K323 / 2027
Thermal processes overview

Lesson 4 of 4

Controlling thermal transfer

  • I can apply conduction, convection and radiation to everyday situations and designs.Syllabus

    Syllabus K323, 8(e). Apply conduction, convection and radiation to everyday situations and designs.

Make a guess. You are not marked.

A vacuum flask keeps a drink hot. Which part mainly stops energy transfer by conduction and convection?

  1. The silvered surfaces, which reflect energy back towards the drink.
  2. The plastic cap, which stops hot air escaping from the top.
  3. The vacuum between the walls, which has no particles to pass energy on.
Show the answer

The vacuum between the walls, which has no particles to pass energy on.

Conduction and convection both need particles. A vacuum has almost none, so it blocks both. The silvered walls deal with radiation, and the cap with convection at the top.

Explain a thermal design by tracing each transfer route and identifying which feature changes it.

Conduction transfers energy through particle interactions; convection carries it with fluid; radiation transfers it by electromagnetic waves. A temperature difference sets the direction of net heating. A real design can involve all three processes.

Why a vacuum flask has several features

A vacuum flask has two walls separated by an evacuated space, reflective surfaces facing that gap, and a stopper at its opening. Each feature addresses a different route.

A flask reduces several routes of energy transfer

This schematic cutaway contains a warm drink in cooler surroundings. Match each numbered feature to its role below.

A vacuum flask with its useful features and remaining conducting routes labelledA warm drink is held in an inner vessel surrounded by an outer wall. Label one points to the evacuated gap between the walls. Label two points to reflective surfaces facing the gap. Label three points to an insulating stopper closing the opening. Label four points to the neck, where solid material joins the inner and outer parts. Label five points to small solid supports beneath the inner vessel. The vacuum reduces conduction and prevents convection across the gap, reflective surfaces reduce radiation, and the stopper reduces transfer through the opening. The neck and supports still provide conducting routes. The drink's headspace is inside the inner vessel and is separate from the vacuum gap.Cooler surroundingsWarmdrink12345Schematic cross-section
  1. Vacuum gap: reduces conduction and prevents convection across the gap.
  2. Reflective faces: reduce radiative transfer between the walls.
  3. Insulating stopper: reduces conduction and movement of fluid through the opening.
  4. Neck: the solid connection still conducts some energy.
  5. Small supports: hold the vessel in place but still provide conducting paths.

The same features also slow energy entering a cold drink from warmer surroundings.

The evacuated gap, reflective surfaces and insulating stopper reduce different transfers. Solid connections at the neck and supports remain possible conduction routes, so the flask does not provide perfect isolation.
Evacuated gap
Removing most gas particles greatly reduces conduction through the gap and prevents an ordinary fluid convection current there. Radiation can still cross the gap because it does not require particles.
Reflective surfaces facing the gap
Shiny surfaces are poor absorbers and emitters in the usual thermal-radiation comparison. They reduce radiative transfer across the gap that the vacuum alone cannot prevent.
Insulating stopper
A low-conductivity stopper slows conduction at the opening. Closing the opening also limits exchange of fluid between the contents and the surroundings.
Neck and supports
Solid parts connecting the inner and outer regions still conduct some energy. The stopper is not a perfect insulator, and the reflective surfaces do not remove all radiation.

Apply the same design twice

A hot drink and a cold drink

  • A drink at 80°C in a 20°C room: net energy transfers out of the drink. The flask slows its cooling.
  • A drink at 5°C in a 25°C room: net energy transfers into the drink. The same barriers slow its warming.

The second case does not require cold to flow out. Reverse the temperature difference and the net transfer direction reverses. The features still reduce the available transfer routes.

Sometimes the aim is faster transfer

A cooling device may use a conducting metal base and fins. Conduction spreads energy from the hotter component into the metal; the fins provide a larger exposed area for transfer to the surroundings.

Air moving past the fins carries energy away. A fan can strengthen this fluid movement when natural circulation is insufficient. Radiation also contributes, with its rate affected by the surface finish, temperature and area. Calling every feature an "insulator" would miss the purpose of this design.

Compare designs under the same conditions

To investigate the effect of a cup's insulating wrap, change that wrap while keeping the cup, amount of liquid, lid, initial temperature, surroundings and measurement method the same. Record temperature against elapsed time using the same sensor position and intervals.

If one cup starts at 80°C and another at 60°C in the same room, they have different driving temperature differences. Their final readings alone cannot isolate the effect of the wrap. Resetting the starting conditions addresses that problem; repeating the unequal comparison does not.

Identify where the remaining energy can go: through the cup and lid, into moving surrounding air, and by radiation. An improvement should reduce a named route, rather than claim that an extra layer must stop every process.

Match the feature to the mechanism. A vacuum suppresses routes across the gap that need a material medium; a shiny surface reduces radiative transfer; a low-conductivity solid slows conduction. None is a complete explanation for the whole flask by itself.

Check your understanding

Air enters the space that was evacuated between the walls of a vacuum flask. The reflective finishes remain unchanged. What new explanation accounts for faster transfer across this gap?

  1. Radiation can now cross the gap for the first time, because air carries the infrared.
  2. The air stops conduction but makes the walls more reflective.
  3. The air now provides a conduction pathway across the gap and may allow convection within it.
Show the answer

The air now provides a conduction pathway across the gap and may allow convection within it.

The gas provides a material pathway for conduction and, if the geometry allows circulation, convection. The vacuum had suppressed those routes; radiation and conduction through solid connections were already possible.

Practise an exam question

Write your answer first, then tick the mark points it makes.

Exam-style question

A vacuum flask keeps hot drinks hot. Explain how each of these features reduces energy transfer: the vacuum between the double walls, the silvered surfaces of the walls, and the stopper. [3]

Saved on this browser only. Anyone using this browser can see it.