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Q & A: Different forms of magnetic force

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Most recent answer: 03/24/2009
Q:
We have studied that magnetic force and field are perpendicular to each other.But when we are performing an experiment to draw the magnetic field of a magnet we first keep a magnetic compass near the north pole of the magnet and slowly bring it to the south pole in a defined manner.M question is that the magnetic force of magnet is toward the needle while the magnetic field which we draw also coincide with that magnetic force.Then how can it be possible that they are perpendicular.
- Prudhvi Raj Borra (age 16)
Machilipatnam,Andhra Pradesh,India.
A:
The magnetic force on a moving charged particle is perpendicular to both the velocity of the motion and to the magnetic field, (in vector notation it's a cross product). 
When you have a compass needle in a magnetic field the needle, a small bar magnet itself, will align itself to the direction of the external magnetic field.  There is no velocity involved.  If you do the calculation, a bit involved, you find that the minimum energy configuration is when the needle and the external field are aligned.  That's where it winds up.

LeeH


To help see that, imagine that instead of a little permanent magnet, you used a little electromagnet. The currents going around in the coils would be going at right angles to the direction of the magnet.  So there would be forces between the other magnet and those currents. /Mike W.

(published on 03/24/2009)

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