# Homework Help: Conducting rail ciruit, against gravity

1. Jun 5, 2010

### Frostfire

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

So my text book has a lot of example of conducting rails moving horizontally with unknown variables of F and B, but how would you approach a problem that has such a device pointed vertically with the moving bar droping? lets say R and B are known and we need to find I(current) sufficient to counteract gravity so it doesn't accelerate past a velocity required to generate that current.
2. Relevant equations
Fb=qv x B
R = delta V/I
Fg = mg (call the mass of the rod m )
I = p/delta V

3. The attempt at a solution

I played with it, but not sure if Im right.The part im stuck on is what relation of the magnetic field do I set equal to Fg

is it Fg= qV x B

2. Jun 5, 2010

### Frostfire

So I found a formula for magnetic levitation that allows me to relate magnetic field strength and gravity

F= IlB=mg

if Im right this allows me to determine the current needed to counteract gravity.Does this sound right? Anybody no where that formula is derived from?

3. Jun 6, 2010

### Frostfire

Can somebody give me some advice here? Its kind of hard to ask questions that aren't in the book, if your unsure of what your doing