# Energy density of electric field

1. Apr 8, 2008

### Cheetox

1. The problem statement, all variables and given/known data

Assume that 10% of the energy dissipated by a 40W light bulb is radiated isotropically in the form of light. What is the magnitude of he E-field at a distance of 1m? What is it for sun light on the Earth's surface, given the Sun provides ~ 1400W/m2>

2. Relevant equations

Energy density = 1/2$$\epsilon$$$$_{}0$$E$$^{}2$$

3. The attempt at a solution

Ok so I know that we have 4W available, but how do I convert this into and energy per unit area on the surface or volume? I assume we have to use the equation for the energy denstiy of a magnetic field given above.

Last edited: Apr 8, 2008
2. Apr 8, 2008

### rohanprabhu

If it spreads out isotropically in all directions, then the power [4 W] is uniform at any point on the sphere of the radius, and can be given by:

$$I = \frac{1}{\pi}~Wm^{-2}$$

You have the same thing i.e. Intensity for the 'sun' question. Now, Intensity is simply:

$$I = \frac{Energy \times velocity}{Volume}$$

so.. i guess i've given u enough hint now.. [at what velocity does e.m energy travel?]

Last edited: Apr 8, 2008