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Ohm’s law was discovered by G.S. Ohm in 1828. If V be the potential difference between the ends of the conductor through which a current I is flowing, then Ohm’s law states that
V I or V=RI,
where R is the proportionality constant, known as the Resistance of the conductor. SI units of resistance are Volt per Ampere, VA-1 or Ohm ( ).
Resistance of the conductor depends on:
- Dimensions of the conductor
- Material of the conductor
- Resistivity ( ) or specific resistance of the conductor
Limitations of Ohm’s law
The proportionality of V and I do not hold for certain materials and devices used in electric circuits. Some deviations are:
- V ceases to be proportional to I for a good conductor.
- Value of I is different for the same potential difference on reversing the direction of V. Example: Diodes.
- The value of potential is different for the same current. Example: GaAs (gallium arsenide).
Now, let us have a look at some questions related to Ohm’s law:
Q1. Find the resistance of a resistor if it has a voltage of 18V across it and a current of 6A flowing through it.
Ans. Using Ohm’s law,
V=IR
Q2. If the current flowing through a 4 resistor is 2 A, what will be the voltage across the given resistor?
Ans. V=IR
2 A= 2 10-6 A
V= 4 2 10-6
V= 8 10-6
V= 8 V
Q3. Which statement regarding Ohm’s law is correct?
- The potential difference would change directly with changes in the current if the resistance were kept constant
- The potential difference will always decrease if resistance increases in an electrical circuit.
- Resistance is directly proportional to the potential difference
- Current increases in a circuit, if there is an increase in resistance and the potential difference remains constant
Ans. Option a) is correct, as it follows Ohm’s law.
Q4. Which of the following is correct with respect to Ohm’s law?
Ans. Option b) is correct as it is in accordance with Ohm’s law.
Q5. Why is Ohm’s law valid only for a constant temperature?
- A potential difference of a battery decreases with a rise in temperature
- A potential difference of a battery increases with a rise in temperature
- The electrical resistance of a battery decreases with a rise in temperature
- The electrical resistance of a battery increases with a rise in temperature
Ans. Option d) is correct. Resistance depends on the temperature of the conductor, as the temperature is dependent on the material of the conductor.
Q6. Find the current flowing through a resistor if it has a voltage of 15V across it and electrical resistance of 3.
Ans. V = IR
15 = 3I
Q7. In a closed electrical circuit, the resistance of a few metres long wire conductor is:
- Very high
- High
- Low
- Nearly equal to zero
Ans. Option d) is correct.
Q8. If there is a drop of IR across a given resistor, it is inferred that the resistor has experienced a change in potential. Does this potential change affect the current as well?
- No, the current flow remains constant. According to Ohm’s law, V= IR. Here, R is constant. At a constant resistance, the current is directly proportional to the potential difference. Hence, without any change in the resistance across the resistor, the current flow will not change as well.
Conclusion
In this article, we saw an overview of Ohm’s law. It gives the expression of V.∝
I or V=RI, where R is the proportionality constant, commonly known as the resistance of a conductor. We also saw some of the limitations of Ohm’s law. Moving on, we took up some sample questions related to Ohm’s law and solved them.
