A convenient measure of the performance of a refrigerator is expressed by the coefficient of performance w, which is the ratio of the heat extracted from the low-temperature resevoir to the work done on the refrigerant. Unlike the thermal efficiency n, w may be considerably larger than unity. Derive an expression for the heat rejected to the high-temperature reservoir. Such a refrigerator is called a "heat pump" and can warm a house in winter by refrigerating the ground, outside air, or water supplied in the mains. Assume a value of 5 for the coefficient of performance and comment on the effectiveness of a heat pump.