AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2024: Question 3

✅ Correct answer

• Voltmeter connected across the LED in parallel.
• Ammeter connected in series with the LED.
• Use the correct circuit symbols: V in a circle and A in a circle.

💡 Key knowledge

• An ammeter measures current, so it must be placed where the current flows through it.
• A voltmeter measures potential difference, so it must be placed across the component.
• For an LED circuit, the meter positions are usually shown with the ammeter in the main loop and the voltmeter across the LED.

🧠 Exam technique

• Marks are for both placement and symbol.
• Draw the voltmeter branching off the LED, not in the main circuit path.
• Draw the ammeter in the circuit line so the current passes through it.

❌ Common errors

• Putting the voltmeter in series loses the mark.
• Putting the ammeter across the LED is wrong.
• Using the wrong symbol shape or forgetting the letter inside the circle can cost the symbol mark.

✅ Correct answer

• Change the number of cells in the battery to change the potential difference.
• Reverse the connections to the LED / battery to make the potential difference negative.

💡 Key knowledge

• Potential difference is changed by altering the power supply.
• A variable resistor can also be adjusted in some setups, but here the key idea is changing the supply and reversing the polarity.
• To go from negative to positive p.d., the polarity must be reversed.

🧠 Exam technique

• Use the idea of polarity: positive and negative are not just “bigger” and “smaller”.
• If asked for a method, give a practical change to the circuit, not just “turn it up”.

❌ Common errors

• Saying “increase the current” is not the same as changing the potential difference.
• Vague answers like “adjust it” without saying what is adjusted often do not score.
• Forgetting that negative p.d. needs the connections reversed is a common mistake.

✅ Correct answer

• Plot the remaining correct points from the table:
• (2.0, 5)
• (2.2, 19)
• (2.4, 41)
• (2.6, 69)
• Draw a smooth line of best fit through the points.

📐 Calculations / graph plotting steps

1. Read the x-axis value: potential difference in volts.
2. Read the y-axis value: current in milliamps.
3. Plot each point accurately using the correct coordinates.
4. Use a neat, thin cross or small dot.
5. Join with a line of best fit, not dot-to-dot.

🧠 Exam technique

• The mark scheme allowed a tolerance of ± ½ small square for each point.
• One mark could still be given for 3 correct points, but 4 correct points is the full answer for plotting.
• The line should be smooth, especially because the LED I-V relationship is non-linear.
• Ignore the zero-current region before about 1.8 V when drawing the best-fit curve through the rising data.

❌ Common errors

• Using the wrong axis: voltage goes on the x-axis, current on the y-axis.
• Plotting the points with swapped coordinates.
• Joining points with straight line segments instead of a smooth curve.
• Forgetting units: current is in mA, not A.

✅ Correct answer

• The current is zero or the current stops.
• This is because the LED / diode has a very high resistance in the reverse direction.
• Accept: the LED only allows current to flow in one direction.

💡 Key knowledge

• An LED is a diode.
• Diodes conduct in forward bias but not in reverse bias.
• In reverse bias, the resistance is very high, so the current is effectively zero.

🧠 Exam technique

• Give both parts: what happens to the current and why.
• A good answer links the idea of reverse direction to high resistance.

❌ Common errors

• Saying “the current becomes negative” is not correct for this question.
• Saying “the LED uses less voltage” is not an explanation of reverse bias.
• Some students forgot that a diode blocks current in one direction only.

✅ Correct answer

The investigation is reproducible.

💡 Key knowledge

• Reproducible means a different person or method still gets similar results.
• Repeatable means the same person uses the same method and gets similar results.
• If two different students get the same pattern using red and blue LEDs, that supports reproducibility.

🧠 Exam technique

• Choose the investigation quality word carefully.
• This is not about accuracy because we are not comparing results with a true value.

❌ Common errors

• Repeatable is wrong here because it refers to the same person / same method.
• Accurate is not the correct conclusion from this information alone.

✅ Correct answer

Resistor at constant temperature

💡 Key knowledge

• A resistor at constant temperature obeys Ohm’s law.
• That means current is directly proportional to potential difference.
• The graph is a straight line through the origin.

🧠 Exam technique

• “Always linear” is the clue: choose the component that behaves ohmically when temperature is constant.
• Other components such as an LDR, thermistor, and filament lamp have non-linear relationships.

❌ Common errors

• Filament lamp: resistance increases as it heats up, so not linear.
• LDR: resistance changes with light intensity, not a linear I-V device.
• Thermistor: resistance changes with temperature, so the I-V graph is not always linear.

💡 What to remember

• Ammeter in series.
• Voltmeter in parallel across the component.
• To get negative p.d., reverse the connections.
• LED current is zero in reverse bias.
• Reproducible = different student/method, same pattern.
• Resistor at constant temperature gives a linear I-V graph.

🧠 Top-mark strategy

• Use precise physics words: series, parallel, reverse bias, high resistance.
• When asked to explain, always include a because.
• For graph questions, check units, axis labels, and point accuracy.