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Recently I took out some soup to eat and got a call from a friend. I covered it with a plate and when i came back later, it was still hot. While i was eating, i wondered how does covering this hot soup prevent heat losses. Well i thought of these three ideas but i'm not quite sure which are correct. Basically conduction, convection and evaporation. Radiation and boiling is out of the question for sure.

So my question is simply: Covering a bowl of hot soup with a plate reduces heat losses by which means? a) Conduction b) Convection c) Evaporation

I would want to believe it'd be more than one. Thanks.

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all of them, + radiation. I guess convection is the most efficient. – Fabrice NEYRET May 23 at 15:49
up vote 2 down vote

It would be more scientific to have compared a covered with an identical non-covered bowl, with temperature measurements over an identical period of cooling time.

But covering a bowl of hot soup with a plate does indeed reduce heat losses and slows down cooling of the soup, because:

  1. Radiative losses are usually small at these temperatures but the plate further reduces them by partial reflection of the radiation. White plates will do that better than dark coloured ones in that respect.

  2. Convective losses too will be reduced because convection relies on air movement which becomes somewhat restricted by the plate.

  3. Evaporative losses become small once the head space between soup and plate has become saturated with water vapour.

Only convective/conductive losses from the body of the bowl will not be affected by the plate cover.

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White and black does not matter for infrared wavelengths in the 5-15 micrometer wavelength range. Smooth metal surfaces do typically reflect infrared radiation, but not if they are covered with water droplets from condensation. A nonmetallic plate would typically lower radiative heat losses nevertheless, because the processes of absorption and re-emission act as a thermal resistance. – Han-Kwang Nienhuys May 23 at 16:55

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