Magnetic Fields
Magnetic fields act on moving charge. The force is sideways to both the charge's velocity and the magnetic field, so magnetism naturally bends trajectories.
Why can a magnetic field bend a charged particle without speeding it up?
For motion perpendicular to the magnetic field, the magnetic force stays perpendicular to the velocity. A sideways force changes direction, not the instantaneous speed.
For perpendicular vectors, . Parallel motion feels no magnetic force.
Send a charged particle across a uniform magnetic field.
Velocity, magnetic field, and force form a right-handed relationship for positive charge. A negative charge reverses the force direction.
When v stays perpendicular to B, the magnetic force acts like a centripetal force and the path becomes circular.
Because the magnetic force is perpendicular to displacement, it changes direction without changing kinetic energy in the ideal case.
A current is moving charge, so wires and coils create magnetic fields around them. Field geometry follows the current geometry.
Ordinary magnetic field lines form closed loops. Cutting a bar magnet produces two smaller dipoles, not isolated north and south poles.
Electric and magnetic field descriptions depend on motion of the observer; relativity reveals them as parts of one electromagnetic field.