# Homework Help: Electric fields and parallel plates

1. Dec 13, 2005

### F.B

I need help on a few questions.

1.An alpha particle has a positive charge of 2e and a mass of 6.6 x 10^-27 kg. With what velocity would the particle reach the negative plate of a parallel plate apparatus with a potential difference of 2.0 x 10^3 V.
a)if it started from rest at the positive plate.
b)if it started from rest at a point halfway between the plates.

I got the answer for a, it 4.4 x 10^5 m/s. But i dont know how to do b.

2. An electron is accelerated through a uniform electric field of magnitude 2.5 x 10^2 N/C with an initial speed of 1.2 x 10^6 m/s parallel to the electric field.
a)Calculate the speed of the electron after it has travelled 2.5 cm in the field.

I dont know how to do this one at all.

3. Two electrons are fired at 3.5 x 10^6 m/s directly at each other.
a)Calculate the smallest possible between the two electrons.

2. Dec 13, 2005

### Astronuc

Staff Emeritus
Well how did you do part 'a' of question 1? In part b, the acceleration takes place over half the distance. Hint: The electric field is contant between paralle plates.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elefie.html

2. What is the force on a charge in an electrical field? Get F, and then solve for a. Then what is the equation for final velocity, initial velocity, acceleration and distance.

http://hyperphysics.phy-astr.gsu.edu/hbase/mot.html#mot1

3. Each electron has a kinetic energy and approach each other until they stop at some distance, at which point the energy is all electrical potential energy.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elepe.html