The fridge in my kitchen

<aside> 💡 COP I didn’t talk about the Coefficient of Performance of refrigerators in my lesson, but just know that it’s very similar to efficiency. It’s defined like this: $COP=\frac{Q_c}{W}$, where $Q_c$ is the heat absorbed from the cold source and $W$ is the work needed to do so. Similar to efficiency that can’t be equal to 1, the COP can’t be equal to $\infty$.

</aside>

The fridge in my kitchen needs has a COP of 3,2. How much electrical energy do I need to give to my fridge to make it absorb 820J of heat from the cold source? Assume that only 90% of the electrical energy can effectively be turned into work.

An ideal machine

I want to build a machine that works with reversible transformations. I already have one cold reservoir at 273K, how can I make my engine have an efficiency of 0.7? Calculate the change in entropy of the universe connected to this transformation.

Some work is lost

Calculate the lost available work when 2410J are transferred from a reservoir at 700K to another one at 400K.

It’s your turn!

Try to demonstrate some of what we talked about in this lesson. You can try with the interdependency of the Kelvin and Clausius statement, or with the efficiency of a Carnot engine. Close your notes and try to write the demonstration on a blank piece of paper. If you can’t do it, don’t worry! Come back here and look for the explanation ;)