IGCSE · 2 · structured
Thermal physics · Question 7
Thermal physics · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- discriminating
- Marks
- 8
- Topics
- 1
- Answer
- Complete
Smoke particles are placed in a small glass cell containing air and viewed through a microscope with a bright light shining from the side. The smoke particles are seen as tiny bright specks.
- (a)
Describe Describe what is seen when the smoke particles are observed through the microscope.
2 marks - (b)
Explain Explain this motion in terms of the molecules of the air.
3 marks - (c)
Explain The air in a sealed metal container is heated. The volume of the container does not change. Explain, in terms of molecules, why the pressure of the air increases.
3 marks
Ready to self-mark?Reveal the detailed answer guide
(a)
Describe Describe what is seen when the smoke particles are observed through the microscope.
Answer the command word directly and use precise physical vocabulary. Include only the distinct features or facts that earn marks, without burying them in unrelated background information.
- 1
the bright specks move continuously
- 2
in random directions / on jerky, unpredictable paths
(b)
Explain Explain this motion in terms of the molecules of the air.
State the outcome first, then link cause to effect with the relevant physical principle. Each link in the reasoning should be explicit enough to earn its own marking point.
- 1
the air molecules are in constant random motion and collide with the smoke particle
- 2
the molecules are much smaller than the smoke particle but move much faster, so each collision transfers a noticeable momentum
- 3
at any instant the collisions on one side outnumber those on the other, so the resultant force on the particle changes direction randomly
(c)
Explain The air in a sealed metal container is heated. The volume of the container does not change. Explain, in terms of molecules, why the pressure of the air increases.
State the outcome first, then link cause to effect with the relevant physical principle. Each link in the reasoning should be explicit enough to earn its own marking point.
- 1
heating increases the average kinetic energy, so the molecules move faster on average
- 2
so they strike the walls more often
- 3
and each collision produces a greater change of momentum, so the average force on unit area of the wall increases
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