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How to boil water faster
soldar:
The question was asked in another forum but there were a lot of just plain ignorant and stupid answers and the topic died but I think it might be a good thought experiment. The question is
Is it faster to boil 10 L of water all at once (in a 10L pot) or 10 L of water in 1 L pots successively?
That was the question but, as always, the devil is in the details. If you boil 10 L of water all at once then all the water is boiling at the end whereas if you boil it in small containers then all containers were brought to a boil but at the end only one is still boiling. So let us stipulate at the start all are at room temperature and the experiment is completed when all the water was brought to a boil, even if not simultaneously.
I think bringing the water to a boil simultaneously in several pots would only be feasible with a source of heat for each pot.
I suppose we may need to specify other things like how it is heated (electric, gas, etc) but for now we can just specify the heating power is the same in both cases.
What say you?
Andy Chee:
This sounds like a surface area/volume problem.
I'm pretty sure the 1L pots will boil quicker, even if all the pots are on the single heat source.
It's almost identical to the ice cube melting problem, which will melt quicker, a massive 10kg block of ice, or 10 x 1kg blocks of ice.
tom66:
I would suggest that a submerged element vs a plate element at the bottom of a pot would vary the results significantly, as would stirring the fluid, but my intuition is that assuming a bottom-heated plate (i.e. a normal kettle) it is quicker to boil 10 pots than 1 larger pot. However, intuition is often wrong!
AndyC_772:
I'll make a case for the opposite, that the 10l container boils first.
Throughout the heating process, the heat (from whatever source) is going into a larger vessel, one with a higher ratio of volume to surface area. This means that relatively less heat is being lost from the walls of the pot, and more of it is retained within the water. If we assume the pots have similar proportions, then the larger base will also receive a greater proportion of the heat from the source, ie. less goes straight from the source to the environment, bypassing the pot entirely.
It's the opposite of the ice cube problem. Melting faster means maximising the rate of heat transfer between the ice and the air. Here we're looking to minimise that heat transfer.
Ian.M:
Basic thermodynamics. The energy required to heat 1 kg of a substance by 1 K (degree Kelvin) is known as its specific heat capacity. It can (and does) vary with respect to temperature but it wouldn't be a useful concept if the energy required wasn't directly proportional to the mass.
Therefore, with the same heating power, in a perfectly thermally insulated vessel of negligible thermal mass, 1L of water will boil exactly ten times faster than 10L of water.
The only differences are the losses, and as the surface area of a container scales with the square of its linear dimensions and the volume with the cube, it is likely that the larger container will loose less heat boiling 10L than a smaller container will loose boiling 1L ten times.
The case when the container is not of negligible thermal mass is more interesting. In real life no-one waits for the kettle to fully cool to room temperature before refilling it to boil it again, so the first time takes longer as the kettle starts from cold, but all subsequent refills boil a little quicker due to stored energy in the kettle.
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