# How long does it take to get heat? (thermodynamics )

1. Aug 15, 2015

### lasha1

Can anyone tell me the formula.
For example: How long does it take to boil 1kg 20°C water in steel container. With 1 kg of coal (or something). I want to understand the formula of time attitude to heat.

2. Aug 15, 2015

### lasha1

I think it's the following formula:
cmwaterΔT=qmfuelt where t is time q is specific heat of fuel.

3. Aug 15, 2015

### FactChecker

That would depend on the burn rate of the coal and the heat transfer properties of the steel container. Don't forget that the transfer rate will change as the water warms. I haven't looked it up, but you should also make sure that the coal contains enough energy to bring the water to a boil. Then you need to assume that the heating is so even that there is no boiling till all the water gets hot enough. Uneven heating changes everything.

4. Aug 16, 2015

### lasha1

Ok FastChecker. But with Joule lenz's law we can get the following formula: Pt=mcΔt where P is power of kettle and t is time that needed to get heat.

5. Aug 17, 2015

### FactChecker

Your second post came as I was writing my reply and I never saw it. So you have equations that you are satisfied with. Good.

6. Aug 17, 2015

### nasu

This equation does not make sense. The units are not consistent. If q is indeed specific heat of fuel you need some temperature.
But is more likely you mean the caloric power of the fuel. In any case, time is not supposed to be there

This equation is OK but you changed the OP problem. Originally you said that you use some coal to heat up that water. Now you seem to switch to some kettle. You cannot apply this to OP question without knowing all kind of details about the burning of coal and heat transfer.

7. Aug 17, 2015

### lasha1

Yes. But I checked lenz's law. I understand it. Thank you.