# Homework Help: An electron is launched between two plates

1. Dec 21, 2016

### sliperyfrog

1. The problem statement, all variables and given/known data
An electron is launched between two parallel, neutral, conducting plates that are each L long and separated by a distance d with a uniform magnetic field of magnitude B permeated between them.

(a) What is the minimum speed the particle must have to traverse the region without striking either plate?

(b) What electric field prevents the electron from deflecting? Indicate the polarity of the plates (once charged) if they are to provide this field.

2. Relevant equations

F = BIL

F = ma

3. The attempt at a solution

So what i did was set the equations equal to each other to get BIL = ma then I turned a into dv/dt and turn I into q/t getting me BqL/t = mdv/dt I then rearrange the equation so look like BLq/mt dt = dv I then took the integral of both sides and got v = (BqL/m)ln(t) the problem is I don't how to get d into the equation and I don't think t should be in the answer.

Last edited: Dec 21, 2016
2. Dec 21, 2016

### Ssnow

Hi, mmm strange combination of formulas ... , the force of an electron $e$ that moves with a velocity $v$ in a magnetic field $B$ is $F=evB$...

3. Dec 21, 2016

### Ssnow

A second hint: think if the trajectory of an electron in a magnetic field will be linear or circular ?