Obviously, this P, and look, p, or pressure, is equal to force comes area. Area remains constant, because if you take the cross sectional area of this cylinder, it doesn't change as the piston moves up or down. So if this guy is constant and this guy is decreasing, the pressure is decreasing, then the force must also decrease. So we see that in a heat engine, when the piston is moving, the force is changing, and so is pressure, and so is volume, but temperature remains the same. Okay, so what this basically means, or what this basically implies, is that heat must be converted to work, and all the heat must be converted to work. But that's actually not true, and only about 10% to 20% normally is converted to work.