Calculate time to raise temperature of water

[polyperson]

The problem I have is in understanding what is meant by heat capacity (as opposed to specific heat capacity).

I have been given the heat capacity (J kg-1) of a kettle plus the power input (IV), the mass of water and given that the water must be raised from room temp to boiling point.

The question asked is how long will this take.

I know that Specific Heat Capacity = I V t / m (change in temp) but where does the heat capacity come in all of this?

Any help appreciated!

 

[Pengwuino]

Well, a specific heat capacity is in J/kg*C while your heat capacity is simply in J/C. It's called a heat capacity basically when you're just using an object as opposed to different masses. For example, I'd use a crucible as a calorimeter that has a heat capacity of 180J/C because the mass doesn't change, its simply an object whose mass remains constant.

 

[polyperson]

Thanks peng. I can understand that part - I just can't see how it fits into the calculation. The way I see it:

The energy required to heat the water = mass x specific heat capacity of water x change in temp

And:

Time to change temp = Energy required / Power input

I know this is not correct but I don't know why

 

[Pengwuino]

Well remember... the water isn't the only thing being heated up...

 

[polyperson]

Yes, I see where you are coming from now. I need to think about it some more.

 

[polyperson]

Time = (Energy to heat water + Energy to heat kettle) / Power input

Energy to heat kettle = Heat capacity x change in temp ?

 

[Pengwuino]

exactly!

good job.

 

[polyperson]

Thanks peng. I got there in the end!