Magnetic field
The key idea
Magnetic fields act on moving charges and magnetic dipoles. Their effects depend on direction as well as magnitude.
Why it matters
The magnetic force on a moving point charge is perpendicular to both velocity and field, so it can bend a trajectory without directly changing the particle’s speed.
A picture to remember
The magnetic force on a moving point charge is perpendicular to both velocity and field, so it can bend a trajectory without directly changing the particle’s speed.