![]() ![]() ![]() The hot and cold objects are called the hot and cold reservoirs. (b) A heat engine, represented here by a circle, uses part of the energy transferred by heat to do work. įigure 12.14 (a) Heat transfers energy spontaneously from a hot object to a cold one, as is consistent with the second law of thermodynamics. The temperature of the hot reservoir is T h, T h, and the temperature of the cold reservoir is T c T c. In physics, a reservoir is defined as an infinitely large mass that can take in or put out an unlimited amount of heat, depending upon the needs of the system. As shown in Figure 12.14, heat transfers energy, Q h Q h, from the high-temperature object (or hot reservoir), whereas heat transfers unused energy, Q c Q c, into the low-temperature object (or cold reservoir), and the work done by the engine is W. Heat engines do work by using part of the energy transferred by heat from some source. Heat now transfers energy from the gas to the surroundings, so that the gas’s pressure decreases, and a force is exerted by the surroundings to push the piston back through some distance.Ī cyclical process brings a system, such as the gas in a cylinder, back to its original state at the end of every cycle. To repeat this process, the piston needs to be returned to its starting point. (c) Heat transfer of energy to the environment further reduces pressure in the gas, so that the piston can more easily return to its starting position. Gas pressure and temperature decrease during expansion, indicating that the gas’s internal energy has decreased as it does work. (b) The force exerted on the movable cylinder does work as the gas expands. Thus, heat transfer of energy to the gas in the cylinder results in work being done.įigure 12.13 (a) Heat transfer to the gas in a cylinder increases the internal energy of the gas, creating higher pressure and temperature. The gas does work on the outside world, as this force moves the piston through some distance. This increases the gas temperature, which in turn increases the pressure of the gas and, therefore, the force it exerts on a movable piston. Fuel combustion releases chemical energy that heat transfers throughout the gas in a cylinder. Gasoline and diesel engines, jet engines, and steam turbines that generate electricity are all examples of heat engines.įigure 12.13 illustrates one of the ways in which heat transfers energy to do work. A heat engine does exactly this-it makes use of the properties of thermodynamics to transform heat into work. One of the most important things we can do with heat is to use it to do work for us. In this section, we’ll explore how heat engines, heat pumps, and refrigerators operate in terms of the laws of thermodynamics. ![]() Heat Engines, Heat Pumps, and Refrigerators Ask students whether they can explain the limits on efficiency in terms of what they have now learned. Review the ideal gas law, laws of thermodynamics, and entropy. Return again to the discussion of efficiency that was begun at the start of the module. ![]()
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