Why do charged particles deflect in magnetic fields?

Charged particles deflect in magnetic fields due to the force exerted by the field on the moving charges.

In more detail, this phenomenon is a fundamental aspect of electromagnetism, governed by the Lorentz force law. This law states that a charged particle moving in a magnetic field experiences a force that is perpendicular to both the direction of its motion and the direction of the magnetic field. The force is given by F = q(v x B), where F is the force, q is the charge of the particle, v is the velocity of the particle, and B is the magnetic field. The 'x' denotes the cross product, which means the force is perpendicular to both v and B.

The direction of the force can be determined using the right-hand rule. If you point your fingers in the direction of v and curl them towards B, your thumb points in the direction of the force for a positive charge. For a negative charge, the force is in the opposite direction.

This force causes the charged particle to move in a circular path, as the direction of the force is always perpendicular to the direction of motion. This is why charged particles deflect when they enter a magnetic field. The radius of the circular path depends on the speed of the particle, the strength of the magnetic field, and the charge and mass of the particle. Faster particles and particles with a larger charge will deflect more sharply.

It's also important to note that a charged particle moving parallel to the magnetic field will not experience a force, as the cross product of parallel vectors is zero. Therefore, such a particle will not deflect. This principle is used in devices like the mass spectrometer, which separates ions based on their mass-to-charge ratio by making use of their different deflection paths in a magnetic field.