 Welcome back. After defining the Carnot cycle, the next question is, what are we going to use it for and how are we going to use it? What we will do is using the definition of the Carnot cycle as shown in the previous part, we will implement it using a working fluid and let us select the working fluid to be for simplicity and ideal gas with constant specific heats. Why this? The equation of state relating p v and t is very simple, p v equals r t. The relation between u and temperature is straightforward, delta u is c v delta t or d u is c v d t. So our algebra or some amount of calculus that we will have to do will not be very complex. Then we will analyze the Carnot cycle in detail. It is an ideal gas with constant specific heats. So we will use the mass m. We will perhaps have an expression containing r, its gas constant, may be c v, may be c p, may be gamma and also the four states. Let me simply call them states 1, 2, 3, 4. May be temperatures at these four states are involved, may be pressures are involved, may be volumes are involved. We do not know to begin with what is the complexity of this expression. But we will definitely note that although there are four states, there will only be two temperature levels. One will be T a and one will be T b. After analyzing this, we will obtain expressions for Q absorbed, Q rejected and hence we will be able to obtain an efficiency expression. Eta is W by Q absorbed, which we know also equals 1 minus heat rejected divided by heat absorbed and this we will be able to obtain in terms of all these. Hopefully it will not contain all of these. It would be a very simple expression, hopefully. So this is let us say equation 1 for relation 1. But then we will also say that since what we have a reversible 2 T heat engine, which is running on that Carnot cycle. Hence by the second law, the Carnot theorem, its corollary and the thermodynamic temperature scale definition. The definition of the thermodynamic Kelvin scale, we will also write theta equals 1 minus theta b by theta a. This comes purely out of thermodynamics. It does not depend on the details of the cycle, the working fluids and the process details. All that we have to assert is that the engine is a 2 T reversible heat engine. This will be expression 2. Notice that expression 2 will contain only thetas, theta b and theta a and expression 1 will contain T a and T b and may be a few other parameters. Hopefully hardly any other parameters. And then using these two expressions, expression 1 and expression 2, we will be able to relate theta 2 T. That is our aim in analyzing the Carnot cycle. Thank you.