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Work-Energy, Acceleration, and Plates

  1. Jan 26, 2013 #1
    1. The problem statement, all variables and given/known data

    34fmf5i.jpg

    The magnitude of the instantaneous acceleration experienced by an electron as it first enters the region containing the perpendicular fields is?

    29elzdy.jpg

    Maximum acceleration of an alpha particle at its closest approach to a nucleus of a gold atom, expressed in s.n is?

    2. Relevant equations



    3. The attempt at a solution

    For the first one, i did [itex]\sqrt{}2vq/m[/itex] to find the Vf as it exits the plates and enters the other plates, Then i assumed Fnet= Fm+Fe = Bqvf+ Eq / Me =a, which i was wrong.

    I dont know how to start? Ek+Ep=Ek? Then how do i find acceleration?



    You obviously dont need to give a straight up answer i would rather try to understand it as i will be writing a diploma monday.
     
  2. jcsd
  3. Jan 26, 2013 #2

    gneill

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    Staff: Mentor

    You should create separate posts for separate questions. This will help to minimize confusion when there are multiple interleaved responses.

    For the first problem, consider that the magnetic and electric forces may have different directions. Either determine the directions before summing (and treat accordingly), or use the vector version of the Lorentz force to handle it.
     
  4. Jan 26, 2013 #3
    Wait whats lorentz force? And did i even set it up right? well magnetic force is down, but i never learned to vector add two fields or forces, well i suppose i did, but isnt the electric field down (+ to -) and mag force is down using the hand rules, and since the electron is being deflected down. But than again the bottom plate is negative too.
     
  5. Jan 26, 2013 #4
    The simple google search of lorentz force using Q[E+[v*b]] divide my mass to find a did not work.
     
  6. Jan 26, 2013 #5

    gneill

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    Staff: Mentor

    It's a vector equation, not a scalar equation.

    What's the direction of the electric force on the electron when it's between the plates?
     
  7. Jan 27, 2013 #6
    Oh Fe is up and Fm is down making the velocity negative in lorentz force, both seemed to work, but for lorentz force, does that work for any charge in any electric and magnetic field or does it have a very specific theoretical aspect to it?
     
  8. Jan 27, 2013 #7

    gneill

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    Staff: Mentor

    The so-called Lorentz Force is just the net force experienced by a charge moving in combined electric and magnetic fields. Yes, works for any charge in any electric or magnetic fields. It takes into account the directions of the fields and the direction of motion of the charge, which is why it is expressed as a vector equation. You can compute and sum the forces separately provided that you take care to determine their directions and sum appropriately.
     
  9. Jan 27, 2013 #8
    Well thank you for that that would save me loads of time in a diploma question, but also thank you for the vector problem i missed in my calculations. Both work, making magnetic force negative, and the velocity in lorentz law yielded the same answers.

    How about the second one, only thing i remember about potential energy is that a positive particle has more potential energy if it moved against the field, so toward the positive plate, and a negative is moved from the negative plate to the positive, that is if i remember correctly. I reposted the second one as you asked. I really dont understand it,
     
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