One particular plug-in power supply for a radio looks similar to the one shown i
ID: 1375746 • Letter: O
Question
One particular plug-in power supply for a radio looks similar to the one shown in the figure below and is marked with the following information: Input 120 V AC 7 WOutput 3 V DC 340 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? (W)
(c) Suppose the input power to the transformer is 7.00 W when the radio is switched off and energy costs $0.145/kWh from the electric company. Find the cost of having foursuch transformers around the house, each plugged in for 31 days.
Explanation / Answer
a)
output power = I*V = 340*10^-3*3 = 1.02 V
efficiency = o.p/i.p = 1.02/714.57 %
b)
wasted power = 7 - 1.02 = 5.98 W
c)
31 days = 31*24*3600 = 2678400 seconds
energy = 7*2678400 = 18748800 J
total cost = 4*18748800*0.145/3.60*10^6 = $3.02
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