Why is a force exerted on a charged particle when it enters a magnetic field?
The force which is exerted on a charged particle in magnetic field is force = B*e*v. 'e' is the charge of the particle, a constant. 'v' is the velocity of the particle when the particle is moving in magnetic field. And last but not least, B is the flow density of magnetic field.*
Say velcotiy is not changed, the force would depend on flow intensity of magnetic field (please note: if the velocity does really NOT changed). That is to say, the force is stronger the more homogen field lines exist. Homogen means that the field lines have the same distance to each other (means that the same field force exists everywhere in magnetic field).
To sum up: higher flow density -> more (homogen) field lines in magnetic field -> stronger the magnetic field -> stronger the attraction to the charge -> stronger the force to particle.
Hope that is helpful for you. **