AQA GCSE Physics Physics Paper 1 (Foundation), November 2020: Question 9

✅ Correct Answer

The first (leftmost) diagram showing:

• Three cells connected in series facing the same way (positive to negative terminals aligned).
• A closed/open switch in series.
• An LED pointing in the forward-biasing direction (triangle pointing towards the cathode/right).

💡 Key Knowledge

• Cells in series: Must face the same direction so their potential differences add up (long line is positive, short thick line is negative).
• LED Symbol: A triangle inside a circle with two small arrows pointing away. Current flows in the direction of the triangle point.

✅ Correct Answer

charge flow = current × time

Or using standard symbols: Q = I × t

🧠 Exam Technique

Always write formulas out in full words or standard accepted symbols. Avoid using ambiguous letters. Word equations are fully accepted by AQA.

📐 Step-by-Step Calculation

Step 1: Convert current into standard units (Amps)

I = 50 mA = 50 ÷ 1000 = 0.050 A

Step 2: State the equation and substitute values

Q = I × t

Q = 0.050 × 14 400

Step 3: Calculate final answer with units

Q = 720 C

❌ Common Errors & Traps

• Unit Blindness: Forgetting to convert milli-amps (mA) to amps (A) by dividing by 1000. (Though error-carried-forward applies if converted incorrectly, correct final numbers require this step).
• Missing the unit C (Coulombs) on the answer line.

✅ Correct Answer

Any two of the following points:

• There is no current in the diode because it is connected in the reverse direction.
• Charge will not flow through a diode in the reverse direction.
• A diode has a very high resistance in the reverse direction.

❌ Common Pitfalls

Examiners note that students frequently write "the circuit is not complete" . This is incorrect—the physical loop of wire is still closed, but the electrical properties of the diode block current flow.

✅ Correct Answer

Efficiency = Useful power output / Total power input

💡 Key Knowledge

Efficiency can be expressed as a ratio between 0 and 1, or as a percentage (though formula triangles and standard equations expect the decimal ratio format unless multiplied by 100).

📐 Step-by-Step Calculation

Step 1: Set up the rearranged equation

Useful power output = Efficiency × Total power input

Step 2: Substitute known values

Useful power output = 0.75 × 0.24

Step 3: Evaluate and state units

Useful power output = 0.18 W

🧠 Exam Technique

Always show your working clearly even for simple rearrangement steps. If you make a calculator slip, intermediate substitution marks can still save your grade.