# Homework Help: Stuck on light intensity problem

1. Mar 21, 2009

### dewdrop714

stuck on light intensity problem....

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

http://s685.photobucket.com/albums/...ew&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. Relevant 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 im using the wrong formula...which one am i supposed to use?

2. Mar 21, 2009

### Delphi51

Re: stuck on light intensity problem....

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.