Accelerated charges radiate.

This bubble chamber picture, which is a few times smaller than life-size, illustrates several phenomena:

Charged particles leaving trails of bubbles in their path as they force their way through a specially prepared liquid (a super-heated mixture of neon and hydrogen).
Moving charged particles (electric currents) experiencing forces in a magnetic field.
Electron (and anti-electron or positron) tracks curling up in a characteristic way.
High energy gamma-rays (or photons) materialising into electron-positron pairs (in the field of a nucleus).
Accelerating (they are turning and therefore not moving in a straight line with steady speed) electrons and positrons emitting high energy photons (tangentially) - which then do something to herald their existence.