AQA GCSE Physics Physics Paper 1 (Higher), June 2025: Question 8

📐 Step-by-Step Calculation

1. Convert mass to standard SI units (kg):
   m = 50 g ÷ 1000 = 0.050 kg [1 mark]
2. Select equation & substitute values:
   Formula: E = m × L
   9950 = 0.050 × L [1 mark]
3. Rearrange to solve for L:
   L = 9950 ÷ 0.050 [1 mark]
4. Calculate final value:
   L = 199 000 J/kg (or 1.99 × 10⁵ J/kg ) [1 mark]

❌ Common Traps to Avoid

• Forgetting mass conversion: Using 50 instead of 0.050 gives 199 J/kg . This loses the first mark (though subsequent substitution and answer marks can still be awarded via error carried forward).
• Wrong latent heat type: Don't confuse latent heat of fusion (melting/freezing) with vaporisation (boiling/condensing).
• Rearrangement slip: Make sure you divide energy by mass ( E / m ), never mass by energy ( m / E ).

✅ Model Answer

• The potential energy of the particles increases. [1 mark]
• The kinetic energy of the particles does not change (because the temperature stays constant during boiling). [1 mark]
• Therefore, the total internal energy increases. [1 mark]

💡 Key Knowledge

Internal Energy is defined as:

Internal Energy = Total Kinetic Energy + Total Potential Energy

• Temperature depends solely on the average kinetic energy of particles.
• During a phase change, thermal energy supplied goes into overcoming intermolecular forces, increasing potential energy, not kinetic energy.

🧠 Exam Technique

Always break "internal energy" down into its two components: kinetic energy and potential energy. Address both separately, then state what happens to the overall internal energy.

❌ Examiner Warning

Many students incorrectly state that "kinetic energy increases because gas particles move faster than liquid particles". At boiling point, the gas and liquid are at the exact same temperature, meaning their average kinetic energy is identical! Note: Mark 3 depends entirely on getting marks 1 and 2 right.

✅ 4-Step Explanation (Model Answer)

1. The mean kinetic energy (or mean speed) of the gas particles increases. [1 mark]
2. Particles collide with the container walls with a greater force per collision. [1 mark]
3. The frequency of collisions between particles and the walls increases (more collisions per second). [1 mark]
4. So the overall pressure increases. [1 mark]

🧠 The Pressure Chain Formula

To score full marks on gas pressure questions, you must hit all four links in the logical chain:

❌ Common Traps & Lost Marks

• Vague collision language: Writing "there are more collisions" gets 0 marks. You must specify that collisions are with the walls of the container and mention the rate or frequency (collisions per second).
• Forgetting force: Students frequently mention collision frequency but forget to mention that particles hit with greater force / momentum change.
• Dependency condition: The final mark for "pressure increases" is dependent on scoring at least 1 of the explanation marks first.

💡 Underlying Physics

Pressure is defined as force per unit area ( P = F / A ). The total force exerted on the sealed walls increases because faster particles experience a larger change in momentum upon impact (greater force) and they reach the walls more often (higher collision frequency).