# Applications of Magnetic Fields

1. Feb 28, 2012

### stonecoldgen

1. The problem statement, all variables and given/known data
An ion with a charge of +1e enters a region where the electric field produced by parallel plates at 630V separated by 7.0mm is at right angles ot a magnetic field B=450mT. The ion moves a straight line. Find the magnitude of the electric field and the speed of the ion.

2. Relevant equations
F=qvB
Ampere's Law B=μ0I/2∏r

3. The attempt at a solution
Is this a mass spectrometer?

first of all, because the plates are parellel the force is maximized (sin90=1)

I am confused, isn't the ion supposed to curve in a radius defined by: r=mv/qB (mv2/r=qvB)

I just don't know what to do since the ion doesn't curve.

Thanks.

2. Feb 28, 2012

### Staff: Mentor

A moving charge in a magnetic field curves because the Lorentz force acts as a centripetal force. In this case the fields are arranged in such a way that the electric force cancels the magnetic force, so the path never bends. But since the force on the moving charge depends upon its velocity, this cancellation will occur only for same-charge particles with a specific velocity.

It's not a mass spectrometer; it behaves as a velocity selector.

3. Feb 28, 2012

### stonecoldgen

sorry, can you please tell me how to work with velocity selectors? thanks I really appreciate it

4. Feb 28, 2012

### Staff: Mentor

Look up the Lorentz Force. It's comprised of the sum of the forces on charge due to electric and magnetic fields. In this problem you're told that the particle moves in a straight line at some velocity v, so what's the net force?

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