My kettle packed up last week and it was rubbish anyway - so slow to boil. The kettle I had before was much quicker so that was a stipulation when buying a new one - a quick boil. Since it had to be white, have a flip open lid and a scale filter, the most powerful that filled the brief was a 2200W Bosch. Job done.
Wifey filled it up to max and set it to boil (to clean it properly). So out of curiosity I said, let’s time it. Let’s see how quick it is. So we did. 1.7 litres and it started to boil and click off at 6m 2s. It seemed ok but I wanted to time it for 1 litre to boil. If 1 litre is 60% of 1.7 litres and 60% of 6m is 3m 36s, then that’s when it should click off. So we set the timer and stared at it and waited for it to boil. 3m 32s. But the volume was closer to 59% so 3m 32s was spot on after all. All very exciting, this boiling a kettle full of water.
But I really am fascinated by the physics of such a simple daily task. Is there an obvious and simple equation where you take 1 litre of water, and 2200W of energy giving you a boiling point at 3m 32s or thereabouts? Of course there’s chaos, I’m slightly above sea level and the water was probably 10 degrees to start with, neither am I in the controlled environment of a laboratory. Any retired physics tutors?
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Exciting stuff on a Sunday afternoon!
My kettle took 2m 29s.
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At the apartment I'm currently staying in Portugal there is only an old hob-top kettle to go on the old electric hob.
I've no idea just how slow it is, but it's certainly a long wait when you are gagging for your first cuppa of the day...!! :-)
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I measured my favourite coffee cup - exactly 300ml (30% of one litre).
So 30% of 3m 30s is 66 seconds to boil.
Guess what. 74 seconds. Doesn't seem to be linear, does it? Too many variables. Too much chaos.
Last edited by: BiggerBadderDave on Sun 24 Mar 19 at 18:36
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Remember each time the kettle needs heated also and that is a constant mass independent of the water within.
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Perhaps the 'clicking-off' mechanism didn't work with any real immediacy what with there only being 300ml of water at the bottom of the vessel...
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Well,
The specific heat capacity of water is 4200 joules per kg per deg C so it takes 4200 joules to raise the temp of 1kg of water by 1 deg C.
1 litre of water is near as dammit 1kg at its maximum density so it will take 4200 joules to increase temp of 1 litre by 1 deg C.
2200 watts is 2200 joules per second so your kettle should take 4200 / 2200 = 1.9 seconds to raise temp of 1L of water by each deg C.
YMMV
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Now it's starting to make sense. To take 1 litre of litre from 10 deg C to 100 deg C (90 degrees x 1.9 sec)
That would be 171 seconds.
It may have been cooler to start with.
I'm also reading the small print that it says 1850-2200W, so it was never running at full throttle after all.
So, if you know the volume, you know the initial temp and you know exactly how long to reach boiling point - you can work out how powerful the kettle really is...?
Tomorrow's task.
Cheers Kevin.
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The 1850-2200 rating is to allow for variation in voltage, nominally 230V across the EU, but actually somewhere in the range 220-240V (plus or minus a little bit) depending on where you are. (I suspect 220V where you are, rather than 240V in the UK, so you'll be at the lower end of the wattage.
The rating is also the input. Electric kettles are reasonably efficient, but you're unlikely to transfer more than, say, 80% of that as heat output.
The heat capacity of the kettle is reasonably constant, but the varying amount of air above different volumes of water will have an effect.
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FFS drink cold beer from the fridge
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Damn you LL. I was just getting in to the thread and then you go and blow it out of the aqua with a sensible idea like that.
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But if the beer you buy from the Super Mercado is 20 deg C how long does it need to be in the fridge before it's drinkable?
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Stop it Kev. You know the answer is - NOW :-)
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Re the experiment being undertaken several times, surely the kettle's element takes time to heat up for sample 1 and then, depending on time lag, could be already hot for sample 2 etc. thus throwing out the times.
Last edited by: zippy on Sun 24 Mar 19 at 23:00
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"But if the beer you buy from the Super Mercado is 20 deg C how long does it need to be in the fridge before it's drinkable?"
I've been thinking about that this morning. I don't have a thermometer so if I want to know all the variables to work out how powerful my kettle is (minus 20% from wastage as tyrednemotional says) the only way to work out the temp of the water is to leave a litre in the fridge first. I know it's 5 degrees, cos it says so on the front of the door. But how long do I have to leave it in the fridge?
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I'm off out now - but will update later with the vintage Dualit kettle
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>> I'm off out now - but will update later with the vintage Dualit kettle
>>
...you'll only be successful if you take all your measurements in °F.....
;-)
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Fascinating...
So a litre of water from the fridge was 5 degrees and my kettle took exactly 4 minutes to click off. Why so long time...?
So to raise the temperature of a litre from 5 to 100 (95 degrees) each degree took 2.5 seconds. (240 sec)
4200/1680 = 2.5 sec
My kettle was 1680W and that's without the wastage factor.
Who can I sue?
I shall buy a thermometer and keep repeating this experiment at different times of the day.
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I think you misunderstand the math......
The kettle is delivering 1680 watts of output.
At (say) 80% efficiency, that is 1680/0.8 watts input, or 2100 Watts - well within the specified range.
Even at an unlikely 91% it would be just about in range.
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Ahh, yes, I see what you mean now...
My result is divided by .8 for the 80% efficiency factor and that must fall between the 1850-2200 quoted?
Got it.
Cheers!
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A watched pot and all that... - Duncan
