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# Specific Heat Capacity Question

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1. Hi,

On my homework there's a question which I thought I'd done right but the answer is 70kW and I got 7000W so I was wondering whether anyone could see where I've gone wrong.

The question says: An instant gas hot water heater is capable of raising the temperature of 2kg of water by 50K in each minute, what is the power?

So I did:
Change in Q = ItV and P=IV so change in Q=Pt
Change in Q = mc change in T so Change in Q=2kg x 4200 x 50K = 420000
420000/60 = P = 7000W

2. From the information given, you're correct, in my opinion - 7kW.
3. (Original post by The Wavefunction)
From the information given, you're correct, in my opinion - 7kW.
Thank you, perhaps it was just a typo! The next question says What problem might there be in designing an instant electric water heater which is to work from the ordinary mains supply and achieve the same rate of heating? Now that I know my answer is correct it would be reasonable to put that the power required is too large for that of a mains supply?
4. seems you are correct

but fwiw you can give it a sanity check by comparing with a normal electric kettle which has a power of 2kW and a capacity of probably around 1.5l (1.5 kg)
5. (Original post by Joinedup)
seems you are correct

but fwiw you can give it a sanity check by comparing with a normal electric kettle which has a power of 2kW and a capacity of probably around 1.5l (1.5 kg)
Thanks
6. (Original post by Blake Jones)
Thank you, perhaps it was just a typo! The next question says What problem might there be in designing an instant electric water heater which is to work from the ordinary mains supply and achieve the same rate of heating? Now that I know my answer is correct it would be reasonable to put that the power required is too large for that of a mains supply?
Yeah, that sounds like a reasonable answer
7. (Original post by The Wavefunction)
Yeah, that sounds like a reasonable answer
Awesome, thanks
8. (Original post by Blake Jones)
Thank you, perhaps it was just a typo! The next question says What problem might there be in designing an instant electric water heater which is to work from the ordinary mains supply and achieve the same rate of heating? Now that I know my answer is correct it would be reasonable to put that the power required is too large for that of a mains supply?
Well 7kW is a fairly standard rating for an instantaneous electric shower, but it'd need a dedicated circuit from the consumer unit (i.e. the fusebox) and it's own 30A fuse. 30A is one of the standard value for fuses in the consumer unit.

I'd say it was more of an installation issue than a design issue really.
9. The question says "ordinary mains supply", which is quite ambiguous, technically you could get that power with another circuit breaker, but my exam intuition tells me that's what they're getting at.
10. (Original post by mik1a)
The question says "ordinary mains supply", which is quite ambiguous, technically you could get that power with another circuit breaker, but my exam intuition tells me that's what they're getting at.
Thanks

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