1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electric field and velocity

  1. Oct 20, 2008 #1
    1. The problem statement, all variables and given/known data

    Two 4.0 cm diameter disks face each other, 2.0 mm apart. They are charged to +-12 Nc.

    Part A
    What is the electric field strength between the disks?
    E = 1.1 * 10^6 N/C

    Part B
    A proton is shot from the negative disk toward the positive disk. What launch speed must the proton have to just barely reach the positive disk?
    2. Relevant equations

    E=(Q/(epsilon zero * A)
    v=x/t

    3. The attempt at a solution

    I was able to figure out part A. I just need help with part B.
    V = ?
    x = 2mm
    t = ?

    Doesn't seem like this part should be difficult, so I must be missing something simple...

    Thanks in advance for any help.
     
  2. jcsd
  3. Oct 20, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Two ways to attack this:

    (1) Using kinematics. Hint: What's the proton's acceleration?

    (2) Using energy. Hint: What's the potential difference between the plates?
     
  4. Oct 20, 2008 #3
    So...this is what I have so far.

    F = qE

    F = (12*10^-9)(1.1*10^6)
    F = (.0132N)

    F = ma

    mass of proton = 1.67*10^-27

    .0132 = (1.67*10^-27) (a)
    7.9*10^24
     
  5. Oct 20, 2008 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Since you need the force on the proton, use the charge of a proton. (Not the total charge on the plate!)
     
  6. Oct 20, 2008 #5
    F=(1.6*10^-19)(1.1*10^6)
    F = 1.76*10^-13

    F=ma

    1.76*10^-13 = (1.67*10^-27)(a)
    1.05*10^14 = a
     
  7. Oct 20, 2008 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Once you've found the acceleration, it's time for kinematics. You'll need a kinematic equation relating speed and distance.
     
  8. Oct 20, 2008 #7
    v = x/t
    f = m/a

    Those aren't it...hm...

    Kinetic energy = .5(mass)(velocity)^2

    so...(if I remember correctly) total energy = Kinetic energy + potential energy

    potential energy = (mgh)

    Sadly, if this is the correct approach I do not remember what variables are on the total energy side.

    *edit*
    O yeah...I forgot I was looking for an equation relating speed and distance. I am having trouble finding one.
     
  9. Oct 20, 2008 #8
    Hm...perhaps this equation
    v^2 = vo^2 + 2a(X - Xo)

    *edit*
    Yup...that would be the correct equation.

    Thanks for the help Doc Al.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Electric field and velocity
Loading...