One particular plug-in power supply for a radio looks similar to the one shown in Figure 33.23 and is marked with the following information: Input 120 V AC 8 W Output 9 V DC 300 mA. Assume that these values are accurate to two digits.
(a) Find the energy efficiency of the device when the radio is operating.
(b) At what rate does the device produce wasted energy when the radio is operating?
(c) Suppose that the input power to the transformer is 8.0 W when the radio is switched off and that energy costs $0.135/kWh from the electric company. Find the cost of having six such transformers around the house, plugged in for thirty-one days.