Electric Field Question

[jeheron]

I was given this question. I have worked through it and get the answers below. I am unsure as to if they are correct or not. The method I have used was never discussed in lectures.

 

An electron with v= 3 x 10^6 m/s enters a region of uniform electric field between two charged plates. The plates are 0.1 meters apart.

 

The diagram included showed the electron entering the field between the plates from the side such that its direction was perpendicular to the field lines which flowed from the bottom plate (drawn horizontally) upward to the top plate. The field is uniform (E=200 N/C)

 

a) Find acceleration:

 

MY WORKING:

 

F= qE

= (1.6*10^-19)(200)

= 3.2*10^-17 N

 

therefore using F= ma

a= F/m

a= (3.2*10^-17)/(9.11*10^-31)

a= 3.513*10^13 m/s

 

b) Find time electron in field:

 

Assuming the electron enters the field halfway between the plates (from diagram):

 

distance travelled by electron (d) = (ut) + (1/2at^2)

 

t= sqrt((2d)/a)

= 1.7*10^-7 s

 

Is my reasoning sound?

I am high school student studying in Australia.

Cheers in adv.

[swansont]

part a: acceleration is a vector

 

part b: you only substituted in for a; the 2d is missing. (You've also assumed that the electron strikes a plate by using that calculation)

[jeheron]

Thanks for your help.

 

I just found the source the question was taken from. In the copy of the question we were given the photocopier had cut off "The plates are 0.1m lin length." That changes the method of calculation, making it alot easier.

 

Thanks again.