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12.4 Heat Engines

12.4 Heat Engines. Heat engines convert heat into other useful forms of energy like mechanical or electrical. A car engine is an example. Heat Engines. Mechanical Energy can be obtained from Thermal energy ONLY WHEN Heat is allowed to flow from Hot to Cold.

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12.4 Heat Engines

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  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

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