Chapter 19 Magnetism Section 3 Magnetic Force

Charged Particles in a Magnetic Field Magnetic fields exert a magnetic force on moving charged particles. Force is greatest when the movement is perpendicular to the magnetic field Force is zero when the particle moves along the field lines Force is in between these values for other directions When the movement is perpendicular, the magnetic force is: Fmagnetic = qvB where q is the charge, v is the velocity, and B is the magnetic field strength. This text limits discussion to situations in which charges move parallel or perpendicular to the magnetic field lines. When the movement is perpendicular, the equation shown on the slide applies. When the movement is parallel, the force is zero.

Charged Particles in a Magnetic Field The right-hand rule for the force on a moving charged particle Thumb in the direction a positive particle is moving Fingers in the direction of the magnetic field The force will be in the direction of your palm For negative particles, the force is out the back of your hand.

Charged Particles in a Magnetic Field So, the magnetic field (B) can be determined from the force on moving charged particles as follows: SI unit: Tesla (T) where T = N/(C•(m/s)) = N/(A•m) = (V•s)/m2 Ask students to explain why all three of the units shown for Tesla are equivalent.

Force on a Charge Moving in a Magnetic Field Click below to watch the Visual Concept. Visual Concept

Classroom Practice Problems An electron moving north at 4.5  104 m/s enters a 1.0 mT magnetic field pointed upward. What is the magnitude and direction of the force on the electron? What would the force be if the particle was a proton? What would the force be if the particle was a neutron? Answers: 7.2  10-18 N west 7.2  10-18 N east 0.0 N For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful. This is a good application of the equation and the right-hand rule. Students will need to know the charge on an electron. Help them use the right-hand rule appropriately for a negative charge like the electron. You can also ask students what the force would be if the magnetic field was north or south instead of upward. (Zero, because the velocity and B field are parallel)

Magnetic Force as Centripetal Force Use the right-hand rule to determine the direction of the force. Which direction would the force be when the charge is at the top? the left side? the bottom? Always directed toward the center Because of this magnetic force, the charge moves in a circle. The force is centripetal. Charged particles entering Earth’s atmosphere get trapped into circular motion by the magnetic field and spiral toward the poles. This phenomenon causes the northern and southern lights.

Current-Carrying Wires Magnetic forces also exist on the moving charges in current-carrying wires. The right-hand rule to is used to determine the direction, as shown in the diagram. The magnitude of the force is as follows:

Parallel Current-Carrying Wires Current carrying wires create a magnetic field which interacts with the moving electrons in the nearby wire. Currents in the same direction produce attraction. Currents in opposite directions cause the wires to repel. Use the-right hand rule to verify the direction of the force for each of the four wires shown. Remind students that the direction for I is that of positive charge. The electrons are moving in the opposite direction. In order to verify the direction of the force, students will need to imagine the B field extending over to the other wire. For example, in the top diagram, the B field from the left wire at the right wire would be into the page. Therefore, since the positive charge is moving upward, the force would be to the left.

Classroom Practice Problem A 4.5 m wire carries a current of 12.5 A from north to south. If the magnetic force on the wire due to a uniform magnetic field is 1.1  103 N downward, what is the magnitude and direction of the magnetic field? Answer: 2.0  101 T to the west For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful. Using the right-hand rule may be difficult for students. They need to point their thumb to the south (direction of the positive charge) and palm downward (direction of the force). This will leave their fingers pointed to the west for the magnetic field.

Applications - Cathode Ray Tube Televisions and computer monitors use CRTs. A magnetic field deflects a beam of electrons back and forth across the screen to create an image. Have students look up more information about how a television produces an image (a CRT television, not LCD or plasma). “HowStuffWorks.com” is a web site with good information.

Applications - Speakers The forces on electrons as they move back and forth in the coil of wire cause the coil to vibrate. The coil is attached to the paper cone, so sound waves are produced by the vibration.

Galvanometer Click below to watch the Visual Concept. Visual Concept