2. A heat engine receives 250 J of heat from the heater in one cycle and gives 100 J to the refrigerator . What is the efficiency of the machine?

The efficiency of a heat engine is defined as the ratio of the useful work output to the total heat input. In this case, the useful work output is the heat given to the refrigerator, which is 100 J. The total heat input is the heat received from the heater, which is 250 J.<br /><br />To calculate the efficiency, we can use the formula:<br /><br />Efficiency = (Useful work output / Total heat input) x 100%<br /><br />Plugging in the values, we get:<br /><br />Efficiency = (100 J / 250 J) x 100% = 40%<br /><br />Therefore, the efficiency of the machine is 40%.