Phys 4, Section 2                                                                 Feb 13, 2002

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 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)

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:

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

but

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