1. Limited time only! Sign up for a free 30min personal 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 potential difference?

  1. Jun 22, 2013 #1
    1. The problem statement, all variables and given/known data

    An X-ray tube is similar to a cathode-ray tube. Electrons are accelerated to high speeds at one end of the tube. If they are moving fast enough when they hit the screen at the other end, they give up their energy as X-rays (a form of nonvisible light).

    (a.) Through what potential difference should electrons be accelerated so that their speed is 1.00% of the speed of light when they hit the other end of the tube?
    (b.) What speed would this potential difference give to protons? Express your answer in m/s.
    (c.) What potential difference would be needed to give protons the same kinetic energy as the electrons?

    This is literally all we are told for this problem so I'm super confused. How am I supposed to go about solving this? Any help would be greatly appreciated. Thank you in advance.

    2. Relevant equations

    ΔV = ΔU/q0 = -WAB / q0


    3. The attempt at a solution

    so if our change in speed is 1%, I think my [itex]\Delta[/itex]V would be .01, but I have no idea what to do from there (or if I'm even on the right track.
     
  2. jcsd
  3. Jun 22, 2013 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Not a "change in speed". The initial speed is zero, and the final velocity is [what]?
    No, that does not work. Just check the units: ΔV is an energy, 0.01 is a number.
     
  4. Jun 22, 2013 #3
    Oh god I feel so dumb now... The final velocity would be 1% of the speed of light, so 1% of 299792458 m/s, which would be 2997924.58 (or 2.99 x 10^6).

    So our ΔV would be 2997924.58 ?

    Am I being really dumb and mixing up velocity and volt?
     
  5. Jun 22, 2013 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Stop guessing, please.
    What is ΔV? It is neither velocity nor Volt. How is that quantity related to the speed of electrons?
     
  6. Jun 22, 2013 #5
    I know that

    V=U/q0

    And that

    VB-VA=UB/q0-UA/q0=-WAB/q0
    ΔV=ΔU/q0=-WAB/q0
     
  7. Jun 22, 2013 #6

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    aszty8, my question is really basic (and I think your problem is exactly at that point): What type of physical quantity is V (and ΔV)? Is it a length? Is it a time? No, it is not, but do you know what it is?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted