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Maxwell's Laws Tour

Lorentz Force Law

Lorentz found that a magnetic field would exert a force on an electric charge only if it was moving. He then found that the force on a moving charge is proportional to the strength of the magnetic field and the speed of the charge. The direction of the force is always perpendicular to both the magnetic field and the velocity of the charge. Thus according to Lorentz a magnetic field can only change the direction of the velocity not the magnitude. In the accompanying animation we see a special case of Lorentz's law. If a charge is moving at a constant velocity in free space and then enters a area of a magnetic field the charge will make a half circle and then leave the magnetic field going in the opposite dirrection that it entered. Since the force on the charge is proportional to the velocity no matter how fast the charge is traveling it will always exhibit the same behavior. However, the radius of the half circle that it traces out will change. How do you think you could tell the velocity of a particle using the Lorentz force equation?