IGCSE Physics 0625: Why a Vacuum Flask Design Reduces Conduction, Convection and Radiation

A camping equipment company designs a new double-walled steel travel mug. It has an inner steel wall and an outer steel wall, with a narrow gap between them. Almost all of the air is pumped out of this gap, leaving a partial vacuum, and both surfaces facing the gap are polished to a shiny, mirror-like finish. A tight-fitting lid closes the top of the mug.

(a) Explain why almost no thermal energy can cross the gap between the two walls by either conduction or convection. [3]

(b) Explain how polishing the surfaces facing the gap to a shiny finish, instead of leaving them dull, further reduces the thermal energy crossing the gap. [2]

(c) The company advertises that this single mug design "keeps hot drinks hot and keeps cold drinks cold." Explain why one design can reduce thermal energy transfer in both of these situations. [2]

Part (a): Conduction and convection across the gap

Conduction relies on neighbouring particles colliding and passing on their vibrational energy (it needs particles that are close enough together to touch. Convection relies on a fluid physically circulating, carrying warmer material to cooler regions and back again) it needs a liquid or gas that is free to flow.

Pumping almost all of the air out of the gap leaves it almost empty of particles:

With (almost) no particles present, there is nothing for conduction to pass vibrational energy between, so conduction across the gap is almost eliminated.
With (almost) no gas left to expand, become less dense, and circulate, no convection current can be set up across the gap either.

Part (b): Why the surfaces are polished shiny

A vacuum does not stop radiation, since infrared waves can travel through empty space perfectly well. To reduce radiation, the mug relies on the surface finish instead:

Shiny, polished surfaces are poor emitters of infrared radiation, so far less radiation leaves the hot inner wall than would leave a dull surface at the same temperature.
Shiny surfaces are also poor absorbers. Most radiation that reaches them is reflected rather than absorbed. So even the small amount of radiation that does cross the gap is largely reflected back rather than being absorbed by the outer wall.

Together, these two effects greatly cut the thermal energy crossing the gap by radiation.

Part (c): Why the same design works both ways

The direction in which thermal energy naturally flows always depends on which side is hotter, from hot to cold. What the mug’s design actually does is make all three transfer mechanisms slow, regardless of that direction:

The vacuum gap removes the particles/fluid needed for conduction and convection, whichever side happens to be warmer.
The shiny surfaces are equally poor emitters and poor absorbers on both walls, so radiation is reduced whichever wall is radiating and whichever wall would otherwise absorb it.

So when the drink is hot, energy escaping outward to the cooler surroundings is slowed down, keeping the drink hot for longer. When the drink is cold, energy entering inward from the warmer surroundings is slowed down by exactly the same mechanisms, keeping the drink cold for longer.

Final answers

(a) The vacuum leaves almost no particles in the gap, so conduction (needs particle contact) and convection (needs a circulating fluid) are both almost eliminated.
(b) Shiny surfaces are poor emitters and poor absorbers of infrared radiation, so they cut both the radiation leaving the hot wall and the radiation absorbed by the other wall.
(c) All three mechanisms are reduced regardless of which side is hotter, so the design slows energy leaving a hot drink and slows energy entering a cold drink equally well.