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Magnetic Field, Potential, Velocity

  1. Aug 28, 2008 #1
    1. The problem statement, all variables and given/known data
    An electron travels with speed 1.0 x 107
    m/s between the two parallel charged
    plates shown in the adjacent figure. The
    plates are separated by 1.0 cm and are
    charged by a 200 V battery. What magnetic
    field strength and direction will allow the
    electron to pass between the plates without
    being deflected?


    2. Relevant equations
    B=([tex]\mu[/tex]/4[tex]\pi[/tex])(qvsin[tex]\theta[/tex]/r[tex]^{2}[/tex])

    3. The attempt at a solution
    well im pretty sure that F=0N, and the the direction the field has to be in the same direction of the velocity, so sin(theta)=0...... but i have no idea how find the field strength, anyone have any ideas???
     
  2. jcsd
  3. Aug 28, 2008 #2
    wat can i do with this Voltage... i need a current dont i?... i cant find the right formulas, and getting really frustrated.. argh
     
  4. Aug 28, 2008 #3

    Defennder

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    The question referred to a picture. It may require a picture or at least an accurate description before we can help. Is the electron travelling in a direction parallel to the plates?
     
  5. Aug 28, 2008 #4
    yes sorry... the electron is traveling parallel through the parallel plates.. i see if i can upload pic
     
  6. Aug 28, 2008 #5

    Defennder

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    Well if that's the case, no picture is required. Just think in terms of how much force is needed to counter-balance the force exerted on the particle by the E-field. Then use the Lorentz force equation to find the magnetic flux density needed.
     
  7. Aug 28, 2008 #6
    oh i dont know if we are meant to do it like that, because we havnt done flux or lorentz....
     
  8. Aug 28, 2008 #7
  9. Aug 28, 2008 #8

    Defennder

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    Well, "magnetic flux density" is another word for "magnetic field strength" and "Lorentz force" is a more general term for "force on charged particle due to B-field".
     
  10. Aug 28, 2008 #9
    oh ok so F=(E+v X B)..... i dont understand because doesnt f have to equal 0, for the electron to pass through undisturbed, as soon as there is a force the electron is going to change direction??? unless its opposing the velocity, which would make it slow down????
     
  11. Aug 28, 2008 #10
    thus E=V/s=200/.01=2000N/C but how do i find B?
     
  12. Aug 28, 2008 #11

    Defennder

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    You're missing out q here. F has to be zero in order for the electron to pass undeflected. Note that the force exerted by the E-field and that by the B-field is perpendicular to its velocity, and hence does not affect its speed in that direction.

    Use the equation you stated earlier. Although another similar approach would be to separate the two equations: F=qE and F=Bqv and this two are acting in opposite directions, so you equate them and solve for B. Note that you have to indicate the direction in which the B-field is applied.
     
  13. Aug 28, 2008 #12
    yes sorry i missed q... so qE=Bqv =>E=Bv=>B=E/v=2000/1x10^7=2x10^-4T in the direction of the velocity?
     
  14. Aug 28, 2008 #13

    Defennder

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    Check your value for E-field. And note note that magnetic force will always act in a direction perpendicular to both the velocity and direction of the B field. Use the right-hand rule to get the direction.
     
  15. Aug 28, 2008 #14
    ya its 20000 not 2000.... oh ok yes i remember now.... duh... lol so the direction of B will be out of the page? since the E travels + to -....
     
  16. Aug 28, 2008 #15

    Defennder

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    Remember this is an electron, not a positive charged particle.
     
  17. Aug 28, 2008 #16
    na im confused... what does this mean... wont the electron want to move towards the positive charged plate??
     
  18. Aug 28, 2008 #17
    oh for an electron its the exact opposite, so use left hand, so its going into the page? but is my maths correct, does 2mT sound right?
     
  19. Aug 28, 2008 #18
    no wait before i was using my left hand ad it was saying out.... my right says in, thereofre it must be out of the page???? plz help im really confused
     
  20. Aug 28, 2008 #19

    Defennder

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    No I meant to say that when you apply the F=qv X B vector equation you must note that the resulting direction using the right-hand rule holds for a positive charge. The negative charge goes in the opposite direction.
     
  21. Aug 28, 2008 #20

    Defennder

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    Don't use your left hand. Your right hand would do, just reverse the direction.
     
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