Can anyone tell me why when a charged particle enters into a magnetic field it takes a circular path?tell me how and when we use rigght hand left...

When a charged particle moves in a magnetic field it
experiences a force that is at right angles to the direction of motion and to the
direction of the magnetic field.  This causes the particle to change direction without
speeding it up or slowing it down.  However when it shifts direction the force changes
direction so that it remains at right angles.  This is precisely what a centripedal
force does to an object traveling in a circle.  If the strength of the magnetic field
doesn't change over the orbit of a circle the particle will be trapped into orbiting in
a circle.  If the magnetic field does change then the path of the particle will only be
curved.