2nd law of thermodynamics

[yjj8817]

How does this law relate to heat engines and heat pumps? I am having a very hard time understanding this. I am having trouble linking entropy with heat engines and pumps

Thanks!

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[Cawolf]

It tells us the change in entropy of the universe must always be positive.

So in the cases of heat engines, pumps, cycles, cannot be 100% efficient. They always use more heat than work they do, or take more work to produce less heat.

The Carnot cycle shows a theoretical 100% efficiency, but it is very slow due to isothermal processes.

The main takeaway is that for work to be converted to heat, or heat to work, we will always get less out that put in, and the universal entropy change will be positive.

 

[yjj8817]

It tells us the change in entropy of the universe must always be positive.

So in the cases of heat engines, pumps, cycles, cannot be 100% efficient. They always use more heat than work they do, or take more work to produce less heat.

The Carnot cycle shows a theoretical 100% efficiency, but it is very slow due to isothermal processes.

The main takeaway is that for work to be converted to heat, or heat to work, we will always get less out that put in, and the universal entropy change will be positive.

This may be kinda dumb question but how does less than 100 % efficiency relate to positive entropy change?

 

[Cawolf]

It is beyond the scope of what I can type out effectively.

I would just take it as the second law statement, or read a thermodynamics text.

 

[sazerac]

The first law says you can't win.
The second law says you can't break even.
The third law says you can't get out of the game.

You want to focus on the second law. You can't break even. What it says is that if you put a dollar's worth of energy into a process, you will only get 98 cents of effort out of it. The other 2 cents are going to be lost to waste heat (entropy).

This applies to all processes. It applies to the process of lifting a rock and putting it on a shelf. It also applies to the process of refrigeration, that is, making a cold interior colder by pumping heat into a warm exterior, making it warmer.

Let's focus on that fridge for a second. Let's say it pumps heat from the interior of th fridge, dropping it from 30 degrees to 20 degrees. It does this by warming the room it is sitting in, from say 70 degrees to 80 degrees. Well, the second law says there is going to be some waste heat associated with this process. It isn't 100% efficient. The genius of refrigerators is that this waste heat is dumped into the room, not into the interior of the fridge. So the exterior probably went up from 70 degrees to 82 degrees, and not 80.

Does the fridge work? Yes, the interior is colder. Did the entropy of the universe increase? Yes, the average temperature of our universe went up from 50 degrees to 51 degrees.


I admit I am playing fast and loose with a lot of numbers here, but I hope we all understand that it is the concepts I am trying to get across. If you have any questions about specific formulas, we can give it a shot.