specific heat
Dogbert
Are negative specific heats possible?
For some reason I thought that specific heats could only be positive.
Here is a problem I am solving. I got a number for the specific heat
but it turned out negative.
A 250 gram sample of metal shot is heated to a temperature of 98 C.
It is placed in 100 grams of water in a brass calorimeter cup with a
brass stirrer. The total mass of the cup and the stirrer is 50 grams.
the initial teperature of the water, stirrer, and calorimeter is 20 C.
The final equilibrium temperature of the system is 30 C. What is the
specific heat of the metal sample? (the specific heat of brass is
092 cal/g*C.
this is what I got:
250g*c*(30-98)=50g(1)(30-20)+50g(.092)(30-20)
mc(delta t)of metal= mc(delta)t of water+mc(delta t) of cup and
stirrer
answer= -.0321cal/g*C for metal
Thanks for any help you can give,
Joy
John Spevacek
The problems states 100 g water, but your calculation used on 50 g.
That's the first problem.
Your solution however, shows a more fundamental misunderstanding. If you
use the convention that delta t = final t - initial t, then the whole
equaiton needs to be set up so that delta Q = 0 (no energy (heat = Q) is
gained or lost by the system). In that case, 0 is on one side of the
equation, and **all* of the mc(delta t)'s are on the other side.
Another solution that is more consistent with the way you've worked
things out is one that mimics : heat lossed by metal = heat gained by the
water/brass. This is close to what you were doing above, but the heat
lost is 250*c*(98-30), since energy is always a positive quantity. (This
mistake is why your answer is negative).
I'd recommend the first route, since it is more systematic and can be
done without much thinking (good during tests when time is short and
pressure is high).
John
By the way, I dind't directly answer your first question, can specific
heat be negative. But I did give a lot of clues. What do you think now?
Daren McCrossan
Dogbert wrote:
>
> Are negative specific heats possible?
>
> For some reason I thought that specific heats could only be positive.
> Here is a problem I am solving. I got a number for the specific heat
> but it turned out negative.
>
> A 250 gram sample of metal shot is heated to a temperature of 98 C.
> It is placed in 100 grams of water in a brass calorimeter cup with a
> brass stirrer. The total mass of the cup and the stirrer is 50 grams.
> the initial teperature of the water, stirrer, and calorimeter is 20 C.
> The final equilibrium temperature of the system is 30 C. What is the
> specific heat of the metal sample? (the specific heat of brass is
> 092 cal/g*C.
>
> this is what I got:
> 250g*c*(30-98)=50g(1)(30-20)+50g(.092)(30-20)
> mc(delta t)of metal= mc(delta)t of water+mc(delta t) of cup and
> stirrer
Should be (I hate putting units in equations!):
250*c*(98-30)=100(1)(30-20)+50(.092)(30-20)
Note that the question referred to 100 (50) g water.
Maybe it's better to think of it as
250*c*(30-98) + 100(1)(30-20)+50(.092)(30-20) = 0
(i.e. conservation of total energy)
BTW, if this homework, you should be looking at examples in your book/notes before attempting to do problems like this on your own.
--
Best regards, D. McCrossan
*********************************************************************
Daren McCrossan, BSc, MSc Mathtrek Systems
Marketing Manager 3-304 Stone Road West, Suite 165
EMail: sa...@mathtrek.com Guelph, Ontario CANADA N1G 4W4
Tel:519-763-1356, FAX:519-763-4525 http://www.mathtrek.com
*********************************************************************
liah...@gmail.com
> Are negative specific heats possible?
>
> For some reason I thought that specific heats could only be positive.
> Here is a problem I am solving. I got a number for the specific heat
> but it turned out negative.
>
> A 250 gram sample of metal shot is heated to a temperature of 98 C.
> It is placed in 100 grams of water in a brass calorimeter cup with a
> brass stirrer. The total mass of the cup and the stirrer is 50 grams.
> the initial teperature of the water, stirrer, and calorimeter is 20 C.
> The final equilibrium temperature of the system is 30 C. What is the
> specific heat of the metal sample? (the specific heat of brass is
> 092 cal/g*C.
>
> this is what I got:
> 250g*c*(30-98)=50g(1)(30-20)+50g(.092)(30-20)
> mc(delta t)of metal= mc(delta)t of water+mc(delta t) of cup and
> stirrer
Poutnik
to a 21 years old post ?
Who do not bother to ask and explain,
should not bother to expect a answer.....
--
Poutnik ( The Pilgrim, Der Wanderer )
A wise man guards words he says,
as they say about him more,
than he says about the subject.
Poutnik
The sign,
the mass,
the heat capacity.