# Homework Help: Sunlit earth

1. Mar 3, 2010

### quietrain

1. The problem statement, all variables and given/known data
At Earth’s location, the intensity of sunlight is 1.5 kW/m^2. If no energy escapes
Earth, by how much would Earth’s mass increase in a day? (Radius of Earth is
6.37 × 10^6 m.)

3. The attempt at a solution

ok so, at any one time, half of the earth is lit.

so surface area lit = 1/2 x 4$$\Pi$$r2 =
2.55x1014m2

intensity = 1500 W/m2

so energy per second on earth = 1500 x 2.55x1014 = 3.82 x 1017 J

energy per day = 3.82 x 1017 x 60 x 60 x 24 = 3.3 x 1022

so E = mc2
so m = E / c2 = 3.67 x 105 kg

but the correct answer is half of this. so where did i go wrong? thanks for help!

2. Mar 3, 2010

### Staff: Mentor

You are treating the light as being perpendicular to the surface area of the sphere, but it's not. The light comes in parallel. (Treat the earth as a disk, not a sphere.)

3. Mar 3, 2010

### quietrain

ooo! thanks a lot doc! now i remembered my lecturer said this too !