Magnetic field lines have no beginning or end, they always form closed loops . The field lines visualizing the magnetic field of a permanent bar magnet are shown below. Outside the magnet the field lines close the loop, from north to south. The density of the field lines is proportional to the strength of the magnetic field. When a bar magnet is bend into the shape of a horseshoe, the magnetic field between the poles is nearly uniform and usually quite strong.
Magnetic fields exert forces on other moving charge.
The force a magnetic field exerts on a charge q moving with velocity v is called the Lorentz force . The force F is perpendicular to the direction of the magnetic field B . It also is perpendicular to the direction of the velocity v . F is perpendicular to the plane that contains both v and B . To find the direction of the force, use the right-hand rule .