Astandard 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?

(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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