Emission of radiation by a charged particle undergoing acceleration An electron is travelling along the x-axis. It then changes its direction by 45 degrees. Will it emit an electromagnetic wave?
 A: Yes.
If a massive particle changes its direction, then it must do so under the action of  acceleration.
Accelerating charged particles emit radiation, according to Larmor's formula, which gives the power emitted by an accelerating charged particle.
A: How does the electron change its direction? Something must be responsible for the distraction from its geodesic orbit.
The electron in a wire as part of an electric current collides with the subatomic particles of the metal and moves in a zigzag manner. Any acceleration of the electron, be it a stop (negative acceleration) or a gain in kinetic energy, emits photons. This is in detail the reason why the wire has an ohmic resistance. The photon emission of the accelerated electrons makes the wire warm or hot.
The electron under the influence of an ion. The electron, when captured be an ion, emits photons.
The electron in an external magnetic field. If the electron moves nonparallel to the magnetic field it gets deflected sideways and by this emits photons. This is called the Lorentz force. Exhausting all its kinetic energy it comes to standstill in the centre of its spiral or helical path.
