Same temperature and entropy


by Antonio2090
Tags: entropy, temperature
Antonio2090
Antonio2090 is offline
#1
Feb10-08, 01:21 PM
P: 1
Does entropy change when temperature remains constant? What if heat is added into a system, while the volume expands and the pressure drops at a constant temperature? Is there any change in entropy?
Phys.Org News Partner Physics news on Phys.org
Physicists design quantum switches which can be activated by single photons
'Dressed' laser aimed at clouds may be key to inducing rain, lightning
Higher-order nonlinear optical processes observed using the SACLA X-ray free-electron laser
Mapes
Mapes is offline
#2
Feb10-08, 02:21 PM
Sci Advisor
HW Helper
PF Gold
Mapes's Avatar
P: 2,532
Does entropy change when temperature remains constant?

Yes, entropy can change even at constant temperature. For example, adding material into a system increases entropy, as does any irreversible isothermal process such as free expansion of a gas into a vacuum.

What if heat is added into a system, while the volume expands and the pressure drops at a constant temperature? Is there any change in entropy?

Yes, heating a system always increases its entropy. Another way to view this process is that the temperature is the same, but the space for atomic motion has increased because the volume increased. There are therefore more available microstates for the system, which is equivalent to saying the entropy has increased.
Andy Resnick
Andy Resnick is offline
#3
Feb11-08, 09:03 AM
Sci Advisor
P: 5,468
Quote Quote by Antonio2090 View Post
Does entropy change when temperature remains constant? What if heat is added into a system, while the volume expands and the pressure drops at a constant temperature? Is there any change in entropy?
It can. For example, let's look at the Gibbs Free energy- this is the change of free energy for processes occuring at constant T and P:

[tex]\Delta G = \Delta H - T\Delta S[/tex]

G is the Gibbs free energy, H the enthalpy, T temeprature, S entropy. There are lots of processes that use this relationship: phase transitions, chemical reactions, etc. The sign of [tex]\Delta G[/tex] tells you if the processes is spontaneous or not- in biochemical reactions, non-spontaneous reactions are generally powered by using the chemical energy in adenosine triphosphate (ATP) or guanine triphosphate (GTP)- but not in the way elementary texts describe.

The Gibbs free energy is related to the chemical potential: specifically, if a chemical is out of equilibrium concentrations. The relative concentration of ATP to ADP is kept about 10^10 times out of equilibrium by organisms (IIRC), and this is the source of energy used to power reactions, and by which we have the ability to locally decrease our entropy.


Register to reply

Related Discussions
Kelvin Temperature - Celsius Temperature Change General Physics 12
Is the Entropy of the Universe Zero?:(Entropy as Entanglement) Quantum Physics 10
Black Hole Temperature and Entropy Astrophysics 5
Differentiation woes with temperature/entropy relations. Introductory Physics Homework 3
The time dilation, entropy and temperature of the foggoid General Physics 0