Chapter 9 review


    Quantitative problems
      Section 9.2
      1. A lightbulb is connected to a 120 V household circuit. How much current does the lightbulb draw if it uses
        1. 100 W of power?
        2. 60 W of power?
        3. 150 W of power?
      2. In the Empire State Building Run-up, participants compete to see who can run up the 320 m Empire State Building the fastest. The current record holder, Paul Crake, who has a mass of 64 kg, covered this distance in 9 min and 33 s. What is the minimum average output power that his body produced while doing this?
      3. Sample EnergyGuide for a product
      4. Easy Does the refrigerator with the EnergyGuide label shown here use more or less energy than comparable appliances?
      5. Easy A man rides a bicycle that is connected to an electrical generator. If he rides as hard as he can, his body can produce a mechanical power of 500 W, but the generator is only 40% efficient at converting mechanical energy into electrical energy.
        1. How many 100 W incandescent light bulbs can he power?
        2. How many 100-W-rated compact fluorescent light bulbs can he power?
      6. Easy A small electric motor produces a force of 5 N that moves a remote-control car 5 m every second. How much power does the motor produce? Give your answer in watts and horsepower.
      7. Easy A fully charged cellphone battery contains 20,000 J of stored energy. If the cellphone uses 2 W of power, how long will the battery last? Give your answer in seconds, minutes, and hours.
      8. Medium In 2009, Usain Bolt set the world record of 9.58 s in the 100 m sprint. A published physical model of his race showed that he exerted approximately a constant horizontal force of 816 N and that 75,200 J of his work was used to overcome drag from the air.
        1. What was his average speed?
        2. How much total work did he do in the race?
        3. What was his average power output?
        4. What was his efficiency?
      1. Medium The Boston Globe reported on an electricity rate change:
        “[The electric utility] NStar filed for electric rate cuts yesterday that would save the average residential customer about $6 a month starting in January.... Counting both the cost of energy and delivery-oriented charges, the typical homeowner using 500 kilowatt-hours monthly would pay... less in Cambridge, NStar said.... Cambridge Electric’s power rate is slated to go from 6.351 to 5.626 cents [per kilowatt-hour], and the delivery cost from 7.282 to 6.782 cents [per kilowatt-hour].”
        1. How much energy is consumed monthly by the typical consumer referred to in the story?
        2. Does electric generation or delivery cost more per kilowatt-hour in Cambridge?
        3. What will be the new cost per kWh for electricity generation plus delivery?
        4. For typical consumers, what will be the change in their total electric bill owing to the rate change?
        5. Do you think that consumers will like or dislike this rate change? Support your argument by communicating evidence found in, or inferred from, the article.
      2. Medium What is the maximum height a 5 hp engine could push a 10 kg box in 5 s? Assume you could attach the engine to an ideal mechanical device with perfect (100% efficient) transfer of energy.
      3. Medium Suppose you have a solar energy conversion system with a sunlight collecting area of 10 m2. On a cloudless day sunlight has an intensity of about 600 W/m2.
        1. How much energy is collected in 1 hr?
        2. A single electric light bulb uses 25 W of power. How long could the collected energy keep the bulb lit?
        3. A very efficient cabin uses about 400 W on average over 24 hr. If the solar conversion to electricity is 15% efficient, can this collector supply the average power draw for the cabin?
      4. Challenging Imagine that a 1,000-acre (400-hectare) farm field can grow enough corn to produce 1.4 million liters (1.4×106 L) of ethanol, an alternative automobile fuel, over the course of a year. Each liter of ethanol can produce roughly 20 million joules (2×107 J) when burned. Now imagine that 20 trillion joules (2×1013 J) of energy had to be used to irrigate, fertilize, transport, and process all of this corn. Assuming that corn can be grown on this field indefinitely, would you consider this a renewable form of energy? Why or why not?

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