Hand rule application (dir of mag force)

  • #1
[SOLVED] hand rule application (dir of mag force)

Homework Statement

You witness an experiment with a wire (a part of a circuit) and a horseshoe magnet. It begins with a wire laying on a table and passing through the mag field of the horseshoe magnet, but with no current passing through the wire. When the current is turned on, the wire experiences and upward force and as a result, jumps vertically out of the mag field. If the current in this experiment was flowing due north (ie, the hypothetical positive charges were flowing due north), in what direction was the magnetic field of the horseshoe magnet at the original location of the wire?

a) north
b) south
c) towards the center of the earth
d) away from the center of the earth
e) west
f) east

Homework Equations


The Attempt at a Solution

the test "answers" state: To create an upward force on a current going
to the NORTH, the magnetic field through which
the current is passing must point towards the

I can apply the hand rule to a wire, but how do I apply it to the horseshoe magnet? This really confuses me.
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Answers and Replies

  • #2
Here is something I found:


So for your horseshoe magnet, the field would flow from N to S. In an arc, but you can assume it's just 1 direction.

Basically, the hand-rule works when you are taking a cross-product, like to find the magnetic field of a current-carrying wire. It will NOT work for a ferromagnet like your horseshoe or bar magnet as far as I can tell. You just need to know that the magnetic fields go from N to S.

However, to calculate force, it's F = q(v x B), so you ARE taking a cross product. your qv is in the North direction, your Force is straight up. When you line it, up, it's thumb (force) straight up, v goes North, so you curl your hand and you get West.
  • #3
The right hand rule for the magnetic force takes into account what direction the magnetic field is pointing, regardless of what is causing it. The fact that it's a horseshoe magnetic is irrelevant, because although the field curves, you are assuming here that it is more or less straight in the region where it intersects the wire.

Version 1 of the Right Hand Rule (for the direction of the magnetic force):

If the index finger points in the direction of the current, and the middle finger points in the direction of the magnetic field, then the thumb points in the direction of the magnetic force, *when* the three fingers are arranged so as to be mutually perpendicular.

In this case, if you point your index finger straight ahead, and your thumb upward, then your middle finger must point to the left (i.e. west, since straight ahead is north).

Version 2

Curl your fingers in such a way that they go from the direction of the current to the direction of the magnetic field. Once you have done so, your thumb will indicate the direction of the magnetic force.

There are only two ways you can curl your fingers...starting out straight ahead and curling to the *left* (in which case your thumb points upwards), or starting out straight ahead and curling to the right (in which case your thumb points downwards). The former arrangement is correct here.
  • #4
Right, just for the sake of clarification, we are using the right hand rule that is normally used to find F, GIVEN that we already know the direction of B. We can't use the other right hand rule to directly find B's direction for a horseshoe magnet, as poop-loops pointed out. You just have to infer its direction based on how F is pointing.
  • #5
Thanks guys! I have been working on this all day and I've just about got it. It makes perfect sense now!

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