Ohm's Law gives a relationship between Potential Difference and Electric Current
As per Ohm's Law
Current flowing through a conductor is
directly proportional to
potential difference across its ends
Provided physical conditions like temperature remains the same
V ∝ I
Example
Electric Current in Amperes | Potential Difference In Volts |
5 | 50 |
6 | 60 |
7 | 70 |
4 | 40 |
3 | 30 |
In this case, if we see
- If electric Current Increases, Potential Difference increases
- If electric Current Decreases, Potential Difference decreases
From above we can say
Potential Difference is directly proportional to Electric Current
V ∝ I
V = constant × I
V/I = constant
This constant is known as Resistance.
V = IR
What is Meaning of Resistance
The Property of Conductor due to which it opposes the flow of current through it
This is because Resistance is inversely Proportional to Electric Current
As per Ohm's Law
Resistance is always constant for a given electric wire of a particular shape at a particular temperature
it is calculated as follows
Resistance = Potential Difference/Current
What is SI Unit of Resistance
It is Measured in ohm
1 Ohm = 1 Volt/1 Ampere
Explanation
We know that
Resistance = Potential Difference/Current
Resistance is Measured in Ohm,Potential Difference in Volt and Current in Ampere
Hence our formula becomes
1 ohm = 1 Volt/1 Ampere
1 Ω = 1 V/1 A
Calculating Resistance
It is the Ratio of Potential Difference to Electric Current
We know that,
V = IR
V/I = R
Electric Current in Amperes (I) | Potential Difference In Volts (V) | Resistance R = V/I |
5 | 50 | 10 |
6 | 60 | 10 |
7 | 70 | 10 |
4 | 40 | 10 |
3 | 30 | 10 |
Note -
Resistance is always constant and is 10 in the above case.
Note - Resistance is Inversely Proportional to Current
It means that if we increase Resistance,Electric Current Decreases
If we decrease Resistance,Electric Current Increases
Explanation
We know that
V = IR
V/R = I
I = V/R
Case 1
Suppose V = 100 V ,R = 20 Ω
I = V/R = 100/20 = 5 A
Case 2
Suppose we increase Resistance from 20Ω to 25Ω
I = V/R
I = 100/25 = 4A
Hence if Resistance increases ,Electric Current Decreases
Case 3
Similarly,if we decrease Resistance from 20Ω to 10Ω
I = V/R
I = 100/10 = 10A
Hence if Resistance Decreases, Electric Current Increases
This shows that Resistance is inversely Proportional to Electric Current
Potential Difference V | Resistance R | Current I=V/R |
(V) | (Ω) | (A) |
100 | 20 | 100/20 = 5 |
100 | 25 | 100/25 = 4 |
100 | 10 | 100/10 = 10 |
Questions
Questions
NCERT Question 7 -
The values of current I flowing in a given resistor for the corresponding values of potential difference V across the resistor are given below –
I (amperes) 0.5 1.0 2.0 3.0 4.0
V (volts) 1.6 3.4 6.7 10.2 13.2
Plot a graph between V and I and calculate the resistance of that resistor.
NCERT Question 8 -
The values of current I flowing in a given resistor for the corresponding values of potential difference V across the resistor are given below –
I (amperes) 0.5 1.0 2.0 3.0 4.0
V (volts) 1.6 3.4 6.7 10.2 13.2
Plot a graph between V and I and calculate the resistance of that resistor.
Example 12.3 - How much current will an electric bulb draw from a 220 V source, if the resistance of the bulb filament is 1200 Ω? (b) How much current will an electric heater coil draw from a 220 V source, if the resistance of the heater coil is 100 Ω?
Example 12.4 - The potential difference between the terminals of an electric heater is 60 V when it draws a current of 4 A from the source. What current will the heater draw if the potential difference is increased to 120 V?