Potential divider practice
Calculate outputs, choose a resistor and predict LDR, thermistor and loading effects.
DC circuit questions
Solve series, parallel and mixed networks; calculate charge, energy and power.
Practise
Parallel resistanceResistors of 30 Ω and 70 Ω are connected in parallel. Find the total resistance. Give your answer to 3 significant figures.
Ideal DC supply, wires and meters. Resistors have constant resistance; heating does not change their values.
Larger diagram
Treat the given values as exact unless an uncertainty is stated.
Hint
Add reciprocals, then invert: 1/R_total = 1/R1 + 1/R2. The result is smaller than either branch resistance.
Worked solution
- R_total = (30 × 70)/(30 + 70) Ω.
- Answer: 21.0 Ω (3 significant figures).
Question type: Parallel resistance
Report a problem
Follow current and potential difference
Series components carry the same current; their potential differences add. Parallel branches have the same potential difference; their currents add. Kirchhoff’s junction rule expresses conservation of charge, and the loop rule expresses conservation of energy.
Reduce a network in stages
Series resistances add. For two parallel resistors, R = R1R2/(R1 + R2); this result is smaller than either branch resistance. Find a parallel pair’s resistance before adding a series resistor. Supply current and branch current need different resistance values.
Charge, energy and power
Q = It, E = VQ, and P = VI = I²R = V²/R for the same resistor. Convert minutes to seconds before finding charge. These tasks use an ideal supply and constant resistances. Internal resistance, sensors and dividers have separate practice pages; multiple-source loops and nonlinear components are not covered here.
A-level relevance
AQA 7408: 3.5.1.1, 3.5.1.4 (selected); OCR A H556: 4.1.1; 4.2.1–2; 4.3.1 (selected). Tasks cover the selected skills listed above.