Entropy changes: Thermal energy to work and (some) back to Thermal energy

In summary, classical thermodynamics says that if you dissipate work as heat, entropy increases. However, if you convert heat into work, the entropy of the surroundings will increase, but never decrease below the amount of entropy that was increased when the heat was converted into work.
  • #1
kmarinas86
979
1
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?
 
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  • #2
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
kmarinas86 said:
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?
Just following on what atyy has said, you are correct that converting heat into work decreases the entropy of the surroundings (by -Qh/Th). The problem is that when you convert thermal energy into work in a heat engine, you have to expel a smaller amount of heat at a cooler temperature. The expelling of heat at a cooler temperature results in an increase in entropy of the surroundings of +Qc/Tc. The second law says that the net change in entropy Qc/Tc - Qh/Th can never be negative. A Carnot engine is the best you can do ([itex]\Delta S = 0[/itex]).

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
 

FAQ: Entropy changes: Thermal energy to work and (some) back to Thermal energy

1. What is entropy?

Entropy is a measure of the disorder or randomness in a system. It is a thermodynamic property that describes the amount of energy that is unavailable for doing work in a system.

2. How does thermal energy change into work?

Thermal energy can be converted into work through a process called heat engine. This involves transferring heat from a hot reservoir to a cold reservoir, causing the expansion of a gas and the movement of a piston, which can then be used to do work.

3. Can work be converted back into thermal energy?

Yes, work can be converted back into thermal energy by running a heat pump or refrigerator in reverse. This process involves compressing a gas, which causes it to release heat and increase in temperature.

4. What is the relationship between entropy and energy?

Entropy and energy are closely related. As energy is transformed from one form to another, the overall entropy of the system increases. This means that the amount of energy available for useful work decreases.

5. How can entropy be decreased?

Entropy can be decreased in a closed system by removing disorder or randomness, but this requires an input of external energy. In an isolated system, entropy can never decrease, it can only remain constant or increase.

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