Choose an incorrect statement from the following about Kirchhoff's rules of electric circuits:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Both the rules can only be used if the temperature is constant
Kirchhoff's rules follow from two conservation laws, so they hold in any circuit, whatever its temperature. A change in temperature only changes the values of the resistances, which are then used in the same rules.
Option (1) \( \Sigma I = 0 \) shows that charge is conserved (Junction rule) — This is true. Charge does not pile up at a junction, so the current entering equals the current leaving.
Option (2) \( \Sigma V = 0 \) shows that energy is conserved (Loop rule) — This is true. Going round a closed loop a charge returns to the same potential, so the sum of the changes in potential is zero.
Option (3) Two rules can be applied to any electrical circuit — This is true. They apply to every d.c. circuit, however complicated.
Option (4) Both the rules can only be used if the temperature is constant — This statement is incorrect, so it is the answer. The conservation of charge and of energy does not depend on temperature.
NCERT (2026-27):
Kirchhoff’s Rules – “… (a) Junction rule: At any junction, the sum of the currents entering the junction is equal to the sum of currents leaving the junction (Fig. 3.15).”
Kirchhoff’s Rules – “… The proof of this rule follows from the fact that when currents are steady, there is no accumulation of charges at any junction or at any point in a line.”
Kirchhoff’s Rules – “… (b) Loop rule: The algebraic sum of changes in potential around any closed loop involving resistors and cells in the loop is zero (Fig. 3.15).”