Ohm's Law, Resistance and Resistivity
Crowded road
When there is too much crowd on a road, it is difficult for us to walk fast.
Similarly, if resistance is high in a conductor, the flow of current decreases.
The relation between current, voltage and resistance is given by Ohm’s Law.
When electrons flow through a conductor, they collide with the ions present in it. And this opposes their flow.
This tendency of a substance to oppose the flow of an electric current through it is called electrical resistance.
A body that offers resistance to the flow of electrical current through it is known as a resistor.
Let us begin with Ohm’s Law.
Ohm’s law was given by German scientist George Simon Ohm.
It state’s that for any conductor, the voltage or potential difference across the conductor is directly proportional to the current flowing.
Mathematically,
It is given as,
$V∝I$
$V=IR$
Where,
$R=$
Proportionality constant known as Resistance
Now let’s learn more about Resistance.
Resistance opposes the flow of current.
The SI unit of Resistance is ohms and it is denoted by the symbol Ω.
Resistance not only depends on the material of the conductor but also on the dimensions of the conductor.
Let us now understand Resistivity.
Consider a conductor of length L and area of cross-section A.
Then resistance R of the conductor is directly proportional to its length L.
And inversely proportional to the area of cross-section A.
Mathematically,
$R∝L$
$R∝A1 $
Therefore,
$R∝AL $
$R=ρAL $
Where
$ρ$
is the constant of proportionality
$ρ$
depends on the material of the resistor and is called resistivity.
Resistivity was previously known as specific resistance. The reciprocal of resistivity is known as conductivity
The SI unit of resistivity is Ohm-meter or Ωm.
Revision
Ohm’s law states that Voltage across a conductor is directly proportional to the current flowing through it.
Resistance opposes the flow of current and it depends on the material of the conductor as well on its dimensions.
The resistivity of a conductor depends only on the material of the conductor.
The End