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Force of Magnetic Fields

  1. Mar 27, 2008 #1
    I was just wondering, since my textbook doesn't really explain it too well, what velocity is part of the equation F = qv x B? Is it the drift velocity, or...?
  2. jcsd
  3. Mar 27, 2008 #2
    A particle of charge q that moving with velocity v through a space where there exists a magnetic field B will experience force F.
  4. Mar 27, 2008 #3
    Ok assume there is an electron in a magnetic field, will the force be affected by said electrons drift velocity, or would it be something else?
  5. Mar 27, 2008 #4
    There is only a force if the velocity of a particle is perpendicular to the direction of the magnetic field. For a positive charge, hold out your hand and put your thumb in the direction of positive current, then put your straightened fingers in the direction of the magnetic field, and your palm will face the direction of force.
  6. Mar 27, 2008 #5
    No I know how to calculate the force, all I want to know is if we put an electron into a magnetic field, so that the electron is moving perpendicular to the field, would the velocity we use be the drift speed of that electron?
  7. Mar 28, 2008 #6
    The velocity used in the calculation of Force of magnetic field is not drift velocity but velocity of the particle. Drift velocity is different from the velocity. Drift is called by the combination of magnetic force and other kind of force, and drift velocity is departed from velocity. You can refer to the definition of drift in magnetic field.
  8. Mar 28, 2008 #7


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    Staff: Mentor

    No. The magntude of the magnetic force is [itex]qvB \sin \theta[/itex], where [itex]\theta[/itex] is the angle between the velocity of the particle and the direction of the magnetic field. The field is zero only when the velocity is along the direction of the field. The force is maximum when the velocity and the field are perpendicular.
  9. Mar 28, 2008 #8
    Whoops, should've said when a component of the velocity is perpendicular to the direction of the magnetic field.
  10. Mar 28, 2008 #9
    Practicly you would use v relative to the charge that causes B, because you can easily calculate B in that system.
    In general you can use any inertial coordinate system and measure the speed of the charge q in that system. You might get different magnetic force F in diferent systems, but this is not a problem, since B and E are also dependent on the system and particle feels both electric and magnetic force. For systems moving relative to each other at nonrelativistic speeds you will get aproximately the same electromagnetic force F. If speeds are relativistic, you get different F in different systems since a relativistic particle has different acceleration in different inertial systems (but those different forces will still describe the same movement).
  11. Mar 30, 2008 #10
    Hello dear,

    Let me help ypu..

    the velocity is the net velocity of CHARGE ,

    Lets take some cases here.....

    1.charge is moving independently with velocity V, so This V will be used in our equation

    2.A charge is moving inside a metal with velocity(drift velocity) V1 (e.g free electrons in semi-conductors) and metal is moving with velocity V2,

    this means-NET velocity is the VECTOR SUM of V1(drift velocity) n V2(metal velocity) ,

    this situation is justified in "ELECTROMAGNETIC INDUCTION" where we use velocity of moving metal(metal velocity) to find magnetic force on CHARGE
  12. Mar 30, 2008 #11
    Sorry, but we dont use relative velocity

    Sorry, but we dont use relative velocity as you said in this post,we only use net ACTUAL velocity of charge
  13. Mar 30, 2008 #12
    You did not read my post carefully. I used the phrase "relative velocity" because it may not be clear which system of coordinates we should choose. In this case "actual velocity" has little meaning.
  14. Mar 30, 2008 #13
    Please correct me if I'm wrong. For a wire, v serves as the drift velocity in equation F = qv x B, even if it isn't an actual velocity. With this assignment, q must be interpreted as the amount of current entering the wire, per unit time.

    Importantly, with these assignments, q is not the charge carried by each particle.

    I realize that this isn't the usual correspondence of variables, but it seems to serve.

    Maybe you were really looking for this equation: F = iL x B, the force on a current carrying wire of length, L within B.
    Last edited: Mar 31, 2008
  15. Apr 2, 2008 #14
    Ah thanks, I get it now.
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