# Final velocity of an electron

1. Nov 15, 2005

### fran008

I have this question that I have been pondering about for over an hour now :
What is the final speed of an electron accelerated through a potential difference of 200V if it has an initial speed of 2.0x10-6C and mass of electron +9.11x10-31.
Now I read the forum rules and I knowI have to help myself if I want others to help me...but Im totally stuck: because the only method I can think of is obviously wrong i.e. finging the weight of electron and that being the electrical force. Also even if I got that and could get the electric field strenght and distance between the pd...what next?
Thanks
Fran008

2. Nov 15, 2005

### whozum

Then you would say that the total force is the electric force (F=qE) and follow newtons second law (F=ma) and combining the two you can find the acceleration (while in the field).

3. Nov 15, 2005

### fran008

yes but then how do you find the resulting velocity once out of the field?

4. Nov 15, 2005

### whozum

With [itex] v_f = v_i + at [/tex] but you'd need to know how long you were in the field or atleast how far youve traveled in the field.

5. Nov 15, 2005

### chroot

Staff Emeritus
This problem is much simpler than you're making it out to be, whozum. I've told you before not to answer if you're not sure you are helping!

An electron accelerated through a potential difference of 200V will gain 200 eV of kinetic energy. All you need to do is use K = 1/2 m v^2 to find its final velocity.

- Warren

6. Nov 15, 2005

### fran008

Oh thanks that was relatively simple! But its always like that when you manage to solve it!Even great scientists agree...Im currently reading "The Double Helix" and it says that after they cracked DNA structure they thought"how simple why didnt we see it before"! LOL!