Register to reply 
Entropy changes: Thermal energy to work and (some) back to Thermal energy 
Share this thread: 
#1
Oct411, 02:39 PM

P: 1,011

In classical thermodynamics, if we dissipated the kinetic energy of an object as thermal energy, then we would increase the entropy.
However, let's say we took 90% of some thermal energy in a reservoir, and converted it into work, and 10% of that is converted back into thermal energy after 1 minute is passed. This would mean that 81% of the thermal energy has been converted into work. If we dissipate work as heat, entropy increases. So what happens if we convert heat into work? Shouldn't the opposite occur  a decline of entropy? I think we should have a sum of changes, an entropy increase in excess of a subsequent decrease. Is this the correct view? 


#2
Oct411, 05:55 PM

Sci Advisor
P: 8,792

In classical thermodynamics, you always need a temperature difference to do work. Every time you do work, you reduce the temperature difference. When a temperature difference is reduced, entropy increases.



#3
Oct411, 06:53 PM

Sci Advisor
HW Helper
P: 6,684

So the second law puts a limit on the efficiency at which you can convert thermal energy into work. That upper limit is the Carnot engine cycle. It can never get better than that. AM 


Register to reply 
Related Discussions  
Enthalpy, thermal energy, and kinetic vs. potential energy  Introductory Physics Homework  6  
Mechanical energy into thermal energy  Introductory Physics Homework  1  
Energy in Thermal processes: Energy Transfer  Classical Physics  0  
Loss of rotational energy to thermal energy  Introductory Physics Homework  4  
Work and Thermal Energy  Introductory Physics Homework  2 