 One term that we'll be using when we study refrigeration cycles is the coefficient of performance and this is what we use in order to quantify the efficiency of the cycle. When we looked at heat engines we talked about thermal efficiency. When we look at refrigeration we look at coefficient of performance. Coefficient of performance is written out in capital letters of COP and we will put a subscript for either refrigeration or heat pump R for refrigeration and it would then be defined as being the amount of heat coming out of whatever system we're trying to cool divided by the amount of work that we have to put in and in the case of a heat pump it would be our desired output and here we're trying to heat a space so it would be QH and then divided by the work going in. Typical values here typically 1.5 to 4.0 are not uncommon. To put this into perspective if we had an electric resistance heater which is sometimes used in areas for space heating where they'll just run electric current through an electrical resistance heater, if you look at the resistance heater the coefficient of performance of that system is 1 because you put in 1 watt and you get 1 watt out so you can see that when you use the heat pump if we're getting numbers ranging from 1.5 to 4 we're doing a lot better and so it's a much more efficient system. Electric resistance heater is really the worst type of space heating you could possibly have. So what we see based on the coefficient of performance is we're actually transferring more heat than the amount of work that we are putting in and sometimes that conceptually can be a little difficult to comprehend. However the working fluid is the thing that's transferring the heat or the thermal energy and so we're just taking it for one place and moving it to another and we get more heat transfer out of it than the one putting in the work which is a good thing. It's a more efficient system than we would have with the electric resistance heater. So that's it for coefficient of performance. You'll be using those to be able to quantify all of the refrigeration cycles you'll be looking at. In the next segment and the last segment of this lecture what I want to do is work an example problem dealing with the ideal vapor compression cycle.