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9 May, 17:29

A standard flashlight battery can deliver about 4.2 W·h of energy before it runs down. (a) If a battery costs 80 cents, what is the cost in dollars of operating a 100 W lamp for 7.0 h using batteries? (b) What is the cost in dollars if power is provided at the rate of 6.0 cents per kilowatt-hour?

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  1. 9 May, 20:48
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    (a) $133.6

    (b) $42

    Explanation:

    (a) First, calculate the energy that the lamp needs for 7 hours with this formula:

    E=Pt

    Where E is the energy, t is the time and P is the potency.

    E=100W*7.0h=700Wh=0.700kWh

    Whit this, calculate the number of batteries: dividing the total energy needed between the energy delivered by one batterie:

    N=700/4.2=166.66≈167 batteries

    The total cost is the number of batteries time the cost of each battery.

    Total Cost=0.8*167=$133.6

    (b) Multiply the energy needed in kilowatts hours by the cost of each kilowatt:

    Total Cost=0.06*0.70=$42
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