Physics/Chem: Why the sign difference in Thermodynamics?

AI Thread Summary
In thermodynamics, the sign convention for work and heat differs between chemistry and physics due to the perspective taken on the system. Physics defines work done by the system as negative, as it represents energy loss, while work done on the system is positive, indicating energy input. Similarly, heat added to the system is positive, while heat removed is negative. Discrepancies in sign conventions may arise from discussing different systems or using distinct potential functions in canonical transformations. Understanding these conventions is crucial for accurate thermodynamic analysis.
ishan001
Messages
2
Reaction score
0
I have been wondering why we take work done on the system +ve in chemistry while -ve in physics. Similar difference exist for many other quantities. Can you explain why the difference exists?
 
Science news on Phys.org
There is no difference unless the Chemists are erring.

Physics always works from the system's point of view. The system is normally your gas or solution or what have you. Work done by the system pushing pistons, swirling wheels counts negative, because the system loses energy. And you can drive stuff with it. When you push the piston or turn the wheel from the outside then you put energy into the system so it counts positive.
Same for heat, heating the system positive cooling the system negative.

The reasons why things may disagree can be:
- You are talking about different systems
- Canonical transforms. If you use a different potential then the formulas look very similar, but are in fact different.
 

Thread 'Entropy and configurations of microstates'

I was watching a Khan Academy video on entropy called: Reconciling thermodynamic and state definitions of entropy. So in the video it says: Let's say I have a container. And in that container, I have gas particles and they're bouncing around like gas particles tend to do, creating some pressure on the container of a certain volume. And let's say I have n particles. Now, each of these particles could be in x different states. Now, if each of them can be in x different states, how many total...
View full post »
Thread 'Why work is PdV and not (P+dP)dV in an isothermal process?'

Thread 'Why work is PdV and not (P+dP)dV in an isothermal process?'

Let's say we have a cylinder of volume V1 with a frictionless movable piston and some gas trapped inside with pressure P1 and temperature T1. On top of the piston lay some small pebbles that add weight and essentially create the pressure P1. Also the system is inside a reservoir of water that keeps its temperature constant at T1. The system is in equilibrium at V1, P1, T1. Now let's say i put another very small pebble on top of the piston (0,00001kg) and after some seconds the system...
View full post »

Thread 'Condensate rate of water for an air conditioning cooling coil'

I need to calculate the amount of water condensed from a DX cooling coil per hour given the size of the expansion coil (the total condensing surface area), the incoming air temperature, the amount of air flow from the fan, the BTU capacity of the compressor and the incoming air humidity. There are lots of condenser calculators around but they all need the air flow and incoming and outgoing humidity and then give a total volume of condensed water but I need more than that. The size of the...
View full post »

Similar threads

I Paradox: Thermodynamic equilibrium does not exist in gravitational fields
3
Replies
135
Views
6K
I Movable Wall in an Adiabatic System: Same Final Temperature?
Replies
8
Views
1K
I Application of the first law of thermodynamics
Replies
46
Views
6K
I Thermodynamic functions and state variables
Replies
8
Views
1K
Absolute zeros other than temperature (e.g. pressure)
Replies
11
Views
1K
I Questions about my Understanding of Thermodynamics and Statistical Mechanics
Replies
2
Views
3K
Sign Convention for Work (In Thermodynamics)
Replies
13
Views
6K
Thermodynamics: sign convention
Replies
3
Views
5K
B How are thermodynamic laws related to one another?
Replies
4
Views
2K
B Understanding thermodynamic equilibrium
Replies
3
Views
2K

Hot Threads

Recent Insights

Back
Top