# Homework Help: Magnetic flux

1. Dec 13, 2015

### tankarish

1. The problem statement, all variables and given/known data
A magnet is used to control negative ions and only allow those with a specific mass to pass through a narrow gap opening . The magnet will bend the ion beam in a quarter circle of radius 50 cm .
In an experiment, a negative ion accelerated by a voltage of 3.0 kV.

Which magnetic flux density must the magnets have for these molecules with mass 277 u to follow the circular path through the magnet and get through the gap opening ?

2. Relevant equations
QU = MV ^ 2 * 1 / r , then mv ^ 2 * 1 / r = QVB

3. The attempt at a solution
Because we have volt , I think QU = MV ^ 2 * 1 / r , then mv ^ 2 * 1 / r = QVB
But we do not have Q, or v. Thus I do not know how I solve this .

Sorry for the English, but it seems the problem cant be solved?

2. Dec 13, 2015

### JeremyG

I'm not sure what equation you are using here. What is U?

You do not need Q to solve this question. You do, however, need v. This v would be the velocity of the negative ions after being accelerated by the voltage of 3kV, right before they enter the magnetic field. Can you find what is this v using the concept of energy?