The energy used to boil water.
Discussion
I'm having a lively discussion with the wife.
If I'm cooking rice, I'll boil the water in the kettle first, just to save time. The boss just sticks it in a pan and brings it to the boil.
I'm convinced that the same amount of energy is used to bring a set amount of water to the boil, whether it's on a gas hob, electric hob, kettle or microwave.
She thinks I'm talking tosh, but I seem to remember from school that I'm not!
Who's right?
If I'm cooking rice, I'll boil the water in the kettle first, just to save time. The boss just sticks it in a pan and brings it to the boil.
I'm convinced that the same amount of energy is used to bring a set amount of water to the boil, whether it's on a gas hob, electric hob, kettle or microwave.
She thinks I'm talking tosh, but I seem to remember from school that I'm not!
Who's right?
TypeR said:
I'm having a lively discussion with the wife.
If I'm cooking rice, I'll boil the water in the kettle first, just to save time. The boss just sticks it in a pan and brings it to the boil.
I'm convinced that the same amount of energy is used to bring a set amount of water to the boil, whether it's on a gas hob, electric hob, kettle or microwave.
She thinks I'm talking tosh, but I seem to remember from school that I'm not!
Who's right?
If I remember my O level physics (!) If I'm cooking rice, I'll boil the water in the kettle first, just to save time. The boss just sticks it in a pan and brings it to the boil.
I'm convinced that the same amount of energy is used to bring a set amount of water to the boil, whether it's on a gas hob, electric hob, kettle or microwave.
She thinks I'm talking tosh, but I seem to remember from school that I'm not!
Who's right?
You are in terms of the absolute amount of energy needed to raise the temperature of a specific volume of water by a specific amount
However, the actual amount of energy used will differ depending on the efficiency of what you are using.
For example I'd think an electric kettle is more efficient (ie. less waste heat) than say a gas hob. If its on the hob the difference in waste heat between having a lid on and a lid off the pan is also pretty large.
Then again, electricity costs roughly 5 times as much as gas so if you are looking for a cost comparison then you'd have to bring that into it too. So although its wasteful in terms of heat loss, a gas hob might still be the cheapest way to do it.
The energy required to raise the temperature of the water is always the same no matter the source of the energy. This is a basic unalterable fact of physics.
The electric kettle is likely to be very efficient at putting energy into the water, the gas hob less so.
However, the energy required to boil the water, and keep the water boiling dwarfs the energy required to raise the temperature of the water. So, the most efficient use of energy is to have the water a touch under boiling.
If you raised the atmospheric pressure, the boiling point could be raised a bit (or even a good bit), so the food would cook quicker, and with less likelihood of the water boiling, and therefore use less energy while cooking quicker, a la pressure cooker.
Similarly, if you were to lower the pressure, the water would boil at very much lower temperatures. This is the principle of freeze drying, where the water boils away at a temperature so low the physical nature / appearance / taste of the food is preserved.
Interesting stuff science.
The electric kettle is likely to be very efficient at putting energy into the water, the gas hob less so.
However, the energy required to boil the water, and keep the water boiling dwarfs the energy required to raise the temperature of the water. So, the most efficient use of energy is to have the water a touch under boiling.
If you raised the atmospheric pressure, the boiling point could be raised a bit (or even a good bit), so the food would cook quicker, and with less likelihood of the water boiling, and therefore use less energy while cooking quicker, a la pressure cooker.
Similarly, if you were to lower the pressure, the water would boil at very much lower temperatures. This is the principle of freeze drying, where the water boils away at a temperature so low the physical nature / appearance / taste of the food is preserved.
Interesting stuff science.
Just to throw something else into the mix, as I’ve contemplated this too... For the record, I used to boil in the kettle first too when I had gas.
Induction hobs, they have a boil setting, which accelerates the time needed to bring water to the boil, this setting has a pan of water up to boil in no time. I’m guessing the amount of electricity used is a lot more than the kettle, however the time needed is less than half.
I guess given time vs cost, I’d go with the hob in this scenario.
Induction hobs, they have a boil setting, which accelerates the time needed to bring water to the boil, this setting has a pan of water up to boil in no time. I’m guessing the amount of electricity used is a lot more than the kettle, however the time needed is less than half.
I guess given time vs cost, I’d go with the hob in this scenario.
bearman68 said:
The energy required to raise the temperature of the water is always the same no matter the source of the energy. This is a basic unalterable fact of physics.
The electric kettle is likely to be very efficient at putting energy into the water, the gas hob less so.
However, the energy required to boil the water, and keep the water boiling dwarfs the energy required to raise the temperature of the water. So, the most efficient use of energy is to have the water a touch under boiling.
If you raised the atmospheric pressure, the boiling point could be raised a bit (or even a good bit), so the food would cook quicker, and with less likelihood of the water boiling, and therefore use less energy while cooking quicker, a la pressure cooker.
Similarly, if you were to lower the pressure, the water would boil at very much lower temperatures. This is the principle of freeze drying, where the water boils away at a temperature so low the physical nature / appearance / taste of the food is preserved.
Interesting stuff science.
I think in a science, thought experiment the same amount of energy is required I don’t think that is the case in the real world. Have a read of Max Torque’s post on the other thread on a similar topic. A lot of the energy in boiling water gets lost in heating the environment. I suspect a kettle is much more efficient. The electric kettle is likely to be very efficient at putting energy into the water, the gas hob less so.
However, the energy required to boil the water, and keep the water boiling dwarfs the energy required to raise the temperature of the water. So, the most efficient use of energy is to have the water a touch under boiling.
If you raised the atmospheric pressure, the boiling point could be raised a bit (or even a good bit), so the food would cook quicker, and with less likelihood of the water boiling, and therefore use less energy while cooking quicker, a la pressure cooker.
Similarly, if you were to lower the pressure, the water would boil at very much lower temperatures. This is the principle of freeze drying, where the water boils away at a temperature so low the physical nature / appearance / taste of the food is preserved.
Interesting stuff science.
The answer to the question is to invest in a boiling water tap so you always have boiling water. Horribly expensive (so not cash efficient) but once you’ve lived with one you won’t want to go back!
Esceptico said:
I think in a science, thought experiment the same amount of energy is required I don’t think that is the case in the real world. Have a read of Max Torque’s post on the other thread on a similar topic. A lot of the energy in boiling water gets lost in heating the environment. I suspect a kettle is much more efficient.
The answer to the question is to invest in a boiling water tap so you always have boiling water. Horribly expensive (so not cash efficient) but once you’ve lived with one you won’t want to go back!
Agree that the kettle is likely to be more efficient in getting energy into the water, but it doesn't change the fact that the water temperature will vary predictably with energy input. (not to be confused with energy expended). The answer to the question is to invest in a boiling water tap so you always have boiling water. Horribly expensive (so not cash efficient) but once you’ve lived with one you won’t want to go back!
Having thought about this problem, I tend to boil the water on the induction hob in a saucepan with the lid on. As soon as it gets to boiling, I switch it down to the lowest possible setting consistent with producing an occasional gas bubble. I reckon the extra energy in heating the kettle is probably significant. But I have no evidence, just a guess, and a good theoretical knowledge.
The energy required to heat up water is simply ….
(Specific heat constant of water) times (The temperature you want to raise it by – delta T) times (the mass of the water)
Specific heat constant of water is 4200 joules/ Kg. °C
So let’s say you have a kettle full of water, 1.5 litres, which is 1.5 kg-ish.
The water out of the tap is - e.g. 10 °C, and it needs raising to boiling-ish - 100 °C – so that’s a delta T of 90 °C.
So I need 4200 * 1.5 * 90 = 567,000 Joules.
I use a 3 kW kettle, and 1 watt = 1 joule/sec, so my kettle delivers 3000 joules/second.
Therefore, the time taken for the kettle to boil is 567,000/3,000 = 189 seconds, or just over 3 minutes, which is about right.
This is a simplistic analysis as we are also heating up the kettle itself, as well as the element.
As the kettle heats up it is losing heat to the surroundings through radiation, and convection and possibly a bit of conduction to nearby things.
As above, heating the water with gas is far more “effective” due to the energy coming out of the hob in the form of the calorific value of (energy contained within) the gas. But some is wasted heating up the air flow around the flames surrounding the pan/kettle on the stove. Seeing how much heat energy goes upwards with a thermal camera pointed at a stove is an eye opener.
How the energy gets transferred to the water to heat it up is irrelevant, it’s always the same amount, it’s a measure of efficiency as to how much you pay for, and how much is transferred to the water that makes it seem as though some methods are more efficient than others.
(Specific heat constant of water) times (The temperature you want to raise it by – delta T) times (the mass of the water)
Specific heat constant of water is 4200 joules/ Kg. °C
So let’s say you have a kettle full of water, 1.5 litres, which is 1.5 kg-ish.
The water out of the tap is - e.g. 10 °C, and it needs raising to boiling-ish - 100 °C – so that’s a delta T of 90 °C.
So I need 4200 * 1.5 * 90 = 567,000 Joules.
I use a 3 kW kettle, and 1 watt = 1 joule/sec, so my kettle delivers 3000 joules/second.
Therefore, the time taken for the kettle to boil is 567,000/3,000 = 189 seconds, or just over 3 minutes, which is about right.
This is a simplistic analysis as we are also heating up the kettle itself, as well as the element.
As the kettle heats up it is losing heat to the surroundings through radiation, and convection and possibly a bit of conduction to nearby things.
As above, heating the water with gas is far more “effective” due to the energy coming out of the hob in the form of the calorific value of (energy contained within) the gas. But some is wasted heating up the air flow around the flames surrounding the pan/kettle on the stove. Seeing how much heat energy goes upwards with a thermal camera pointed at a stove is an eye opener.
How the energy gets transferred to the water to heat it up is irrelevant, it’s always the same amount, it’s a measure of efficiency as to how much you pay for, and how much is transferred to the water that makes it seem as though some methods are more efficient than others.
I've got it in mind that a 'leccie kettle is far more efficient than a saucepan on a traditional (esp. gas) hob. This because there is far less energy wasted by transfer of heat from the kettle element to the water than in heating up the air and convection around a pan.
For cost a gas hob and an electric kettle are roughly the same as the lower cost of gas mitigates the additional energy used.
I'd suggest an induction hob is better than gas (but not as good as a kettle) as the pan is heated not the air around it. Next halogen hob as long as the pan is flat of course. Halogen transfers by conduction not convection.
For cost a gas hob and an electric kettle are roughly the same as the lower cost of gas mitigates the additional energy used.
I'd suggest an induction hob is better than gas (but not as good as a kettle) as the pan is heated not the air around it. Next halogen hob as long as the pan is flat of course. Halogen transfers by conduction not convection.
jet_noise said:
I've got it in mind that a 'leccie kettle is far more efficient than a saucepan on a traditional (esp. gas) hob. This because there is far less energy wasted by transfer of heat from the kettle element to the water than in heating up the air and convection around a pan.
I agree. As well as the direct effect of the kettle heating efficiency resulting in a lower proportion of the power being wasted, this also means that the water boils more quickly so that waste power is being lost for less time. You still have to raise the temperature of the pan and hob materials if you heat the water in the kettle and then transfer it to the pan.
If I'm in a hurry I'll sometimes boil a small amount of water in the pan to get that up to temperature while I boil the rest of the water in the kettle.
Agree in efficiency it's probably kettle>induction>gas
If I'm in a hurry I'll sometimes boil a small amount of water in the pan to get that up to temperature while I boil the rest of the water in the kettle.
Agree in efficiency it's probably kettle>induction>gas
RizzoTheRat said:
You still have to raise the temperature of the pan and hob materials if you heat the water in the kettle and then transfer it to the pan.
If I'm in a hurry I'll sometimes boil a small amount of water in the pan to get that up to temperature while I boil the rest of the water in the kettle.
Agree in efficiency it's probably kettle>induction>gas
This is exactly what I do, although I did boil a pan dry once when I went looking for a corkscrew.If I'm in a hurry I'll sometimes boil a small amount of water in the pan to get that up to temperature while I boil the rest of the water in the kettle.
Agree in efficiency it's probably kettle>induction>gas
Thanks for all the answers, O Level Physics was a very long time ago, I'm glad I hadn't forgotten too much of it!
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