Condition for validity of Ohm’s law is that

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(a) temperature remains constant
(b) current is proportional to voltage
(c) resistance is wire wound type
(d) all the above

Correct Answer: (a) temperature remains constant

Explanation

Resistance varies with temperature; therefore constant temperature is essential.

Notes for More Information

CONCEPT: Ohm’s Law

  1. Ohm’s law states that at constant temperature, the potential difference (V) across a current-carrying conductor is directly proportional to the current (I) flowing through it.
  2. Mathematically:



  1. Hence,



  1. Where:
    • V = potential difference (volt)
    • I = current (ampere)
    • R = resistance (ohm)
  2. Metallic conductors generally obey Ohm’s law under normal conditions.

LIMITATIONS OF OHM’S LAW

  1. Ohm’s law is not applicable to unilateral networks.
  2. Unilateral networks allow current to flow only in one direction.
  3. Such networks contain devices like:
    • Diodes
    • Transistors
  4. These devices do not show a linear V–I relationship.
  5. Ohm’s law is also not applicable to non-linear networks.
  6. In a non-linear network, circuit parameters such as:
    • Resistance
    • Inductance
    • Capacitance
    • Frequency
      do not remain constant with voltage or current.
  7. Since these parameters change with operating conditions, the V–I relationship is non-linear, and Ohm’s law fails.

EXPLANATION: Validity of Ohm’s Law

  1. For Ohm’s law to be valid, two conditions must be satisfied:

(a) Constant Temperature

  1. The temperature of the conductor must remain constant.
  2. If temperature changes, the resistance of the conductor changes, and Ohm’s law is no longer strictly valid.

(b) Proportionality Between V and I

  1. The potential difference must be directly proportional to the current.
  2. In practice, voltage is applied first, and then current flows in the circuit.
  3. Hence, voltage is considered proportional to current, not the other way around.

FINAL STATEMENT OF OHM’S LAW

  1. Ohm’s law states that the current flowing through a conductor is directly proportional to the potential difference across its ends, provided that physical conditions, especially temperature, remain constant.
  2. If these conditions are violated, the ratio V/I is no longer constant, and Ohm’s law does not apply.

KEY TAKEAWAY

  1. Ohm’s law is valid only for linear, bilateral, metallic conductors at constant temperature.
  2. It is not valid for diodes, transistors, and non-linear circuits.



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