Electric-charge relativistic motion causes weak electric force {magnetic force}| transverse to motion direction. Magnetic fields are electric fields caused by relativistic charge motions that make excess electrons or protons appear. Magnetic fields have no net charge to stationary observers.
examples
Wire in magnet field, tube and magnet, TV tube and magnet, two wires with current, and carpenter's bubble illustrate magnetic fields.
force
Magnetic force F equals moving charge q times velocity v times stationary-object magnetic field B times sine of angle A of approach to stationary object: F = q * v * B * sin(A). Magnetic force F equals wire current I times wire length L times stationary-object magnetic field B times sine of angle A between wire and stationary object: F = I * L * B * sin(A). Magnetic force F equals space magnetic permeability k' times wire current I1 times current I2 in other wire divided by distance r between wires: F = k' * I1 * I2 / r.
distance
Magnetic force depends on distance between wires, not distance squared, because relativistic effects are transverse to current motion.
Torques require moments {magnetic moment}. Magnetic moment M equals current i times coil area A: M = i * A. Magnetic moment equals pole strength p times path length l: M = p * l.
If positive current points in right-hand finger direction {right hand rule, magnetism}|, magnetic-field direction {north magnetic pole} points in thumb direction. The opposite direction is the other pole {south magnetic pole}.
5-Physics-Electromagnetism-Magnetism
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Date Modified: 2022.0225