Ohm's Law class 10 | cbse24.com

Note -Before going to understand ohm's law first understand what is resistance?


Resistance is a measure of the opposition to current flow in an electrical circuit and other word property of a substance to resist the flow of current through it, is called resistance.

Picture to understand the flow of current of current

Resistance is a property of material on the basis of which it opposes the flow of electrons through the material. It restricts the flow of electrons through the material. It is represented by (R) and is measured in ohms (Ω).
When the voltage is applied across the resistor, the free electrons start accelerating. These moving electrons collide and therefore resist the flow of electrons.

Ohm's Law:

Ohm's law gives a relation between current and potential difference. According to ohm's law: At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends.

If I is the current flowing through a conductor and V is the potential difference across its ends, then according to ohm's law.

I∝ V (At constant temp.)

This can also be written V∝ I

or V=R x I

where R is a constant called resistance of the conductor. The value of this constant depends on the nature, length, area of cross-section and temperature of the conductor. The above equation can also be written as:


V=potential difference(volt)
R=resistance (Which is constant)(ohm)

we find the ratio of the potential difference applied between the ends of a conductor and the current flowing through it is a constant quantity called resistance

the strength of an electric current in a given conductor depends on two factor
  • the potential difference across the ends of the conductor
  • the resistance of the conductor

Factors affecting the resistance of a conductor:

  • Length of the conductor(R∝l)
  • Area of the cross-section of the conductor(thickness of the conductor)(R∝1/A)
  • Nature of the conductor( mean which material of this form)
  • The temperature of the conductor(R∝T)

A mixed question of S.Chand and NCERT of this topic

Q1.(a) Define the unit of resistance.

(b) What happens to the resistance as the conductor is made thinner?

(c) keeping the potential difference constant, the resistance of a circuit is doubled. By how much does the current change?

Ans:(a) Ohm

(b) R∝l so making thinner wire resistance decrease

(c) V=IaxRa--------(1)



(voltage is constant for both equation)

so equating both equation eq(1) and eq(3)


so Ib=Ia/2

cureent become half

An electric circuit consisting of a 0.5m long nichrome wire XY, an ammeter, a voltmeter, four cells of 1.5V each and a plug key was set up.

(i) Draw a diagram of this electric circuit to study the relation between the potential difference maintained between the points X and Y and the electric current flowing through XY.

(ii) The following graph was plotted between V and I value.

what would be the value of the V/I ratio when the potential difference is 0.8V,1.2V and 1.6V respectively? What conclusion do you draw from these values?
(iii) What is the resistance of the wire?


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