One particular plug-in power supply for a radio looks...

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 $\mathrm{V}$ AC 8 $\mathrm{W}$ Output 9 $\mathrm{V}$ DC 300 $\mathrm{mA}$ . Assume these values are accurate to two digits. (a) Find the energy efficiency of the device when the radio is operating. (b) At what rate is energy wasted in the device when the radio is operating? (c) Suppose the input power to the transformer is 8.0 $\mathrm{W}$ when the radio is switched off and energy costs $\$ 0.135 / \mathrm{kWh}$ from the electric company. Find the cost of having six such transformers around the house, each plugged in for 31 days.

Key Concepts

Energy Efficiency

This concept refers to the measure of how effectively a power supply converts input power into useful output power. It is generally calculated as the ratio of the output power to the input power, often expressed as a percentage. This foundational idea helps in understanding losses in conversion processes and overall system performance.

Energy Losses

Energy losses occur in any power conversion system due to inefficiencies, such as the dissipation of energy as heat. The difference between the input energy and the useful output energy represents the wasted energy, and calculating this difference is crucial for determining the rate at which energy is lost during operation.

Power and Energy Conversion

This involves the relationships and conversions between energy, power, and time. Power, measured in watts (W), is the rate at which energy is used or converted. Converting between watts and kilowatt-hours (kWh) is essential when evaluating energy consumption over time, particularly for cost analysis and energy management.

Energy Cost Calculation

When evaluating the cost of operating devices over time, the energy consumption in kWh is multiplied by the cost per kWh provided by the utility. This conversion is critical for understanding how energy use translates into monetary expense, particularly in situations where devices consume power even when not actively in use (standby power).

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