# Energy and intensity problem

1. Sep 17, 2008

### Aeighme

1. The problem statement, all variables and given/known data
A super nova releases 2.0×1045 J of energy. It is 1500 ly from earth. If you were facing the star in question, and your face was a circle 7 cm in radius, how much energy would reach your face?

2. Relevant equations
I=P/A

3. The attempt at a solution
I used I=P/A to get the intensity of the sun at the point of the face, but was unsure of how to use that number to determine the energy on 'my' face.

2. Sep 17, 2008

### edziura

Assume that the energy is uniformly distributed over the face of a sphere of radius 1500 ly.

3. Sep 17, 2008

### Aeighme

but which equation would connect the energy to the intensity? =/

4. Sep 17, 2008

### edziura

I = P / A isn't relevant here. You are given the total energy released, not the power (rate of energy release).

5. Sep 17, 2008

### Aeighme

oh, well I feel silly.
hopefully I can find the equation I should use in my book.