Magnetic Fields
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Phys 4, Section 2                                                                 Feb 13, 2002


Magnetism (Permanent Magnets)

Magnetism: first noticed as effect of iron ores by Greeks in Magnesia.
Magnetic Poles: always N and S in a pair, never a separate monopole.
Use of Magnet as compass

Magnetic Field

Magnetic field B is caused by moving charge (current); this field is experienced by other moving charges.

F = qυ B (Units of B are Tesla: 1 T = 1 N/A m; also gauss: 1 G = 10-4 T)

Lorentz Force: F = q(E + υ B)

Magnetic Field Lines and Magnetic Flux

Field lines come out of N poles and go into S poles, except within the magnet itself where the lines go from S to N. Magnetic field lines have no origination or destination points, but always form closed loops.

Magnetic flux is defined:

Gauss' Law for Magnetism:

Charged Particles in B

Magnetic force F is at right angles to motion υ of charged particle q, so does no work, but can only change direction of motion, it acts as a centripetal force driving the particle in circular motion of radius R:

F = qυB = mυ2/R


ω = υ/R

so get  ω = qB/m (cyclotron frequency).

Crossed B and E fields (balance of E-force and B-force):

velocity selector: υ = E/B
Thomson's e/m experiment: mv2/2 = eV e/m = E2/2VB2
Magnetic Force on current-carrying conductor of length L:    F = IL B




Last modified on February 27, 2002