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IGCSE · 2 · structured

Thermal physics · Question 9

Thermal physics · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand
demanding
Marks
7
Topics
1
Answer
Complete
Cut-away view of a vacuum flask holding a hot drinkhot drinkinsulating stoppervacuuminsulating supportssilvered surfacesouter case
Fig. 9.1A cut-away view of a vacuum flask. Inside an outer case stands a double-walled glass container, sealed at the neck, with a narrow gap between the two walls that runs down both sides and under the base; the gap is labelled vacuum and the two facing glass surfaces are labelled silvered. Hot drink fills most of the inner container, an insulating stopper closes the neck at the top, and the glass container rests on small insulating supports standing on the floor of the outer case.
structured7 marks

A vacuum flask keeps a hot drink warm. It has a double-walled glass container with a vacuum between the walls, both facing surfaces of the glass are silvered, the flask has an insulating plastic stopper, and the glass container rests on soft insulating supports inside an outer case.

  1. (a)

    Explain Explain how the vacuum between the two walls reduces the transfer of thermal energy.

    2 marks
  2. (b)

    Explain Explain why both facing surfaces of the glass are silvered.

    2 marks
  3. (c)

    Explain Explain, in terms of the particles in a metal, why a metal spoon left in the drink becomes hot along its whole length far more quickly than a plastic one.

    3 marks
Ready to self-mark?Reveal the detailed answer guide
Answer overviewKey answer: there are (almost) no molecules between the walls so energy cannot be transferred by conduction or convection, both of which need a medium a shiny silvered surface is a poor emitter of infrared radiation, so the inner wall radiates less energy away
01

(a)

2 marks

Explain Explain how the vacuum between the two walls reduces the transfer of thermal energy.

How to approach it

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. 1

    there are (almost) no molecules between the walls

  2. 2

    so energy cannot be transferred by conduction or convection, both of which need a medium

02

(b)

2 marks

Explain Explain why both facing surfaces of the glass are silvered.

How to approach it

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. 1

    a shiny silvered surface is a poor emitter of infrared radiation, so the inner wall radiates less energy away

  2. 2

    and a shiny surface is a good reflector, so radiation crossing the gap is reflected back rather than absorbed

03

(c)

3 marks

Explain Explain, in terms of the particles in a metal, why a metal spoon left in the drink becomes hot along its whole length far more quickly than a plastic one.

How to approach it

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. 1

    in both, particles at the hot end vibrate more and pass energy on by colliding with neighbouring particles

  2. 2

    a metal also contains free (delocalised) electrons

  3. 3

    these gain kinetic energy at the hot end, move rapidly through the metal, and transfer energy by collision far along the spoon, so conduction is much faster

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