E23: Circuit Calculation Questions
How to approach numerical questions using V=IR, P=VI and other formulas
Circuit Calculation Questions
How to approach numerical questions using V=IR, P=VI and other formulas
Key Fact: Always write the formula first, then substitute values, then give the answer with units — this secures method marks even if the final answer is wrong.
Key Fact: Ohm's Law: V = IR can be rearranged to I = V/R or R = V/I depending on which quantity is unknown.
Key Fact: Power formulas: P = VI, P = I²R, P = V²/R — choose the one using the two known quantities.
Key Fact: For potential dividers: V_out = V_in × R2/(R1 + R2); always identify which resistor is R2 (the one V_out is measured across).
Key Fact: Convert all units to base SI before calculating: mA to A (×10⁻³), kΩ to Ω (×10³), μF to F (×10⁻⁶), mV to V (×10⁻³).
Key Fact: Show each step of working on a new line; examiners award marks for correct method even if arithmetic errors occur later.
Key Fact: Check your answer is sensible: resistance should be positive, current should not exceed supply capacity, power should be within ratings.
Key Fact: For multi-step calculations, keep intermediate values to at least 3 significant figures to avoid rounding errors in the final answer.
Key Fact: When a question gives component values in mixed units (e.g. kΩ and mA), convert everything to base units before substituting.
Key Fact: Charge: Q = It; Energy: E = Pt = VIt; always check which formula applies to the quantities given.
Key Fact: For capacitor calculations: Q = CV, E = ½CV², τ = RC — remember the ½ in the energy formula.
Key Fact: Label the circuit diagram with known and unknown quantities before starting calculations.
📋 Key Vocabulary and Concepts
For Circuit Calculation Questions, you must know:
- Rearranging: Changing the subject of a formula; e.g. from V = IR to I = V/R by dividing both sides by R.
- Substitution: Replacing the symbols in a formula with the numerical values given in the question.
- Significant figures: The number of reliable digits in a measurement or calculation; typically give answers to 2 or 3 significant figures in exams.
- Base SI units: The standard units for calculations: volts (V), amperes (A), ohms (Ω), farads (F), seconds (s), watts (W).
- Order of magnitude: A rough check that an answer is in the right range; e.g. a resistor current should be mA, not kA.
- Intermediate value: A result calculated during a multi-step problem that is used in the next step; keep extra significant figures to avoid rounding errors.
❓ Practice Questions
Q: A 12 V supply drives 150 mA through a resistor. Calculate its resistance showing full working.
Q: A 9 V battery powers a 4.5 V, 0.5 W Zener diode regulator. Calculate the series resistor value if the load current is 40 mA and Zener current is 10 mA.
Q: Why should you keep intermediate values to 3 significant figures in a multi-step calculation?
Q: A resistor carries 20 mA and dissipates 0.1 W. Calculate its resistance.
Q: What is wrong with this working: 'I = V/R = 12/10k = 0.0012 A'?
✅ Answers
- R = V/I = 12/0.150 = 80 Ω. (Convert 150 mA to 0.150 A first.)
- R_s = (V_in − V_Z)/(I_Z + I_load) = (9 − 4.5)/(0.01 + 0.04) = 4.5/0.05 = 90 Ω.
- Rounding too early introduces cumulative errors; keeping 3 significant figures ensures the final answer is accurate to at least 2 significant figures.
- P = I²R → R = P/I² = 0.1/(0.02)² = 0.1/0.0004 = 250 Ω.
- The value 10k has not been converted to base units; it should be I = 12/10000 = 0.0012 A. The answer is correct but the method should show R = 10000 Ω explicitly.
🎯 Exam Tips
- Write the formula → substitute values → calculate answer → add units. This pattern secures method marks.
- Always convert to base SI units before substituting: the most common error is mixing kΩ, mA or μF with formulas.
- Label every quantity on your circuit diagram to make your working clear and easy to follow.
- Check answers for sanity: if you calculate a current of 500 A through a small resistor, something is wrong.
- In multi-step problems, state the result of each step clearly before moving to the next.
📝 Exam Technique
GCSE Electronics Exam Tips — Circuit Calculation Questions:
1. For Circuit Calculation Questions questions, use correct electronic symbols and terminology
2. Always show your working in calculations, including units at each step
3. When analysing circuits, state which law or rule you are applying first
4. For evaluation questions on Circuit Calculation Questions, compare component choices and consider cost, reliability and tolerance
5. Draw circuit diagrams neatly with conventional symbols
1. For Circuit Calculation Questions questions, use correct electronic symbols and terminology
2. Always show your working in calculations, including units at each step
3. When analysing circuits, state which law or rule you are applying first
4. For evaluation questions on Circuit Calculation Questions, compare component choices and consider cost, reliability and tolerance
5. Draw circuit diagrams neatly with conventional symbols
⚠️ Common Errors
✗ Substituting values with mixed units (e.g. V in volts but R in kΩ). ✓ Convert all values to base SI units first: 10 kΩ = 10000 Ω, 150 mA = 0.150 A, 47 μF = 47 × 10⁻⁶ F.
✗ Missing the ½ factor in capacitor energy calculations (using E = CV² instead of E = ½CV²). ✓ Always use E = ½CV² for capacitor energy and E = ½LI² for inductor energy; include the ½.
✗ Rearranging V = IR incorrectly (e.g. writing R = IV). ✓ R = V/I: divide voltage by current. Check by confirming the units: V/A = Ω.
✗ Forgetting to include units in the final answer. ✓ Every numerical answer must have the correct SI unit: V, A, Ω, W, J, F, H, s or Hz.
✍️ Model Answer
Full-Mark Response
Describe the method for solving a circuit calculation question in an exam, using an example that involves finding the current through and power dissipated by a resistor.
The systematic method for solving circuit calculations has four steps. First, identify what is known and what is unknown: read the question carefully and list the given quantities with their values and units. Second, convert all values to base SI units: for example, 12 V stays as 12 V, but 10 kΩ becomes 10000 Ω and 5 mA becomes 0.005 A. Third, choose the correct formula and write it down before substituting. To find current, use I = V/R; to find power, use P = VI (or P = I²R if I and R are known). Fourth, substitute the values, calculate and state the answer with correct units. Example: A 12 V supply is connected across a 4 kΩ resistor. Current: I = V/R = 12/4000 = 0.003 A = 3 mA. Power: P = VI = 12 × 0.003 = 0.036 W = 36 mW. This method ensures method marks are earned even if a later arithmetic error occurs, because each step is clearly shown.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of circuit calculation questions, including electronic components, circuit theory and systems concepts relevant to WJEC Eduqas C690QS.
AO2 (Application): Apply knowledge and understanding of circuit calculation questions to analyse, design and construct electronic circuits and systems.
AO3 (Evaluation): Evaluate electronic circuits and systems, making reasoned judgements about design choices, performance and practical considerations, constructing supported arguments.
📝 Exam Questions by Topic
🎬 Video Resources
Ready to ace your GCSE Electronics exams?
Get the best revision books and guides to boost your grades.