AQA GCSE Physics Physics Paper 1 (Higher), November 2021: Question 10

✅ Model Answer (Full 3 Marks)

• Mark 1: Very high potential difference results in a very low current (for a given power transmission).
• Mark 2: A lower current means less thermal energy is transferred to the surroundings (less power lost as heat due to wire resistance).
• Mark 3: This increases the efficiency of power transmission.

💡 Key Physics Principles

Remember the two core power equations:

• P = V × I → If transmission power is constant, increasing V forces I to be much lower.
• P = I² × R → Cable heating loss depends on current squared. Halving current cuts heating loss by a factor of 4!

🧠 Exam Technique: Link the Chain

Always write this 3-step causal chain in order:

High p.d. → Low current → Less heat loss / Higher efficiency.

Missing out "low current" is the most common reason students fail to secure all 3 marks.

❌ Common Errors to Avoid

• Stating "electricity travels faster at high voltage" (it does not!).
• Saying "so homes receive more volts" (step-down transformers reduce voltage to 230 V before it enters homes).
• Vague statements like "less electricity is lost" instead of less thermal energy is transferred.

✅ Model Answer (Full 3 Marks)

• Mark 1: The electric field strength between the cable and kite/earth is very high (due to the large potential difference).
• Mark 2: This high electric field causes the air to become ionised (meaning the air breaks down and conducts electricity).
• Mark 3: Charge is then conducted through the kite/string to the person and into the earth.

💡 Why Does Air Break Down?

Air is normally an electrical insulator. However, when the potential difference across a small gap is extremely high, the electric field exerts a large electrostatic force on air molecules, stripping away electrons. This is ionisation, which turns the air into a conductor, creating a spark or arc.

🧠 Top-Level Terminology

Use proper scientific vocabulary:

• Use the term "electric field strength" rather than just "high voltage".
• Use "ionisation" or "air breaks down" to describe how air allows charge to flow.

❌ Examiner Warning

Do NOT mention the kite touching the cable!

The prompt explicitly says: "very close to, but not touching". Any answer assuming direct contact scores 0 marks for that point.

✅ How to Draw the Line

• Feature 1: Draw a perfectly straight line passing through the origin (0, 0) using a ruler.
• Feature 2: The line must be below the existing line at all points across the entire grid (lower gradient).

📐 Visualising the Graph

Original Line (Dry Air): Starts at (0, 0) and goes up to (0.20 m, 600 000 V).

Your New Line (Humid Air):

• Starts at (0, 0).
• Has a gentler slope (e.g. passing through 0.20 m at roughly 300 000 V to 400 000 V).
• Remains strictly below the original line everywhere.

🧠 Scientific Reasoning

The prompt states: "When the humidity of air increases the air becomes a better conductor."

If air conducts better, it requires less potential difference to cause charge to flow across any given distance. Therefore, for every distance value on the x-axis, the required p.d. on the y-axis is lower.

❌ Common Errors

• Drawing a curved line (the relationship remains directly proportional, so it must be a straight line).
• Starting the line above zero on the y-axis instead of through (0, 0).
• Drawing the line above the original line (confusing "better conductor" with "needs more voltage").

✅ Model Answer (Full 2 Marks)

• Mark 1: The potential difference across the steel wire and the aluminium wire is the same (because they are connected in parallel).
• Mark 2: The resistance of the steel wire is greater, so there is less current in the steel wire (or: aluminium has lower resistance, so it has more current).

💡 Parallel Circuit Rules

The question states: "The individual wires behave as if they are resistors connected in parallel."

• In parallel branches: V₁ = V₂ = V_total
• Ohm's Law: I = V / R
• Since V is identical, current I is inversely proportional to resistance R .
• Because aluminium is a "better conductor", it has lower resistance, hence carrying higher current than the steel wire.

🧠 Exam Technique: Two Components, Two Marks

Questions comparing currents in parallel always require you to state:

1. What happens to potential difference (it stays the same).
2. How the resistances differ and how that changes the current.

❌ Common Misconceptions

• Thinking the wires share voltage (voltage is shared in series, not parallel).
• Stating current is the same in both (current only stays the same in series components).
• Forgetting to state that the current in steel is lower (or in aluminium is higher).