1. Magnetic Fields and Currents The crossover between topics

2. The force of a magnetic field on an electric charge So, like, yeah, a magnetic field can exert a force on an electric charge, so long as two conditions are met: The charge has to be moving The velocity of the charge has to have a component perpendicular to the field. The force is greatest if the velocity is completely perpendicular to the field

3. Which way will the force go? To find the direction of the force on a moving particle, use the right hand rule Using your RIGHT HAND, point your four fingers in the direction of the field Point your thumb in the direction of the velocity of the charge Your palm will them face the direction of the force Remember, THIS ONLY WORKS FOR YOUR RIGHT HAND.

4. Note on conventions for B-Fields B-fields are vectors, just like all the others So they are represented by arrows This works fine for left, right, up and down To represent B-field going into the page, use X’s (to represent the arrows’ feathers). To represent B-field going out of the page, use circles with dots (to represent the arrows’ points).

5. Example Pictures http://www.analog.com/library/analogdialogue/archives/41-06/AD41-06_FIG-01.jpg http://session.masteringphysics.com/problemAsset/1003319/16/152258.jpg

6. Picture of moving charged particle in B-field http://www.physics.sjsu.edu/becker/physics51/images/28_13A_Orbit_in_B_field.jpg

7. Charged particle in E field Note the direction of the force on the moving particle in the E-field. If + and – plates were north and south poles of B-Field, what would the direction of the force be? http://knol.google.com/k/-/-/3m2gdefbt6ovt/nd458g/1.jpg

8. Check for Understanding Once more with the volunteers

9. Magnetic Field The symbol for magnetic field is B Don’t ask me why. It just is. Units for magnetic field are Tesla 1 Tesla = 1 Newton/Ampere·meter Also have defined 1 gauss = 10 -4 Tesla Many magnetic phenomena, like the earth’s magnetic field, are much smaller than 1 Tesla B = F or F = Bq 0 (v sin(θ)) q 0 (v sin(θ))

10. Example A proton is in an accelerator going 5X10 6 m/s. It encounters a magnetic field of 0.4T and is moving at an angle of 30 0 with respect to the field. What is the direction and magnitude of the force on the proton? What would these be if it were an electron? F = 1.6X10 -13 N upward if field is to the right Same force downward

11. Circular Trajectory The magnetic force on a charged particle moving through a B-field will always be perpendicular to the path So it can cause particles to move in circles The circle has a radius of r = mv/qB

12. Bubble chambers http://www.particlephysics.ac.uk/news/picture-of-the-week/picture-archive/tracks-in-a-hydrogen-bubble-chamber/000329_med.jpg

13. Check for Understanding Once more with the volunteers

14. Force on a current carrying wire If a B-field exerts a force on a moving charge, it shouldn’t come as a surprise that it also exerts a force on a current, since currents are just lots of moving charges F = I L B sin(θ) I = current, L = length of wire, B = field

15. Loudspeakers http://www.practicalphysics.org/imageLibrary/jpeg500/607.jpg

16. Torque on a current-carrying wire If you can exert a force, you can exert a torque. When a current carrying loop is placed in a B-field, the loop tends to rotate so that the perpendicular becomes aligned with the field This is how DC motors work

17. DC motors http://cache.eb.com/eb/image?id=63375&rendTypeId=4

18. Check for Understanding Once more with the volunteers

19. Fields produced by currents In addition to magnetic forces on currents, currents will produce magnetic fields themselves In a long straight wire, the magnetic field will curl in a circle around the wire New RHR: curl your fingers into a half circle. Point your thumb along the direction of the current. Your fingers will be curled in the direction of the field.

20. A picture will help Field created has the equation: B = μ 0 I/2Πr Where μ 0 stands for the permittivity of free space (don’t ask). μ 0 = 4 Π X 10 -7 T·m/A http://www.school-for-champions.com/science/images/electromagnetism-magnetic_field.gif

21. Example A wire carries a current of 3 amps. What is the B-field 0.5 meters perpendicularly away from the wire? B = μ 0 I/2Πr I = 3 amps, r = 0.5 m

22. Magnetic field of a loop of wire http://image.tutorvista.com/content/magnetic-effects-electric-current/circular-loop-magnetic-field.jpeg

23. Solenoid A solenoid is a coil of wire that produces a magnetic field inside. N = # of turns of wire http://www.siemon.com/uk/white_papers/images/06-05-01-magnets5.gif

24. Check for Understanding Once more with the volunteers