Magnetic fields - which direction

  1. 1. The problem statement, all variables and given/known data
    If a flat wire lies in a north-south direction will the MAXIMUM magnetic force be felt if the field lies either in a east-west direction OR an up-down direction (i.e: ppd to the direction of the current?)??


    2. Relevant equations



    3. The attempt at a solution
    I know that the maximum magnetic force is felt when the current and field are perpendicular
    BUT does it matter which plane the magnetic field is in so long as it is perpendicular?

    Also, in exams will it be clear which direction the field is in
    (e.g: they might say it acts at a 40 degree angle to the horizontal)
    therefore if the current is straight ahead I calculate the component perpendicular to the direction of the current to find the force??
     
  2. jcsd
  3. berkeman

    Staff: Mentor

    I think the key to this question is the use of the word "flat" in describing the current-carrying wire. That shape should make the maximum B-field happen in only one of the two choices given...
     
  4. so you're saying that if I had some height as well then it would exclude the up-down direction?

    so a block facing in an east west direction would need a mag field in a south-north direction??

    also what would happen with an aerial
    it has height but basically no depth.
    "a car with a vertical aerial 1.2m is moving at 35 m/s. The horizontal componenet of the earths magnetic field is 22 * 10-6T. Find the emf induced"

    they give me the horizontal compoenent but surely I could also use the forward backward componenet as that too is ppd to the aerial?
     
  5. berkeman

    Staff: Mentor

    Sorry, I'm not understanding what you are saying.

    What I was saying was that if you draw a flat wire of some width, going in the N-S direction, then at the same radius R away from the wire, you will get a slightly different B field if you are out at the side (say on the East side, where the B field points up and down), or up towards you (where the B field points East and West).
     
  6. well what I was trying to understand was how the direction of the mag field is important

    going back to this question:
    a car with a vertical aerial 1.2m is moving at 35 m/s. The horizontal componenet of the earths magnetic field is 22 * 10-6T. Find the emf induced

    would the MAXIMUM EMF only be possible if I was travelling ppd to the horizontal compoenent of the earths mag field (assuming this was the ONLY component)

    therefore could I either be travelling straight ahead OR upwards to achieve this maximum EMF??
     
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