Heat Engines PDF

Heat Engines Topic 11.5 Pg 401 - 404 Waterwheel  Water in high reservoir flowing downhill can be used to produce useful energy. Water-flow vs Heat-flow  Using the concept of waterwheel, the heat also always flow from hot reservoir to cold reservoir.  This t...

Heat Engines Topic 11.5 Pg 401 - 404 Waterwheel  Water in high reservoir flowing downhill can be used to produce useful energy. Water-flow vs Heat-flow  Using the concept of waterwheel, the heat also always flow from hot reservoir to cold reservoir.  This thermal energy flow can be also used to convert it into some useful form of energy.  Heat Engine  A heat engine converts thermal energy from a hot reservoir into useful work. Some heat is exhausted into a cold reservoir, limiting efficiency.  Schematically, all heat engines are same. Heat engine Part of the thermal energy from the hot reservoir is turned into work while the rest is expelled to the cold reservoir. Operation of a Heat Engine  Work done by a Heat Engine No net energy is transferred into or out of the heat engine. The useful work done is equal to the difference between heat energy transferred from the hot reservoir and the heat exhausted into the cod reservoir. 𝑊𝑜𝑢𝑡 = 𝑄𝐻 − 𝑄𝐶 Thermal Efficiency The thermal efficiency e of a heat 𝑤ℎ𝑎𝑡 𝑦𝑜𝑢 𝑔𝑒𝑡 engine is defined as the work done by  𝑒= 𝑤ℎ𝑎𝑡 𝑦𝑜𝑢 ℎ𝑎𝑑 𝑡𝑜 𝑝𝑎𝑦 the engine, Weng, divided by the energy absorbed during one cycle: 𝑊𝑜𝑢𝑡  𝑒= 𝑄ℎ 𝑇𝑐  𝑒𝑚𝑎𝑥 =1 − 𝑇𝐻 Thermal Efficiency  The maximum possible efficiency is fixed by second law.  The theoretical maximum efficiency of an ideal heat engine 𝑇𝑐  𝑒𝑚𝑎𝑥 =1 − is given. 𝑇𝐻  The actual efficiency of real heat engines is usually noticeably less that the given  Heat Engine with 100% efficiency No heat engine can operate without exhausting some fraction of the heat into a cold reservoir.  Quiz Time Which of the following changes would increase the maximum theoretical efficiency of the heat engine? 𝑇𝑐 𝑒𝑚𝑎𝑥 = 1 − 𝑇𝐻 Increasing Increasing Decreasing Decreasing temperature of temperature of temperature of temperature of hot reservoir cold reservoir hot reservoir cold reservoir