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## Homework Statement

7. In Fig. 28-35, an electron accelerated from rest through potential difference [tex]{V}_{1} = 1.00{\textcolor[rgb]{1.00,1.00,1.00}{.}}kV[/tex] enters the gap between two parallel plates having separation [tex]d = 20.0{\textcolor[rgb]{1.00,1.00,1.00}{.}}mm[/tex] and potential difference [tex]{V}_{2} = 100{\textcolor[rgb]{1.00,1.00,1.00}{.}}V[/tex]. The lower plate is at the lower potential. Neglect fringing and assume that the electron’s velocity vector is perpendicular to the electric field vector between the plates. In unit-vector notation, what uniform magnetic field allows the electron to travel in a straight line in the gap.

http://img228.imageshack.us/img228/4953/platepicgl6.jpg [Broken]

## Homework Equations

[tex]

{\vec{E}}_{p1} = {\frac{\vec{F}_{21}}{q_{2}}}

[/tex]

[tex]

{\Delta{V}_{p}} = {-}{\int_{r_{0}}^{r_{1}}}{\vec{E}_{p1}(r)}{\cdot}{d{\vec{r}}}

[/tex]

[tex]

{\vec{F}_{B}} = q{\vec{v}}{\times}{\vec{B}}

[/tex]

## The Attempt at a Solution

Ok, this problem does not seem that hard, however the wording isn’t really clear. So let me see if I understand this problem, you have an electron being accelerated by [tex]{V}_{1}[/tex] in to an electric field produced by two plates. Where the bottom plate has potential [tex]{V}_{2}[/tex].

Ok, so my question is if the bottom plate has potential [tex]{V}_{2}[/tex], does the top plate have a potential?

Also, what about the electric field between the two plates how can you tell in what direction the electric field points in?

Any help is appreciated.

Thanks,

-PFStudent

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