Thermodynamics of Gas: Work Done by vs. Work Done On

  • Context: High School 
  • Thread starter Thread starter hoyy1kolko
  • Start date Start date
  • Tags Tags
    Gas Thermodynamics
Click For Summary

Discussion Overview

The discussion revolves around the conventions of work done by and on a gas in thermodynamics, exploring the definitions and implications of these terms. Participants examine the conflicting information regarding whether work done by gas is positive or negative and seek clarification on the conventions used in different contexts, such as physics and chemistry.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants note that the sign convention for work done by gas being positive and work done on gas being negative is commonly accepted in physics, while others argue that in chemistry, the opposite convention is often used.
  • One participant emphasizes that the choice of convention is a matter of consistency, suggesting that as long as one is clear about the definitions used, the results will be the same.
  • Another participant explains that when a gas expands and does work, it exerts force outward, leading to a positive work value, while contraction results in negative work from the system's perspective.
  • A later reply attempts to clarify the context of energy transfer between two systems, suggesting that work done by the gas indicates a transfer of energy away from the gas, while work done on the gas indicates an increase in the gas's energy.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the sign conventions for work done by and on gas, with multiple competing views remaining regarding the definitions and their implications in different scientific contexts.

Contextual Notes

The discussion highlights the potential confusion arising from differing conventions in physics and chemistry, and the importance of clearly defining terms at the outset of discussions or calculations.

hoyy1kolko
Messages
9
Reaction score
0
I read from my book work done by gas is positive sign and work done on gas is negative sign.

I read through the information on internet,it states that work done by gas is negative and work done on gas is positive.I am confusing now.Please explain to me.Thank you.
 
Science news on Phys.org
It's a matter of convention whether incoming energy is taken as positive or negative. That is, you can choose either definition and get the same answer as long as you're consistent. (However, it does make it confusing if the definition isn't made clear at the start of a solution.)
 
Indeed, its a matter of convention. Usually in physics we take the work done by the gaseous system as positive. The motivation behind this convention comes the definition of work. When the system expands, its exerting some force in the outward direction and the displacement of the boundary is also outward. Hence, the work done by the gas is positive. On the other hand, when the gas contracts, it is exerting the force in the outward direction, yet the displacement of the boundary is inwards. Hence, the work done on the system is negative work from the standpoint of the system.
Usually in chemistry we take the work done on the system as positive. The reference taken in this case is the energy of the system. If something increases the energy, it is assigned positive value.
 
hoyy1kolko said:
I read from my book work done by gas is positive sign and work done on gas is negative sign.

I read through the information on internet,it states that work done by gas is negative and work done on gas is positive.I am confusing now.Please explain to me.Thank you.
I'll stick to the standard convention and take a guess at the context you read this in. Let's say we have two bodies or systems, A and B. A might be a bucket of water or a cylinder with a piston in it. B might be the environment.

When A does work on, transfers heat to or otherwise transfers energy to B, the energy in A decreases and the energy in B increases an equal amount.

- conversely -

When A has work done on it, has heat transferred to it or otherwise has energy transferred from B, the energy in A increases and the energy in B decreases an equal amount.

So when you read that the "work done by gas is positive" (or an increase) that probably was to say that there was a transfer of energy to something and that something had an increase in energy.

When you read that "work done by gas is negative", that probably was to say that the total energy of the gas decreased when it did work.
 

Similar threads

  • · Replies 5 ·
Replies
5
Views
4K
  • · Replies 60 ·
3
Replies
60
Views
10K
  • · Replies 8 ·
Replies
8
Views
4K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 22 ·
Replies
22
Views
6K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 3 ·
Replies
3
Views
1K
  • · Replies 30 ·
2
Replies
30
Views
5K