1. 12.4 Heat Engines Heat engines convert heat into other useful forms of energy like mechanical or electrical. A car engine is an example.

2. Heat Engines Mechanical Energy can be obtained from Thermal energy ONLY WHEN Heat is allowed to flow from Hot to Cold. Heat engines can never be 100% efficient (meaning you can’t convert all the heat into mechanical work). Heat engines are typically repeating cycles so the change in internal energy (u) is equal to zero.

3. Heat Engine It’s a cyclic process so the change in internal energy U = 0. Therefore, the Work done by the heat engine = net heat flowing in. Qnet = Qhot – Qcold = Work If the working substance is a gas, the net work is the area under the PV diagram.

4. Heat Engines & efficiency Efficiency e = net work done/heat input e = (Qhot-Qcold) / Qhot = W / Qhot. e = work you get out divided by the heat you put in, ok that makes sense, elmo. e = 1 – Qcold/Qhot. So the only way e = 1 (or 100% efficient) is if there is no Qcold or no heat output. But you can’t convert all the input heat into work, must have some Qcold, ain’t not no such thing as 100% efficiency!!!! There ain’t no free lunch!!!, you can’t get nothing for free!!!

5. Carnot Engine Sadi Carnot, French engineer, died 1832. Developed the idea of reversible heat engine. “No real engine operating between 2 heat reservoirs can be more efficient than a Carnot engine, operating between the same 2 reservoirs.” In real language, because heat flows from hot to cold and an engine must have some exhaust at lower temp, you can’t make a 100% efficient heat engine. Carnot’s equation: e = (Thot – Tcold)/Th Or e = 1 – Tc/Thot