[quote author=Fu-Kwun Hwang link=topic=33.msg7430#msg7430 date=1287670211]
There is an potential different between the gap which will form electric field \vec{E}
You can calculate acceleration \vec{a}=\frac{\vec{F}}{m}=\frac{q\vec{E}}{m}
And you can calculate how the charge will move (velocity,displacement)from acceleration.

thx , but how can I study the effect of magnetic  field in the  gap ??
and how the charge particle  will move in the gap ?? circular motion or straight one ?
does it effect with the magnetic field ??