How does entropy affect heat engines?

In summary, an engine that uses heat from a hot reservoir to create work has to expend more energy to get rid of the entropy than it does to bring it in. This happens because you're dividing the temperature of the hot reservoir by the colder temperature of the ambient air.
  • #1
wakko101
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I'm having trouble trying to get my head around the role entropy plays in a heat engine. When the energy from a hot reservoir enters the engine, it brings a certain amount of entropy with it. Then, the "waste heat" (which is energy) gets rid of the entropy. But, why does it take less energy to get rid of the entropy than it does to bring it in?
 
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  • #2
Heat engines obviously need a source and sink. The source being the hot reservoir , and the sink being the cold reserviour(ambient air). The second law of thermodynamics says that the direction of temperature flow/transfer is one way (unless work is supplied) so with heat engines you are going from the hot reservior to the cold reservior. When you go from a hot body to a cold body, entropy decreases in the hot body, that is the level of molecular disorder in the heat engine decreases. (because heat is released to the ambient air)

So, in order to bring entropy into the heat engine you would have to reverse the process of heat flow which would require some type of work input, which means more entropy will needed to be generated..

Most of the entropy generated in actual heat engines is caused by friction and vibration.

maybe that helps?
 
  • #3
wakko101 said:
But, why does it take less energy to get rid of the entropy than it does to bring it in?

Because you're dividing by a smaller temperature, that of the cold reservoir. Let's assume reversibility. Entropy is conserved via

[tex]S_\mathrm{in}=\frac{Q_H}{T_H}=\frac{Q_C}{T_C}=S_\mathrm{out}[/tex]

and [itex]Q_H>Q_C[/itex], [itex]T_H>T_C[/itex]. Energy is conserved via

[tex]Q_H-Q_C=W[/tex]

You delivered all the entropy to the cold reservoir, but some energy is left over and can be extracted as work. Does this help?
 
  • #4
thanks for the clarification...I think the thing that was confusing me most was the idea that the engine itself (or the gas inside, I guess) has to change temperature for the part of the cycle where heat is being expelled. I'm a little clearer now...I think.

cheers. =)
 

1. What is entropy?

Entropy is a measure of the disorder or randomness in a system. In other words, it measures the amount of energy that is unavailable to do work.

2. How does entropy relate to heat engines?

Entropy plays a crucial role in the functioning of heat engines. The second law of thermodynamics states that the total entropy of a closed system will always increase over time. This means that in a heat engine, as energy is converted from heat to work, some of that energy will inevitably be lost as entropy increases.

3. How do heat engines work?

Heat engines work by converting thermal energy into mechanical work. This is achieved through a cycle of processes, such as heating a gas to create pressure, and then using that pressure to move a piston or turbine. The efficiency of a heat engine is determined by the amount of energy that can be converted into work, compared to the total amount of energy supplied.

4. What is the Carnot cycle?

The Carnot cycle is a theoretical model of a heat engine that operates at maximum efficiency. It consists of four reversible processes: isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression. The Carnot cycle is used as a benchmark to compare the efficiency of real heat engines.

5. How does entropy impact the efficiency of a heat engine?

As mentioned earlier, the second law of thermodynamics dictates that the total entropy of a closed system will always increase. This means that entropy must also increase in a heat engine, leading to a decrease in efficiency. In order to improve the efficiency of a heat engine, efforts are made to reduce the amount of energy lost as entropy, such as insulating the system to minimize heat transfer.

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