Magnetic Forces on Electric Charges Or from meters to motors 5g Students know how to determine the direction of a magnetic field produced by a current flowing in a straight wire or in a coil. 5f Students know magnetic materials and electric currents (moving electric charges) are sources of magnetic fields and are subject to forces arising from the magnetic fields of other sources. 5k* Students know the force on a charged particle in an electrical field is qE where E is the electric field at the position of the particle and q is the charge of the particle. 5n* Students know the magnitude of the force on a moving particle in a magnetic field is qvB sin theta, and can use the right hand rule to find the direction of this force.

The Lorenz Force Force occurs between magnetic field and electric charge only when charge moves Direction predicted using 3rd RHR Force max when motion perpendicular to magnetic field, minimum when moves parallel F=qvB sin q Question: When does an electric charge feel a force in a magnetic field? In what direction is that force? Under what conditions is the Lorentz force maximum? When is it minimum? Write the equation used to find the magnitude of the Lorentz force. Activities: Demonstrate that a magnet can exert a force on an electric charge using a television or oscilliscope and a powerful magnet. Demonstrate several versions of Lorenz force using strong magnet and wire. Have students predict direction of force using 3rd Right Hand Rule. Be sure to demonstrate that force is 0 when charges move parallel to magnetic field. Write Lorenz equation on board and define terms. Draw two opposite plate charges on board and place positive charge between them. Ask when the charge will feel a force from the plates and which direction is it. Then draw two opposite poles and a charge and ask the same questions. Conclude by saying electric force is different from Lorenz force.

Check Question An electric charge is between two opposite plates. When will the charges feel a force and in what direction? Always and to the right Always and upward Only when charge moves and downward

Check Question 2 Which direction is the Lorentz force on the charge when it is: Stationary Moves to the right Moves upward Moves outward North South

The Evil Parallel Wire Problem Which direction will two parallel wires move if the currents are in the same direction? Towards one another Away from one another No motion at all How about if the currents are in opposite directions

Meters Current in loops creates magnetic field which interacts w/ permanent magnet to cause motion of pointer More current means more rotation Rotates 180 degrees or half way at max. Question: Draw a simple meter. Explain how the pointer is made to move. What is the maximum amount of rotation possible with a meter? Activities: Show transparency of simple meter. Explain two magnetic fields and how they interact to move the pointer. Show transparency of real meter and again explain why current causes movement Demo large real meter w/ video camera Ask how far rotation can go.

Meter Picture 

DC Motor Current moves thru loop which is inside magnetic field. Different parts of loop will feel Lorenz force in opposite directions causing rotation. Commutators change direction of current allowing for full rotation. Motors convert electrical energy into kinetic energy Questions: How does a motor use the Lorenz force to convert electrical energy to kinetic energy? How do motors make complete rotations? Activities: Show transparency of single loop motor and discuss parts and purpose, Use fingers and clothes hanger wire loop to demonstrate different parts. Discuss how force is different in different parts of the loop. Use wire loop if necessary. Discuss function of commutators to complete rotation. Talk about current turning on and off and force changing direction in specific part of loop.

The Aurora Most charged particles are deflected away from Earth by field. + charges moving inward on left of Earth and – charges moving outward on right experience inward force resulting in them circling earth in opposite directions Charges that come from above and below poles move parallel to field and are not deflected. When they impact atmosphere they create aurora Question: What happens to most of the charged particles that hit our magnetic field? What are the Van Allen radiation belts? Why does the aurora only occur at the poles? Activities: Draw Earth and its magnetic field. Remind students that actual field is 3 D and has the general shape of a donut. Show what happens to positive and negative charges coming from right or left. Show how radiation belts form from negative charges moving inward on left side of Earth, and positive charges moving inward on the right side of the Earth. Comment on how Lorentz force is providing a centripetal force for these charges. Belts rotate in different directions and are at different heights well above space shuttle territory. Show that charges that head for poles move parallel to field lines and thus will not be deflected. When these particles collide with the atmosphere they excite electrons to higher energy levels. When they fall back they emit light. For oxygen the color will be red, nitrogen green so these are the primary colors of the aurora. Other planets have been found to have auroras including Saturn which has auroras 10x the size of the entire earth.