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Magnetic field from a wire

  1. Aug 28, 2011 #1
    Hi everyone,

    If I have a current going up a straight wire, and use the right hand rule (the one where you stick your thumb up and curl your other four fingers), then the magnetic field is in the right direction, correct (my fingers curl to the right)? However, according to this video: It's to the left. Which one is correct?

    If the magnetic field is to the right, then could I use the other right hand rule to find the direction of the force? And, since the charges are negative, would I have to invert that result?


    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Aug 28, 2011 #2
    The direction of the field would depend on which area of the wire you are looking at. If you look to it's right, you will see the field going left, and if you look to the left of the wire, the field will go right. There isn't just a single direction a magnetic field goes in like electric fields; the magnetic field lines do not have end points since they meet themselves always. The right hand rule tells you which direction they curl in, so if you view different parts of the wire's neighborhood, you will see different directions, but the curling direction will be the same (shown by your wrapping fingers).

    Yes, you can use the other rule to find the direction of force for positive chargers. Negative charges require you to invert the direction of force.
  4. Aug 29, 2011 #3
    It's more helpful to describe the direction as clockwise or anticlockwise rather than left or right.
  5. Aug 29, 2011 #4
    When I said what direction is the magnetic field flowing, I meant by convention, as in, if it's flowing clockwise, then if I wanted to use the other right hand rule, what direction would I point my middle finger (which represents the magnetic field)?
  6. Aug 29, 2011 #5
    Yes, but you also have to state which end of the wire you're looking from.

    If you look down the wire so that the current is flowing away from you (e.g. from the positive terminal) the the magnetic field flows clockwise arround the wire.

    If you look down the wire so that the current is flowing toward from you (e.g. from the negative terminal) the the magnetic field flows anticlockwise arround the wire.

    To use the right hand rule, point your thumb in the direction of the current. Then your fingers, wrapped arround the wire, show the direction of the magnetic field (from the base of your palm to the tips of your fingers).
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