A charged particle moving through a magnetic field experiences a force that is at right angles to both the direction in which the particle is moving and the direction of the applied field. This force, known as the Lorentz force, develops due to the interaction of the applied magnetic field and the magnetic field generated by the particle in motion. The phenomenon is named for Dutch physicist Hendrik Lorentz, who developed an equation that mathematically relates the force to the velocity and charge of the particle and the strength of the applied magnetic field.
The Lorentz force is experienced by an electric current, which is composed of moving charged particles. The individual magnetic fields of these particles combine to generate a magnetic field around the wire through which the current travels, which may repel or attract an external magnetic field. This tutorial demonstrates the Lorentz force exerted on a wire that carries current through the field of a permanent horseshoe Magnet (field lines always move from a magnet's north pole to its south). The wire is arranged in a kind of pendulum so that it can move back and forth. Click on the Knife Switch to start the flow of current. The wire will swing in a direction perpendicular both to the magnet's field and to the movement of the charged particles. Changing the direction of current flow by clicking the Flip Battery button, or the direction of the magnetic field by clicking the Flip Magnet button, will reverse the direction of the Lorentz force. Using the Show Wire Field Lines and Show Magnet Field Lines radio buttons will reveal the interplay of these invisible forces. The Reset button can be used to return the magnet and battery to their original positions.
You can predict which way the wire will move by using the left-hand rule. You need to contort your hand in a bit of an unnatural position for this rule: If your index finger points in the direction of a magnetic field, and your middle finger, at a 90 degree angle to your index, points in the direction of electrical current, then your extended thumb (forming an L with your index) points in the direction of the Lorentz force exerted upon that particle, and the direction in which the wire shifts in the tutorial.