

Introduction

Resistance plays a major role in the study of electricity. Electricity is so important for various applications in our daily life. Electric current, which is the flow of electrons, passes through the electric circuit with the component’s resistor, capacitor, inductors are used. The electrical component resistor is used in the circuit to allow current of a particular amount and stops the remaining.

This was done by the circuit element called resistor by producing some resistance in the circuit.

What is Resistance?

We know a conductor material produces electric current by movement of free electrons in a particular direction when voltage is applied to the conductor. In order to control the flow of current, some resistance is produced within the material. Hence, resistance is the variable parameter which opposes the flow of current when a voltage is applied across an electric circuit. It is denoted by R. Resistor is the device that produces resistance in an electric circuit.

• Fixed resistor: It is a linear resistor with a fixed resistance value.
• Variable Resistor: It is a resistor which contains various resistance values. This can be obtained by varying the current in the electric circuit with voltage constant. The variable resistor is also called Rheostat.

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Factors affecting Resistance:

There are four factors that affect the resistance of a resistor.

• Depends on the nature of the material the resistor is made up of. For example, the material is a conductor made up of copper, gold etc. Then it contains less resistance due to high conduction of free electrons and hence produces more electric current. If the material is an insulator made up of rubber, plastic etc exhibits less conduction and hence their resistance will be low. Semiconductors produce very high resistance than conductors and insulators.
• Depends on Temperature: If the temperature of the material or the resistor increases, there will be an increase in resistance. If R0R0 is the resistance at 00C00C temperature and RfRf is the final resistance at t0Ct0C temperature then equation for the final resistance RfRf is,

Rf=R0(1+αt+βt2)Rf=R0(1+αt+βt2)

αα and ββ are the temperature coefficient of resistances that vary for different materials and their unit is 1k1k or 1t0C1t0C

• Depends on the length of the material used in the resistor. If the length of the material increases, the resistance also increases. It means they both are directly proportional to each other. i.e.,

RαlRαl

Here l is the length of the material.

For example, if there is a long wire of conductor material connected to the electric circuit, then due to more space there will be an increase in flow of electrons and also collisions between them that opposes the current flow and vice versa.

• Depends on the area of the cross section of the material. If the area of the material increases, the resistance decreases. It means they both are inversely proportional to each other. i.e.,

Rα1ARα1A

A is the area of the cross section of the material. Thicker the material less will be the resistance and thinner the material high will be the resistance.

For example, if the area of cross section of a conductor material connected to the electric circuit is high, it means wider space, then the flow of electrons will be at high rate which can overcome the resistance in the circuit. Hence, the resistance will decrease.

Unit of Resistance

The S.I unit of resistance is ‘ohm’, which is named after a German physicist and mathematics scientist Georg Simon Ohm. Its symbol is ΩΩ.

Ohm’s Law

Important variable parameters in electrical circuits or devices are current, voltage and resistance. Georg Simon Ohm while studying electric current passing through conductors, he found that the current variable increases due to increase in voltage. It means that both variables are directly proportional to each other when the temperature variable is constant. i.e.,

VαIVαI

For removing the proportionality, we need a constant,

V=IRV=IR

This equation is called Ohm’s law and here, R is that proportionality constant, which is called Resistance.

Hence, the resistance R, by using ohm’s law is −

R=VIR=VI

By this the unit of resistance ohm can also be written as, ohm=Voltampereohm=Voltampere i.e.,

If a 1 volt voltage is applied to an electric circuit for a material then 1 ampere current is produced in it with 1 ohm resistance.

What is Resistivity?

We know from the topic, factors affecting resistance, length and area of the conductor are directly and inversely proportional to the resistance. i.e,

RαlandRα1ARαlandRα1A

Combining both the resistance R mathematically can be written as,

RαlARαlA

For removing the proportionality, we need a constant,

R=ρlAR=ρlA

Here, ρ is that proportionality constant, which is called Resistivity or specific resistance of the conductor.

For finding S.I units of resistivity, rewrite the above equation for resistivity.

ρ=RAl⇒ρ=1Ω1m21m⇒ρ=Ωmρ=RAl⇒ρ=1Ω1m21m⇒ρ=Ωm

For a conductor of 1 m length and 1m21m2 area then the resistance produced by the conductor will be 1Ω m. Resistivity measures the resistance limit of any material produced for a given length and area cross section of that material.

Conclusion

In this tutorial we learned about resistance and its units. Resistance is the property of materials that opposes the electric current in the electric circuit and its unit is ohm (Ω) . Ohm’s law gives relation between voltage, current and resistance. Resistivity is the parameter which gives the amount of resistance offered by the conductors. Good conductors exhibit less resistivity value and increase the flow of electrons leading to good conduction of electricity