Cracking the AP Physics B Exam, 2012 Edition by Princeton Review

Author:Princeton Review
Language: eng
Format: mobi
ISBN: 9780307944467
Publisher: Random House, Inc.
Published: 2011-12-12T16:00:00+00:00

THE MAGNETIC FORCE ON A MOVING CHARGE

If a particle with charge q moves with velocity v through a magnetic field B, it will experience a magnetic force, FB, with magnitude:

where θ is the angle between v and B. From this equation, we can see that if the charge is at rest, then v = 0 immediately gives us FB = 0. This tells us that magnetic forces only act on moving charges. Also, if v is parallel (or antiparallel) to B, then FB = 0 since, in either of these cases, sin θ = 0. So, only charges that cut across the magnetic field lines will experience a magnetic force. Furthermore, the magnetic force is maximized when v is perpendicular to B, since if θ = 90°, then sin θ is equal to 1, its maximum value.

The direction of FB is always perpendicular to both v and B and depends on the sign of the charge q and the direction of v × B (which can be found by using the right-hand rule).

If q is positive, use your right hand and the right-hand rule.

If q is negative, use your left hand and the left-hand rule.

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