55. One particular plug-in power supply for a radio lookssimilar to the one shown in Figure 33.20 and is markedwith the following information: Input 120 V AC 8 WOutput 9 V DC 300 mA. Assume these values are accu-rate to two digits. (a) Find the energy efficiency of thedevice when the radio is operating. (b) At what rate isenergy wasted in the device when the radio is operat-ing? (c) Suppose the input power to the transformeris 8.00 W when the radio is switched off and energycosts $0.110/kWh from the electric company. Find thecost of having six such transformers around the house,each plugged in for 31 days.

We have Vi=120 VPi=8 WVo=9VI=300 m A=300 ×10-3Aa)Po=VoI=9×300×10-3=2.7 WEfficiency…

55. One particular plug-in power supply for a radio looks similar to the one shown in Figure 33.20 and is marked with the following information: Input 120 V AC 8 W Output 9 V DC 300 mA. Assume these values are accu- rate to two digits. (a) Find the energy efficiency of the device when the radio is operating. (b) At what rate is wasted in the device when the radio is operat- energy ing? (c) Suppose the input power to the transformer is 8.00 W when the radio is switched off and energy costs $0.110/kWh from the electric company. Find the cost of having six such transformers around the house, each plugged in for 31 days.

55. One particular plug-in power supply for a radio looks similar to the one shown in Figure 33.20 and is marked with the following information: Input 120 V AC 8 W Output 9 V DC 300 mA. Assume these values are accu- rate to two digits. (a) Find the energy efficiency of the device when the radio is operating. (b) At what rate is wasted in the device when the radio is operat- energy ing? (c) Suppose the input power to the transformer is 8.00 W when the radio is switched off and energy costs $0.110/kWh from the electric company. Find the cost of having six such transformers around the house, each plugged in for 31 days.