Physics Intensity Question?

[Casey6930]

The intensity of electromagnetic waves from the sun is 1.4kW/m^2 just above earths atmosphere. 80% reaches the surface. If you model your back as a 30cm * 50cm rectangle. How many joules of solar energyfall on your back as you tan for 1.00 hour? Any help on the problem would be great.

[bvalle]

If 80% of the solar irradiance at the top of the atmosphere reaches the surface, then the irradiance at the surface is0.8 * 1.4 kW/m^2 = 1.12 kW/m^2Now multiply by the surface area to get the power1.12 kW/m^2 * 0.3m *0.5m = 168 WattsA Watt is a Joule/s, and in 1 hour there are 3600 seconds.168 Watts * 3600 s = 604 800 joules.

[billrussell420657]

80% of 1.4kW/m^2 is 1.12kW/m^2Your back is 30cm x 50cm or 1500cm2 or 0.15m2Radiation hitting your back is 1.12kW/m^2 x 0.15m2 = 0.168kW or 168 watt1 joule is 1 watt sec168 watts x 3600sec/hr = 605,000 W-sec or joulesedit corrected arithmetic