# Homework Help: Light in a Box

1. Oct 3, 2008

### andrewm

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

A flashlight is shining into a box. There are no reflections. We know the power of the flashlight in Watts. We know the volume of the box.

Question: How much energy (associated with the EM field) is in the box?

2. Relevant equations

3. The attempt at a solution

Integrate the power density (J s^-1 m^-3) over the volume of the box. But what is the power density?

Hints would be preferred over full solutions :)

2. Oct 3, 2008

### HallsofIvy

You don't need the power "density" if you already know the power! A Watt is one Joule per second. Assuming all the light energy of the is absorbed by the box, which is implied in your question, just multiply the given wattage of theflashlight by the length of time the flashlight is shined on the box.

3. Oct 4, 2008

### andrewm

The flashlight is left on indefinitely, and the light is transmitted (100%) out of the box. I should have been clear that there is no absorption or reflection.