[Brian Holden]

Brian Holden  asked the Naked Scientists:
   
Dear Chris,

Thank you for your part in an excellent site.

Does it take more total energy to heat water quickly in say, an electric kettle, than to achieve the same on a gas stove?

I can see that there would be efficiency questions and production of the energy used, that would effect the out comes in the 'normal world'; but down at a theoretical/all other things being equal, place, is there some truth to my 'intuition' that heating quickly must use more energy?
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Regards Brian

What do you think?

[Geezer]

Hi Brian,

Good question! Assuming no heat escapes (which in the case of a kettle is obviously not the case), in general, it makes no difference whether you heat something slowly or quickly. All substances have a certain heat capacity, so it takes a certain amount of heat energy to raise their temperature by a certain amount, regardless of the time taken to raise the temperature.

Of course, if some of the heat escapes and heats the surroundings, then it might seem as if it takes a different amount of time.

[SeanB]

No, just that heating quickly requires a higher energy input rate, you still ( assuming there are no losses or that they are very small compared to the input energy) have to supply a certain number of Joules of energy. If you apply for example 100000 joules (100 kilojoules, which is not the same as the joule in food BTW) in a period of 100 seconds, you have supplied energy at a rate of 1kW for 100 seconds, which would boil enough water to make a cup of tea very hot. If you apply at a rate of 0.5 kW, then the water will take 200 seconds to reach the same temperature. Conversely, if the rate is 5kW, the water will be hot in 20 seconds. This corresponds roughly with a kettle ( 1kW element), a microwave oven ( being around 500W applied to the cavity for a small one) or a gas or electric instant boiler respectively.

[syhprum]

Over a reasonable range of speeds it makes little difference but if you go to extremes various losses arise.
For instance if you tried to heat your kettle inputting 1 watt into the element thermal losses due to inadequate insulation would be such that if you left it on for days you would input a fair amount of energy but it would never boil.
Conversely if you inputted say 50 Kw there would be much energy wasted producing steam and although it would boil quickly the actual energy consumed would be greater than if you inputted the normal 3 Kw.

[techmind]

Brian Holden  asked the Naked Scientists:
...Does it take more total energy to heat water quickly in say, an electric kettle, than to achieve the same on a gas stove?
...

Heating quickly requires a higher power (energy per unit time) but for less time. Assuming the thing you're heating (kettle) is perfectly insulated, then it takes the same total amount of energy whether you heat it quickly or slowly.
In the real world your kettle won't be perfectly insulated, so it takes less energy to heat it quickly as it spends less time losing energy when it's hot-but-not-yet-boiling.

Note that if you're considering this from an "environmental" perspective then you should also be aware that although the electric kettle is almost 100% efficient (converting electrical energy into heated water), there are lots of inefficiencies in generating the electricity in the first place (energy-for-energy delivered to the consumer, very roughly electricity costs 3-4 times as much as gas, and creates 3-4 times as much CO2).

For your pan on the stove, be sure to use a lid - otherwise you'll lose even more energy through evaporation. The pan/kettle on the (gas) stove is likely to couple less of the energy into the pan/kettle and heat the kitchen more - than an electric kettle. If this heating is useful, then it's no great loss... but if you're pumping it away with air-con then it'd be far better not to create the excess heat in the first place!

[Soul Surfer]

Although the energy input to the material is the same the important difference is the efficiency of the heating process.  Using an electric immersion element in water like a kettle or an instant electric shower is very efficient the only heat lost is what goes into warming the container.  Induction hobs are almost as efficient  however with an electric hotplate or gas cooker transfer efficiencies can be much lower firstly because there is more material to heat and secondly because hot gas can escape round the sides of the container.  This is particularly true if gas flames are very high and lap round the edge of the container.  There used to be highly efficient gas kettles that had a wide bottom covered with a complicated heat sink fins or metal spirals these could be much more efficient and boil extremely quickly.