Magnetic Forces and Magnetic Fields [video]

Abstract

Magnetic Forces and Magnetic Fields Hi. I’m Dr. Bruce Denardo here in the Physics Department of the Naval Postgraduate School in Monterey, California. When a charged particle moves through a magnetic field, a force on the particle can occur. This is the fundamental magnetic force. In this video, we will present a demonstration of the effect. In other videos, we will use the concept of the magnetic force to explain other demonstrations, including a jumping wire and a loudspeaker. 2. CATHODE RAY TUBE This is a cathode ray tube, which produces a beam of electrons. The beam is visible due to argon gas in the tube. The electrons are moving so fast here that gravity is negligible. Watch what happens when I bring a magnet near the beam. The beam deflects, which shows that there is a transverse force due to the magnet on the moving electrons. In general, any charged particle that is moving in a magnetic field will experience this fundamental magnetic force. The beam is horizontal outward. When the magnetic field points to the right, the beam bends downward. Switching the direction of the field by flipping the magnet switches the direction of bending. When the magnetic field points downward, the beam bends to the left. But, of course! We can think of this as just rotating the first demonstration clockwise by 90 degrees. Note that the beam always deflects perpendicular to both the magnetic field and the velocity of the electrons. Finally, there is no force when the magnetic field is parallel to the beam. The fundamental magnetic force law quantitatively accounts for our observations. The force is the charge times the vector (or cross) product of the velocity and the magnetic field. The force is perpendicular to both the velocity and the magnetic field, and the direction is given by the right-hand rule. The force is a maximum when the velocity and magnetic field are perpendicular to each other, and is zero when they are parallel. Let’s check the magnetic force law for the cathode ray tube in the case where the magnetic field points to the right. Using the right-hand rule, the force should be up, but the deflection is down! What’s wrong? Electrons have negative charge, which flips the direction of the force. So the magnetic force law does work here! 3. CONCLUSION A charged particle that is moving in a magnetic field can experience a force, which is the fundamental magnetic force. The force is given in general by the charge of the particle times the vector product of the velocity of the particle and the magnetic field. What if we have the common situation of a wire that is carrying an electric current? If the wire is in a magnetic field, there will in general be a force on the moving charges and thus a force on the wire. This leads to many demonstrations and applications. We will show some of these in other videos. It is important to remember that the physics behind these observations comes from the fundamental magnetic force law that can be demonstrated by a cathode ray tube and a magnet. Physics lecture demonstrations are always fascinating, and the quest for them never ends. This is the Physics Department of the Naval Postgraduate School, and I’m Dr. Bruce Denardo. Thank you.