Series vs Parallel Power (Most Frequent Error): Students write P_series = 3P when P_parallel = P for 3 identical resistors. Correct: P_series = P/9. The factor is $n^{2}$ (here n = 3, so P_parallel = 9 × P_series).

Potentiometer vs Voltmeter: The reason a potentiometer is better is NOT "higher resistance" — it is that NO CURRENT flows at the null point, so there is no Ir drop, and true EMF is measured.

Terminal Voltage at Short Circuit: Students say terminal voltage = ε when R = 0. Actually, V = ε − I(ε/r)×r = ε − ε = 0. Terminal voltage is zero at short circuit.

Metre Bridge Interchange: If balance is at l = 60 cm, swapping R and S gives l′ = 40 cm (NOT 60 cm). The formula l′ = 100 − l must be remembered.

Temperature Coefficient Sign: Semiconductors have α < 0 (R decreases with T). This is the opposite of metals. NTC thermistors are semiconductors.

Charging vs Discharging: V = ε + Ir during charging (current enters positive terminal); V = ε − Ir during discharging. Students often apply ε − Ir for both.

Resistivity vs Resistance: Resistivity is a material property — it is the SAME for a thin wire and a thick wire of the same material at the same temperature. Resistance changes with geometry; resistivity does not.

Power formula selection: In a series circuit, use P = $I^{2}$R (same current). In a parallel circuit, use P = $V^{2}$/R (same voltage). Using the wrong formula for the wrong configuration is a very common error.

Potentiometer consistency: Before solving, always verify that V_wire ≥ ε_test. If V_wire < ε_test, balance is not possible. This step is commonly skipped, leading to physically impossible (negative) answers.