# Homework Help: Flux at Earth's surface and Poynting vector

1. Dec 11, 2011

### Silversonic

(On phone typing this, so preset subheadings haven't shown)

My first question asks me to use the Poynting vector, N, to calculate the average flux of radiation, knowing that the electric field of sunlight arriving at the surface of the Earth is 500Vm^-1

I use that the average flux of radiation is given by the average value of the Poynting vector, which is

<N> = E^2/(2Z), where Z is the impedence of free space, 377 Omhs.

This gives me an answer of 332Wm^-2

The next question asks me to find the ratio of the radiation pressure force on the earth to the gravitational pull of the sun. And it tells me that the flux of radiation in free space at the Earths orbit is 1400Wm^-2.

So apparently the flux at the orbit is greater than the flux at the surface? Considering the relationship of flux is inversely proportional to the distance squared, this is impossible. So why is my answer to the first part wrong?

Last edited: Dec 11, 2011
2. Dec 11, 2011

### SammyS

Staff Emeritus
Is the electric field of the sunlight arriving at the surface of the Earth, an average of some kind, or is it the peak value?

3. Dec 11, 2011

### Silversonic

It just says it is the typical value arriving at the Earth's surface. By typical I assume it means average.