Test Number 1

$\Delta U=q+w$

Hailey Johnson
Posts: 31
Joined: Wed Nov 16, 2016 3:04 am

Test Number 1

I need someone to explain a concept question from test 1. For the question that asks if heat is being transferred during the expansion of a balloon when temperature is constant I answered that heat wasn't being transferred, but got it wrong. Can someone tell me how heat is being transferred even though the temperature stays the same?

Danah Albaaj 1I
Posts: 50
Joined: Fri Sep 29, 2017 7:07 am

Re: Test Number 1

The volume of the balloon decreased, which means there was expansion work done. The definition of expansion work is from the change in the volume of the system, which is how we get the equation w = -P∆V

Posts: 54
Joined: Thu Jul 27, 2017 3:00 am
Been upvoted: 1 time

Re: Test Number 1

Based on what you've said in your post it appears that the system did work; therefore the balloon expanded. Can you post the full question?

miznaakbar
Posts: 55
Joined: Thu Jul 13, 2017 3:00 am
Been upvoted: 4 times

Re: Test Number 1

The question stated: A balloon filled with gas is expanded to a larger volume and lower pressure while maintaining the same temperature. Answer yes or no if heat is being transferred in the process.

My TA said that you do expansion work against the constant atmospheric pressure, which increases the volume. However, your temperature stays constant, which means that delta U = 0. This means that q = -w, so the heat flows into the system to balance the loss of energy due to work.

Yeyang Zu 2J
Posts: 58
Joined: Fri Sep 29, 2017 7:06 am

Re: Test Number 1

you can figure it out it two ways
(question:A balloon filled with gas is expanded to a larger volume and lower pressure while maintaining the same temperature. Answer yes or no if heat is being transferred in the process. )

1:since it is a isothermal expansion, delta T=0, then delta U=0. there is not energy transferred.
2:since temperature stays the same, there is no temperature between difference outside and inside, no heat is transferred.

Christina Bedrosian 1B
Posts: 33
Joined: Fri Sep 29, 2017 7:05 am

Re: Test Number 1

doing work while maintaining a constant temp needs to include a heat transfer in order to maintain the constant temperature (q=-w or w=-q)

Peri Bingham 1G
Posts: 50
Joined: Fri Sep 29, 2017 7:03 am

Re: Test Number 1

Heat transfer does not necessarily imply that there is a change in temperature. The heat transfer is due to the work being done by the system.

Rachel Formaker 1E
Posts: 86
Joined: Fri Sep 29, 2017 7:04 am
Been upvoted: 2 times

Re: Test Number 1

There is heat transferred in this situation in order to keep temperature constant.
As a gas expands, it cools. Because the question tells us that temperature is constant, the gas must have absorbed heat from the surroundings to maintain its temperature and counteract the cooling from expansion.

Bansi Amin 1D
Posts: 51
Joined: Fri Sep 29, 2017 7:05 am
Been upvoted: 1 time

Re: Test Number 1

So the heat was transferred to make up for the change in energy in the system?

Wenjie Dong 2E
Posts: 53
Joined: Fri Jun 23, 2017 11:40 am

Re: Test Number 1

Bansi Amin 1D wrote:So the heat was transferred to make up for the change in energy in the system?

Heat is transferred because of a temperature different. The work is done by the system through giving off heat and the work is done to the system by increasing its heat.