Evaluation of heat and work for a stirred liquid

[guyvsdcsniper]

Homework Statement

Stirring. A liquid is stirred in a well- insulated container until its
temperature rises.
(a) Has heat been transferred to the liquid?
(b) Has work been performed on the liquid?
(c) What is the sign of ΔE?

Relevant Equations

ΔE=Q+W

a.)I would say the stirring, a form of mechanical work, is increasing the internal energy of the liquid, releasing heat into the system, which thereby raises the temperature.
b.) Work is obviously being done by stirring the liquid.
c.)Well we are doing work on a system so W>0 and with the increase of internal energy, we can say Q>0. So ΔE should be positive.

These are my answers and the thought process behind them. Just want to cross reference my answers and see if I am on the right path.

 

[Doc Al]

a.)I would say the stirring, a form of mechanical work, is increasing the internal energy of the liquid, releasing heat into the system, which thereby raises the temperature.

Has heat been transferred to the system? You just said that work was done and therefore the temperature increased but was any heat transferred?

Note: "a well- insulated container"

 

[guyvsdcsniper]

Has heat been transferred to the system? You just said that work was done and therefore the temperature increased but was any heat transferred?

Note: "a well- insulated container"

I was really unsure about my answer to a, and it was regarding exactly what your response says.

So doing work and releasing heat to a system is not the same as transferring heat?

 

[Doc Al]

So doing work and releasing heat to a system is not the same as transferring heat?

Not sure what you mean by "releasing heat to a system". Work was done (the stirring) and the temperature rose. But did heat transfer from outside the system?

 

[guyvsdcsniper]

Not sure what you mean by "releasing heat to a system". Work was done (the stirring) and the temperature rose. But did heat transfer from outside the system?

Ok it seems like I may be incorrect in regards to releasing heat to a system.

I guess I am trying to say the work is being converted into heat by the stirring, but that does not equate to transferring heat. The transfer of heat has to come from outside the system.

Is that correct?

 

[Doc Al]

I guess I am trying to say the work is being converted into heat by the stirring, but that does not equate to transferring heat.

Don't say "converted to heat" (that's what we might say in 'everyday speak', but it's not accurate). The stirring did work, which increased the internal energy and raised the temperature. No "heat" flow was involved.

The transfer of heat has to come from outside the system.

Exactly!

Big hint: "a well-insulated container" means that no heat could flow.

 

[guyvsdcsniper]

Don't say "converted to heat" (that's what we might say in 'everyday speak', but it's not accurate). The stirring did work, which increased the internal energy and raised the temperature. No "heat" flow was involved.

Exactly!

Big hint: "a well-insulated container" means that no heat could flow.

Got it, I will be more meticulous with my wording going forward.

Does this mean my answer to c is still correct? I know that W>0, but since not heat was transferred, Q = 0 , which means the overall sign of ΔE is positive?

 

[Doc Al]

Got it, I will be more meticulous with my wording going forward.

Good. I'm busting your chops a bit, but I want you to get it right.

Does this mean my answer to c is still correct? I know that W>0, but since not heat was transferred, Q = 0 , which means the overall sign of ΔE is positive?

Yes, ΔE is positive (and Q = 0). Good!

 

[guyvsdcsniper]

Good. I'm busting your chops a bit, but I want you to get it right.

Yes, ΔE is positive (and Q = 0). Good!

Haha understood. Thank you for your help I truly feel I have a better understanding now.