1. Not finding help here? Sign up for a free 30min 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!

Magnetic fields and current flows

  1. May 27, 2003 #1

    Dx

    User Avatar

    Hiya ppls!

    I have some questions about magnetic fields and current flows in general.

    Lets say in these eg., A vertical wire carries a current straight up in a region where the magnetic field vector points due NORTH. What direction of the resulting force on this current?

    EX1) No using the right hand method if my thumb points in the positive flow of current then my fingers should curl in the B of the force, correct? SO I am correct to say that the force on this currrent would be EAST.

    EX2) An electron has an initial velocity to the south but is observed to curve upward as the result of the magnetic fied. The direction of the field is? My answer using the right hand method would be upward.

    I am somewhat confused. Am I correct to say that when I want to find a force on a magntic field I use the rt hand method with the thumb points with the current. There are so many ways to use the rt hand methid for different scenarios I am confused with it?

    Something lse that confuses me and this is an actual problem here.

    An electron moving along the +x axis enters a region where there is a uniform magnetic field in the +y direction. What is the direction of the magnetic force on the electron? (+x to right, +y up, +z out of the page)

    I am suppose to find its direction which I believes travels in the _x direction traveling CW, am I correct to say this.

    The answers are multiple choice for this particluar problem.
    a) +z
    b) -z
    c) -y
    d) -x

    Any help with these problem and my questions would be immensely appreciated.

    Thanks!
    Dx :wink:
     
    Last edited by a moderator: Feb 5, 2013
  2. jcsd
  3. May 27, 2003 #2

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    EX1) No using the right hand method if my thumb points in the positive flow of current then my fingers should curl in the B of the force, correct? SO I am correct to say that the force on this currrent would be EAST.

    Are you sure you are using your right hand? :)

    If you do as you say you are, with your fingers point north in the direction of the field and your thumb pointing upward, then your fingers will curl toward the WEST, not east.


    EX2) An electron has an initial velocity to the south but is observed to curve upward as the result of the magnetic fied. The direction of the field is? My answer using the right hand method would be upward.

    I am beginning to think you have a very odd right hand! The whole point of the "right hand rule" is that the force is at right angles to both the motion of the electron and the direction of the magnetic field. The field direction CAN'T be in the same direction as the force!
    Point the fingers of your right hand "south" so that the fingers curl upward. (Your hand is flat with palm up, fingers pointing south.) When I do that I find that my thumb is pointing WEST.

    An electron moving along the +x axis enters a region where there is a uniform magnetic field in the +y direction. What is the direction of the magnetic force on the electron? (+x to right, +y up, +z out of the page)

    I am suppose to find its direction which I believes travels in the _x direction traveling CW, am I correct to say this.

    Again, the force cannot be in the direction of either electron motion or magnetic field. Place your hand so that your fingers point up the y axis and your thumb points along the positive x axis. Your fingers curl "upward" so the force is along the positive z axis.

    It is not only "incorrect" to say the electron "travels in the -x direction traveling CW (clockwise?)", it makes no sense. To say that something travels in any particular direction means it travels in a straight line and then "clockwise" doesn't apply.
    Here, there is a constant force so the electron CAN'T move in a straight line. For this problem, the electron moves in a circle, with center on the positive z axix. Whether it is moving clockwise or counter-clockwise depends upon your viewpoint. Viewing the electron from the positive y-axis, it is moving counter-clockwise, from the negative y-axis, clockwise.
     
  4. May 27, 2003 #3

    Dx

    User Avatar

    Okay!

    I see!
    Dx :wink:
     
  5. May 28, 2003 #4

    Dx

    User Avatar

    Just want to ask to make sure!

    If a long straight wire carries current towqard the east. a proton moves forward toward the east alongside and just south of the wire. what is the direction of the force on the proton?

    SOUTH, correct?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Magnetic fields and current flows
Loading...