Class 10 Physics



Electricity

Ohm's Law

Ohm’s Law states that the potential difference between two points is directly proportional to the electric current.

This means; potential difference V varies as electric current.

Or, `V∝I`

Or, `V/I=R`-----------(1)

Or, `1/I=R/V`

Or, `I=V/R`----------(2)

Or, `V=RI` ----------(3)

Where R is constant for the given conductor at a given temperature and called resistance. Resistance is the property of conductor which resists the flow of electric current through it.


SI Unit of resistance is ohm. Ohm is denoted by Greek letter ‘Ω’.

1 ohm (Ω) of Resistance (R) is equal to the flow of 1 A of current through a conductor between two points having potential difference equal to 1 V.

This means `1Ω=(1V)/(1A)`

From the expression of Ohm’s Law it is obvious that electric current through a resistor is inversely proportional to resistance. This means electric current will decrease with increase in resistance and vice versa.

The graph of V (potential difference) versus I (electric current) is always a straight line.

potential differnce electric current graph

Example 1: Calculate the resistance if 5 A of electric current flows through a conductor having potential difference between two points is equal to 15 V.

Solution: Given, electric current (I) = 5 A

Potential difference (V) = 15 V

Resistance (R) =?

We know from Ohm’s Law that

`R=V/I`

Or, `R=(15V)/(5A)=3Ω`

Example 2: If the potential difference between the terminals of an electric motor is 220 V and an electric current of 5 A is flowing through it what will be the resistance of electric motor?

Solution: Given, electric current (I) = 5 A

Potential difference (V) = 220 V

Resistance (R) =?

We know from Ohm’s Law that

`R=V/I`

Or, `R=(220V)/(5A)=44Ω`

Example 3: An electric current of 15 A is flowing through an electric fan. If the potential difference between two terminals of electric fan is 240 V, what will be resistance?

Solution: Given, electric current (I) = 15 A

Potential difference (V) = 240 V

Resistance (R) =?

We know from Ohm’s Law that

`R=V/I`

Or, `R=(240V)/(15A)=16Ω`


Example 4: If the resistance of an electric iron is 48 Ω and an electric current of 5 A is flowing through it, what will be the potential difference between two terminals of electric iron.

Solution: Given, electric current (I) = 5 A

Resistance (R) = 48Ω

Potential difference (V) =?

We know from Ohm’s Law that

`R=V/I`

Or, `V=RxxI`

Or, `V=48Ωxx5A=240V`

Example 5: Calculate the potential difference between two points of a terminal, if an electric current of 10 A is flowing through it having resistance of 20Ω.

Solution: Given, electric current (I) = 10 A

Resistance (R) = 20Ω

Potential difference (V) =?

We know from Ohm’s Law that

`R=V/I`

Or, `V=RxxI`

Or, `V=20Ωxx10A=200V`

Example 6: If the resistance between two terminals of an electric heater is 15Ω and an electric current of 15 A is flowing through it then what will be the voltage of electric current?

Given, electric current (I) = 15 A

Resistance (R) = 15Ω

Voltage, i.e. Potential difference (V) =?

We know from Ohm’s Law that

`R=V/I`

Or, `V=RxxI`

Or, `V=15Ωxx15A=225V`