E3201_Homework_2018_00.pdf

# E5 a cable tv box is measured to consume 15 w of

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leaving the - terminal into the load? E5. A cable TV box is measured to consume 15 W of standby power when it is turned off and 20 W of power when it is turned on. (a) If 10 million such boxes are in use, and an average of half are turned on at any time, how much power do they require together? (b) If electric energy costs 10 cents per kWh to generate and distribute, how much does it cost to keep these boxes running for a month? (c) If standby power could be reduced from 15 W to 5 W, how much energy and money would be saved each year? E6. A homeowner wants to install enough solar panels on her roof to fully offset, on average, the roughly 7000 kWh that her utility bills say she uses each year. Solar panels are marketed using their maximum power output under ideal conditions, but given her roof pitch, tree shading, and climate, the average power generated by each panel would only be 1 / 7 of its maximum power level when averaged over the year, day and night. If each panel has a maximum power of 250 W, how many panels does she need? E7. Consider a battery with a constant voltage that can supply 13 MJ of energy and 94 Ah of charge before needing recharge. If it is connected to a load that is absorbing 3 W of power, what is the magnitude of the current flowing from the battery to the load? E8. The voltage of a battery can vary somewhat during use. If it might be 5% different than nominal, and it is driving a load with a resistance that might be 5% different than 1

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estimated, what % error might there be in the estimate of power supplied? Given this un- certainty, explain why it could be misleading to state voltage, current, and power estimates using all 10 digits that might be generated by a calculator. Please hand in by 4PM Friday 9/7 in the TA mailbox marked ”E3201” just inside the EE student lounge, 1301 Mudd. Don’t forget your name! 2
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