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Electrons move, causing magnetic force, find v?

  1. Jun 13, 2017 #1
    1. The problem statement, all variables and given/known data
    IMG_1174_1.png

    IMG_1175_1.png

    2. Relevant equations
    f = q.v.B.sin tetha
    I = q/t = v/R

    3. The attempt at a solution
    (1) q =a.b.n.v.t is it right?
    (2) I = q/t = abnvt/t = abnv is it right?
    (3)F = q.v.B is it right?
    (4) ?
    (5) ?

    is it right?
     
  2. jcsd
  3. Jun 13, 2017 #2

    mfb

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    Right so far.
    What do you think about (4) and (5)?
     
  4. Jun 13, 2017 #3
    4&5 can you give me clue?
     
  5. Jun 13, 2017 #4

    scottdave

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    nabvt gives you the number of electrons passing in a given amount of time, t. So nabv gives you the number of electrons per second. Each electron has a charge -q, so this must be multiplied to get the current (charge/time). Volts are Joules/Coulomb = Newton*meters/Coulomb, if that helps you on #4.
    You can rearrange that so that Force = Volts*Coulomb / meter, then see if you can find what fits that. Once you have an expression for V, you should be able to find the last one.
     
  6. Jun 13, 2017 #5
    4) Volts = joules/Coulomb
    Volts = F/q
    V=Bvq/q
    V=Bv??

    5) q=abnvt
    I=q/t
    I=abnv
    n=I/abv
    n=I/ab.V/B
    n=IB/abV

    What's wrong on this?
     
  7. Jun 14, 2017 #6

    mfb

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    A force is not an energy.
     
  8. Jun 14, 2017 #7
    I've looked at the link you gave me.
     
  9. Jun 14, 2017 #8
    V = joule (energy)/coulombs --- which is the energy?
    joule can be force * distance
    V = force * distance / coulombs
    which is the distance?
     
  10. Jun 14, 2017 #9

    mfb

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    In which direction does the force (calculated in 3) go? What is the corresponding distance where the voltage difference appears?
     
  11. Jun 14, 2017 #10

    scottdave

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    Using the right hand rule, in which direction is the force acting on the electrons (from F = qv x B)? Since work is the dot product of Force and the distance traveled, I think you should look for the distance in the same direction done. This is an interesting problem. I have not seen some of these terms expressed like this, but it gets you to thinking about how things are related.
     
  12. Jun 14, 2017 #11
    I understand.
    4) Voltage = Joules/C = force*distance / coulombs
    V = f . distance / q
    V= B*v*q . b / q
    V = B*v . b
    distance found by right hand rule, F go trough width which is b.

    5) q = abnvt
    n = q/abvt I=q/t
    n = I/abv V = B*v*b --> v = V/Bb
    n = I / (ab)V/Bb
    n = IBb/abV
    n = IB/aV ---> but answer is still wrong.. which is wrong?
     
  13. Jun 14, 2017 #12

    mfb

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    Right.
    It is useful to put brackets around denominators: n=q/(abvt). Otherwise you could misinterpret it as ##n=\frac q a bvt##.
    Right.
    Why do you think it is wrong?
     
  14. Jun 14, 2017 #13
    why... you sure it's right?
     
  15. Jun 14, 2017 #14

    mfb

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    If you replace n by ##\frac 1 {A_H}##, it is the usual formula for the Hall effect. The Hall constant as inverse electron density takes into account that not all electrons contribute to current flow.
     
  16. Jun 14, 2017 #15
    I meant, the key answer is n = BI/aqV... but I don't think the key answer is wrong.
     
  17. Jun 14, 2017 #16

    mfb

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    Ah, n is the electrons per volume, not the charge density. Then you need the factor q in the answer (this also applies to question 2).
    In that case your notation for q is a bit strange.
    q in the problem statement is a charge. What you calculate here is a number.
     
  18. Jun 15, 2017 #17
    i still don't get it. What is n stands for? And n actually what? And my answer for 1 and 2 are they right?
     
  19. Jun 15, 2017 #18

    mfb

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    n is something like "5/m3" (just with a larger number for actual materials) - the density of electrons.

    qn is something like "5C/m3" - a charge density.
    1 is right, 2 needs an additional factor q.
     
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