Charged particles are affected by electric fields. An electric field is the area around a charged object. it forms a 'landscape' for charged objects rather like hills and vales in the 'charge dimension'. This makes charges accelerate as they move in a potential gradient, just like a ball would accelerate if it rolled down a hill.

If two parallel plates (one negative and one positive) form an electric field that particles from radioactive decay are made to travel through the particles that are charged will accelerate towards the plate with opposite charge. An alpha particle will therefore accelerate towards the negative plate and the beta particle towards the positive plate.

The gamma ray has no charge so it is not affected by this 'electric landscape' and will just continue on its straight path.

Moving charged particles experience a force when they travel in a magnetic field that is at right angles to their path. The force acts mutually at right angles to the direction the particle is travelling and the direction of the field so it takes a circular path.

The direction it will go in can be found using Flemings left hand rule. At GCSE you do not need to know which way it will move but at A level you do.