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Magnetic force and gravity

  1. Oct 12, 2015 #1
    1. The problem statement, all variables and given/known data
    A straight wire (length Lcm, mass mg) lies horizontally on top of a sensitive scale. The wire is oriented so it runs north-south, and it is part of an electric circuit that includes a battery and an open switch. There is a uniform magnetic field everywhere, magnitude bT and running east to west. Suddenly the switch is closed, and current IA runs through the wire from north to south. Find N, the reading of the scale, in Newtons, after the switch is thrown. As usual, use: g = 9.803 m/s^2.

    2. Relevant equations
    3. The attempt at a solution
    I feel a little confused about the question where it stated "magnitude b T and running east to west" this is the direction of B? So according to right-hand rule, since I is from North to South, the direction of Force due to magnetic field will go outward? So does it will affect the reading of the scale? I only need to consider gravity ?

    thank you for the help.
  2. jcsd
  3. Oct 12, 2015 #2


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    Try your right hand rule again. If the current is running N-S and B is pointing E-W, the force is perpendicular to both of those, so it is pointing in which direction??
  4. Oct 12, 2015 #3
    right hand: thumb points to south(down) forefinger points to west (left) and the middle finger is inward, so the force is inward?
  5. Oct 12, 2015 #4


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    No. Think of it like a map laying on a table. N-S are one direction on the table, E-W are a perpendicular direction, but still in the plane of the table. So what direction is perpendicular to both of those?
  6. Oct 12, 2015 #5
    thank you for the explanation, this is exactly what I feel confused. In the question, it stated "magnitude bT and running east to west" I thought the direction of B is from E-W, in other words, it points to the right, but actually it is not. It is either inward or outward, right?
  7. Oct 13, 2015 #6


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    I still don't think you're getting it. Again, thinking of the map on the table, if the current is running N-S, and B is running E-W (i.e left to right), the cross product of these is downward, toward the floor. I don't know what you mean by 'inward' and 'outward' in this context.
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