Laboratory Exercise 12 – THERMAL EFFICIENCY（实验室锻炼12 热效率）.pdf

Laboratory Exercise 12 – THERMAL EFFICIENCY Heat Engine A heat engine uses the temperature difference between a hot reservoir and a cold reservoir to do work. Usually the reservoirs are assumed to be very large in size so the temperature of the reservoir remains constant regardless of the amount of heat extracted or delivered to the reservoir. This is accomplished in the Thermal Efficiency Apparatus by supplying heat to the hot side using a heating resistor and by extracting heat from the cold side using ice water. In the case of the Thermal Efficiency Apparatus, the heat engine does work by running a current through a load resistor. The work is ultimately converted into heat , which is dissipated by the load resistor (Joule heating). A heat engine can be represented by a diagram (Figure 1). The law of Conservation of Energy (First Law of Thermodynamics) leads to the conclusion that Q = W + Q , the heat input to the H C engine equals the work done by the heat engine on its surroundings plus the heat exhausted to the cold reservoir. Figure 1: Heat Engine The efficiency of the heat engine is defined to be the work done divided by the heat input So if all the heat input was converted to useful work, the engine would have an efficiency of one (100% efficient). Thus, the efficiency is always less than one. NOTE: Since you will be measuring the rates at which energy is transferred or used by the Thermal Efficiency Apparatus all measurements will be power rather than energy. So P = dQ /dt and then the equation Q = W + Q becomes P = P + P and the efficiency H H H C H W C becomes Carnot showed that