Be Excellent To Each Other

I need 3 litres of boiling water, my kettle will only safely boil 2 litres at a time. I am concerned about the environment and want to know what the most efficient way of boiling all 3 litres is.

Do I fill it all the way up, boil that, pour it into whatever container I need it in, and then boil the last litre?

Or perhaps I should boil 2 litres, pour 1/2 of it into the container, and then refill the kettle, and boil that?

Maybe I should boil 1 litre at a time?

I'll make this into a vote (everyone loves votes don't they?) but if you could explain your vote with reasoning, and values, that would be excellent.

Malc

I reckon adding 1 litre to 2 boiling litres will make the boiling process much quicker.

CUE: SCIENCE GUY!

Boil all three litres in a saucepan.

FAIL

Malc

I reckon it doesn't much matter.

It makes very little difference. The energy input to take 3 litres of water from room temperature to 100 deg C is the same in all instances, so you just need to consider which of your methods will lose the most heat to the environment whilst you wait for the rest of the water to boil; but that won't be very much at all.

Newton's law of cooling states that the rate of temperature transfer from one object to another is proporitional to the difference in their temperatures. So it would be marginally more efficient to minimise the amount of water held between the first and second boiling. You don't have an option for that -- boil one litre, then boil another two.

Why? If you need three litres, use a vessel that'll fit all the water in. This is a silly science question.

It's an Oxford Physics entrance interview question. This is the kind of random stuff they ask.

The question clearly states I have a 2litre kettle, It does not mention anything about saucepans, or stoves, or whatnot, and besides, whenever I pour from a pan I always spill some.

Malc

Have you considered where the element is in a kettle, and the volume of water?

Malc

PANFAIL. Get one of those ones with a little spout thingy. Easy.

The answer, I believe, is to boil 2 x 1.5 litre loads. This is because the greater the volume of water boiled, the lower heat losses through evaporation, convection and latent heat are by proportion: the ratio between the surface area presented by the top of the water in the kettle vs. total water mass is minimised.

Imagine if you tried doing it via 20 x 0.15 litre loads. In that case, the amount of heat lost through convection, evaporation and latent heat via the high surface area (unchanged from if a much larger amount of water was boiled), would be greatly increased by proportion in each case, leaving less 'useful' energy in the water to actually increase temperature.

In which case, the correct answer is "Get the scout to do it". Standards have slipped, it seems.

Using a metal pan with an open gas burner as the heat source is much less efficient that using a plastic electric kettle with completely submerged heating element in full contact with the water. The much higher temperature of the flame and relatively low contact area with the saucepan base all make for large heat losses through radiation and convection, and the metal pan itself will also radiate heat much more readily than a plastic vessel. If the pan is open, the heat loss will be even more marked.

These disadvantges are likely to outweigh the advantage of boiling your 3 litres of water in one go, in pure energy expended terms.

You're probably best off doing the 2 then the 1 as the 2l will cool down less than 1.5L or 1L would do while you wait for the rest to boil.

That's true, but the scope of the question was specifically limited to what is the most efficient way of boiling 3 litres of water using a 2 litre kettle, i.e. uses the least amount of energy due to concerns for the environment.

But if you require your water boiling, you'd have to reboil part of it anyway as it will have cooled below 100°C in the time the rest had boiled.
Only having to heat up the 2L which had cooled less as the 1L boiled would use less energy.... (I think with no calculations)

It's 4.2J of energy to raise the temperature of 1ml of water by one K.

I think we are meant to assume negligible heat loss for the held over 'first batch', otherwise you'd be forever re-boiling either the first or second lot of water since getting both of them at 100 deg C at the same time, using one kettle, would be impossible lol. Like I said, the clear intended emphasis of the question is on minimising the energy expended to boil the water using the device specified, purely in terms of how to split the required total volume of water.

You could always put the first 1.5 litres of water into a thermos flask.

That wouldnt take into account the size of the heating element, limescale build up or any other variables.

No, I'm pretty sure it's a universal constant.

No, but other things would affect the heating of the water.

Im saying that is a constant, but you need more information before you can make an appropriate calculation.

You heat up a big saucepan of water on the hob.
You put it on the small hob, it takes longer than when on the big hob.

That would only change the effeciency of it (ie, you would use more electricity J to create that much heat J)

Malc

You can't get 3 litres of *boiling* water if your kettle only holds 2 litres. Fact.

Otherwise I'd guess 1) is the most efficient way.

But wasnt the question regarding energy? So it would come into affect then?

You're both right; that is indeed the specific heat capacity of pure, non boiling water for a 'perfect' heating system with no losses, but for real world situations such as boiling a kettle of water, more energy is obviously needed due to many potential inefficiencies.

Nickachu is seriously over-complicating the question though; I don't think he'd be very good at giving concise answers in an examination situation!

Um, doesn't it change depending on atmospheric pressure?

I've just finished my degree! I've got no experience of real world applications of it due to not being able to find a job!

Surely 1 litre then 2 litres, rather than the other way around - given that your first boil will be cooling while you reboil the second?

Also, Grim... is correct. The correct first step is surely to travel to the Himalayas?

At 1 atmosphere.

Nope, that's the boiling point, not specific heat capacity. It always takes the same amount of energy to heat 1kg of non boiling, pure water by 1 deg C, using a 'perfect' heating system, whether you're at sea level, summit of Mount Everest, deep space vacuum or Mars.

Buy a second kettle, and boil 1.5 litres at exactly the same time.

Sorry chap, I was only joking, no offence meant at all.

But if the BP is lower... Higher pressure and whatnot, then the amount of energy to boil it would be less.....

Yes, so you need to raise the temperature by fewer degrees. The amount of enrgy needed to change the temperature by one degree does not change.

No worries, and far from it!

I always answer concisely in exams, I dont think I stayed to the end of any of mine at my whole time at uni due to being finished super fast!

True, but if the water boiling temperature is constant for all scenarios of Malc's question, it can be ignored.

Throw away your water and boil ethanol instead. As it has a lower boiling point, you'll use less energy.

Im not disputing that. The amount of energy to get the water to BOIL would be less as the BOILING POINT would be lower. So you'd only have to raise it to say 80°C rather than 100°C so thus using less energy.

I don't think re-boiling comes into it. Asking for 3 litres of *boiling* water is impossible, given only a 2l kettle.

Asking for 3 litres of *boiled* water is perfectly reasonable, and then we can assume it doesn't matter what temperature it ends up at.


However I guess if you do 1l first, then more of the thermal inertia in the kettle from the first boil will be transferred to the 2l fill, so yes, 1 then 2, so "other".

Not working is great, isn't it? THINGS BECOME VERY IMPORTANT ALL OF A SUDDEN!

Im just incredibly bored

Rig a pump to circulate the extra water into and out of the kettle to a sump, that way you'll boil the full three litres. An engineering answer rather than a physics one, mind.

I don't think you can hope to answer this question without some numbers around things like how much energy will be lost in the 1-to-2 litres you have hanging around during the second boil, how much energy the kettle will lose during the boil (although that's mostly just down to conduction through the shell I think, and some modest loss of steam up the spout, but not a great deal), etc etc. So: "it depends".

Me, too.

I'm even postponing my morning wanks now, so I've got something to look forward to.

Nope, Kalmar sums it up nicely Doc:



... this whole question of heat loss in the 'held over' body of water is to completely miss the point of the (pretty simple) question, an obfiscatory complication/red herring.

Like I said, I believe 2 x 1.5 litre is the correct answer, for the reasons originally stated. Seems fairly straight forward to me.

Question is... why does Malc need 3 litres of boiled water?

He's got a humungous mug.

You're out of work and still getting up in the mornings?

Lots of tea/coffee for a tea party?

A giagantic pot noodle?