Charged Particles in Electric and magnetic fields

[sarahdee]

Homework Statement

An electron, initially at rest, is accelerated through a potential diff of 15.0 kv. It is then allowed to circulate at right angles to a uniform magnetic field of strength 2.35 T.
Calculate the electrons final speed before entering the magnetic field.

Homework Equations

f= eq
f=bqv

The Attempt at a Solution

(15000)q = q(2.35)v
(15000)= 2.35v
v= approx. 6383

 

[Tusike]

Yeah OK so what do you know about this topic?
If I understand this correctly, the question is what will the speed of the electron be after it has been accelerated by 15kV's, if it started with v = 0.

 

[sarahdee]

I just edited my original post,
to show how I worked it out before.

 

[Tusike]

But how does the magnetic field have any significance? Doesn't the question ask the speed BEFORE the electron goes into the magnetic field?

 

[sarahdee]

But won't that leave me with insufficient info?

because I then have to calculate a with (f=ma), using the f=eq formula.
But i will require the time that it is in the field to know it's final speed

 

[Tusike]

All you need to know is the total work done on the electron (W=U*q) and and the work-theory or whatever you call it when dE = W. So this gives you the equation U*q = .5*m*v^2 - 0, where q is the charge of the electron and m is it's mass.