NumericalModerate2 marks
2(iii)
In the below circuit diagram, calculate:
the external resistance of the circuit.
Topic: Current, resistance and circuits
Previous-year question practice database
Answer
Exam answer
From the circuit, the 12 Ω and 8 Ω are in parallel, and the 2 Ω is in series with that combination.
Step 1 — the parallel pair: R_p = (12 × 8) ÷ (12 + 8) = 96 ÷ 20 = 4.8 Ω
Step 2 — in series with the 2 Ω: R = 2 + 4.8 = 6.8 Ω
External resistance of the circuit = 6.8 Ω
Explanation
Parallel first, then series. Check the parallel step against the rule: 4.8 Ω is less than 8 Ω, the smaller branch ✓.
The question asks for the EXTERNAL resistance, so this is everything outside the cell. If it had asked for the total resistance of the circuit you would add the cell's internal resistance on top.
More from Current Electricity
In the circuit given below, identify the lamp (L1, L2, L3 and L4) whose failure would not interrupt the power supply to the other lamps.
(a) L1
(b) L2
(c) L3
(d) L42026The correct formula to calculate the equivalent resistance of two resistors R₁ and R₂ when connected in parallel, is:
(a) (R₁ + R₂) / (R₁R₂)
(b) (R₁R₂) / (R₁ + R₂)
(c) (R₁ − R₂) / (R₁R₂)
(d) (R₁R₂) / (R₁ − R₂)2026A substance whose resistance becomes almost negligible at a temperature near absolute zero is called a ............... [semiconductor / superconductor].2026How will the voltmeter reading change? (increase / decrease / remain the same)2026Justify your answer stated in (a) above.2026Calculate the ratio V1 : V2.2026