What is the Light Intensity 3 Meters from the Bulb?

[dewdrop714]

stuck on light intensity problem...

1. Homework Statement

http://s685.photobucket.com/albums/vv212/dewdr0p714/?action=view&current=physicsexam2q5withoutphotocell.jpg

A conducting rod slides down 2 frictionless copper tracks at a constant velocity.
There is a .50 T magnetic field perpendicular to the rod. The rod maintains electrical contact at all times. The rod has a length of 1.0 m and a mass of .25kg. A 1.5 Ohm light bulb is attached between the tops of the tracks.

What is the intensity of the light from the light bulb at a distance 3.0 away from the bulb?



2. Homework Equations

S = c / (magnetic permeability of free space) * B^2


3. The Attempt at a Solution

S = ?
B = .50
c = 3.00 *10^8 m/s
(magnetic permeability of free space) is a constant... = 4pi *10^-7 T*m/a

if all these variables are plugged into the equation i can solve for S and get the intensity. But "3 meters" is mentioned in the problem. I know I am using the wrong formula...which one am i supposed to use?

 

[Delphi51]

The rod moving through the magnetic field acts as a generator. The generator formula is
V = LvB (potential = length x velocity x magnetic field).
Knowing the potential, you can use electricity formulas to find the current and power delivered to the light bulb.

Unfortunately no efficiency is given for the light bulb - perhaps you are supposed to look up the efficiency of a typical incandescent bulb in order to estimate the light power produced. The light will spread out in every direction, illuminating an imaginary sphere 3 m in radius, and you want the number of watts of power per square meter on it.