[ruffrunnr]

Hey all..I'm stuck on a few problems

1) A tank containing 20 kg of water at 20 C is fitted with a stirrer that delivers work to the water at the rate of 0.25KW. How long does it take for the temperature of the water to rise to 30 C if no heat is lost.

Here's my thinking...

(Delta)U + KE +PE = Q + W

KE, PE, and Q go to zero...

U = W...

then I'm lost...I know you have to use Cp of water being 4.18 KJ/Kg C somewhere...


2) Heat in the amount of 7.5 KJ is added to a closed system whiles its internal energy decreases by 12 KJ. How much energy is transferred as work? For a process causing the same change of state but for which the work is zero, how much heat is transferred?

I think I got the first part...

7.5 KJ is being added as heat...

U = Q + W
-12 = 7.5 + W
W=-19.5 KJ

I don't know about the second part...

Thanks in advance

[eugenedakin]

Hi ruffrunnr,

Here are a couple of hints...

1) 1 watt = 14.34034 Calories/minute
2) 1 Calorie = 4.1819 Joules
3) Find the specific heat of water

I look forward to your next post, and this should help you go in the proper direction (your doing well).

Sincerely,

Eugene

[ruffrunnr]

ok..i think i got it..

was it just canceling of units?

0.25kw x 1000w/1kw x 14.34cal/min / 1w x 4.1819J/cal x 1/836000J = 1/0.179min

inverse that...

55.76 min??

[Borek]

55.76 looks OK even if it is not inverse of 0.179.

[ruffrunnr]

Ok..thanks...what about number 2?

[eugenedakin]

Hi ruffrunnr,

I got 55.76 also.... Well done!

The second part... I had to read this question a few times before I understood what they were asking. In my opinion, here is what I think they are asking:

If the interal reaction happens but 0 KJ is used internally instead of 12 KJ, how much heat is transferred?

I hope this helps,

Eugene