 So in this video, I want to talk a little bit about how heat engines and heat pumps relate back to PV diagrams. Now I only touched on this a little bit here when we talked about work done with gases and how there's one equation for work done by a gas and another equation very similar but with a minus sign for the work done on a gas. And here I've used the average pressure just so that we can avoid the whole integral formula for right now. Now if the gas expands, that means that what we've got going on over here is we have a a delta V that's a positive number. And so if we look at that as the gas expands, we're going to end up having a positive value for the work done by the gas and the energy is transferred out of the system. If you've got a process moving towards the right, the gas is contracting or being compressed, then the change in volume is actually a negative number. Now if I put the negative into my equation up here on top, then I would end up having a negative work done by the gas or a positive work done on the gas. So instead of thinking about it as by the gas or on the gas, you can also think about it whether if the gas expands the work is transferred out and if the gas is compressed or contracts, then that means the energy is being transferred in.