The power of sunlight reaching each square meter of the earths surface on a clea

Question

The power of sunlight reaching each square meter of the earths surface on a clear day in the tropics is close to 1000 W. On a winter day in Manitoba, the power concentration of sunlight can be 100 W/m^2. Many human activities are described by a power per unit area on the order of 10^2 W/m^2 or less. (a) Consider, for example, a family of four paying $66 to the electric company every 30 days for 600 kWh of energy carried by electrical transmission to their house, which has floor dimensions of 13.0 m by 9.50 m. Compute the power per unit area used by the family. (b) Consider a car 2.10 m wide and 4.90 m long traveling at 55.0 mi/h using gasoline having "heat of combustion" 44.0 MJ/kg with fuel economy 25.0 mi/gal. One gallon of gasoline has a mass of 2.54 kg. Find the power per unit area used by the car. (c) Explain why direct use of solar energy is not practical for running a conventional automobile. (d) What are some uses of solar energy that are more practical?

Explanation / Answer

Part A) Area: 13 * 9.5 = 123.5 m^2 Power consumed = Energy used / Time = 600 KWh / 30 days = 600*1000*3600 J/(30*24*60*60 seconds) = 833. 33 W PPA(power-per-unit-area) = 833.33 W/123.5 m^2 => 6.75 W/m^2

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