• Support PF! Buy your school textbooks, materials and every day products Here!

Electron speed

  • Thread starter kasse
  • Start date
  • #1
366
0

Homework Statement



The potential between the cathode and the screen of a computer is 12kV. What is the speed of an electron when it reaches the screen if it starts at rest at the cathode?

The Attempt at a Solution



First I found that the force that acts on the electron is 1.922E-15 N. I can then find the acceleration, but I need the distance between the cathode and the screen to find the speed.

Can I use energy conservation? Probably. Gravitational forces can be ignored. (1/2)mv^2 = Epot But what is the the expression for potential energy? Not mgh. Can I simply replace m with charge, g with E and h with the potential?

1/2)mv^2 = eEU

?
 

Answers and Replies

  • #2
275
2
Generally the electric and gravitational potentials will NOT take the same form - good try though. The answer is much simpler - it is contained in the problem statement (hint hint: what are the first 2 words?).
 
  • #3
366
0
The potential. Probably eV then? But why?
 
  • #4
249
0
The idea of replacing mgh with eEh is a bit careless but has some meaning if understood properly: Work done by constant forces is F*s, where F is -mg in the first case and eE in the second case. Since the change of energy equals -work, the analog expression of mgh is actually -eEh. However the electric field is not always uniform, so -Eh must generaly be replaced by Integral(-E*dh)=U
 
Last edited:

Related Threads for: Electron speed

  • Last Post
Replies
3
Views
4K
  • Last Post
Replies
11
Views
3K
Replies
1
Views
1K
Replies
2
Views
2K
Replies
0
Views
2K
Replies
1
Views
4K
Replies
3
Views
7K
Replies
2
Views
9K
Top