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Understanding Magnetism and Magnetic Forces in Physics

Learn about magnetic forces on moving charges, examples, x and o diagrams, deflection animations, hand rules, and more in this comprehensive guide to magnetism and induction.

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Understanding Magnetism and Magnetic Forces in Physics

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  1. Magnetic Force • Introduction • Magnetic force on moving charge • Examples • Magnetic deflection animation • x and o diagrams • More examples • Magnetic force on current-carrying wire • Still more examples

  2. Magnetism and Induction • Magnetism is weird, both for the force involved and its direction. It’s like a correction to electric forces when charges are in motion, and correction terms are always weird.* • Electric affects charges, magnetism affects charges in motion • Electric causes linear accelerations, magnetism causes sideways deflections (RH rule) • Magnetism and Induction are very, very (societally) important! • Motors, generators, loudspeakers, credit cards, hard drives, transformers….. *Can derive magnetic force from electric force and Einstein’s theory of relativity

  3. Magnetic Force on moving charge • Formal vector definition • Force Magnitude • Maximum in v perpendicular to B • Zero when v parallel to B • Like torque, only cares about component • Force Direction • Direction right-hand rule • Positive vs. negative charge • B units Tesla = 1 N/A-m (show units) • Units

  4. Right-hand rule • Hand in direction of v • Bend in direction B • Thumb points in direction of F • Alternative right hand rules

  5. Example - Magnetic Force on moving charge • A proton/electron moves straight north at 105 m/s in a 1 T magnetic field pointed west to east. What is the magnitude and direction of the force? • Direction • Proton: straight down into the ground • Electron: straight up in air

  6. Example 2 - Magnetic Force on moving charge • It feels no force when its moving in the direction or against the direction of the field (ϴ = 0 or 180). Thus the field must point north to south, or south to north • Direction • v up • B to north • gives F to west • Magnitude F v

  7. Magnetic force video • mp4 video

  8. X’s and O’s • Simplify diagrams – B field out of or into plane of paper B coming out B going in (+) deflects to right (-) deflects to right

  9. Circular motion in magnetic field • RHR (flip sign) center of circle • Set mag force = mv2/r

  10. Circular motion in magnetic field http://objetoseducacionais2.mec.gov.br/bitstream/handle/mec/23847/ciclotronE.swf?sequence=1

  11. Examples – Force on Moving Charge • Problem 11 – Direction of force for (-) charge • Problem 12 – Direction of field for (+) charge

  12. Problem 11-12 Answers

  13. More examples – Force on Moving Charge • To accelerate in electric field • To deflect in magnetic field • Double mass, radius goes up by • Quadruple mass, double charge, radius goes up by

  14. Magnetic Deflection http://neutrons.ornl.gov/about

  15. Magnetic Deflection

  16. Force on Current-carrying wire • For single charge • For many charges • Rewriting velocity • First term is just current

  17. Summary – 2 magnetic force laws • For single charge • For current-carrying wire

  18. Example 20-1 • Direction: into page

  19. Example 20-2 • Current direction: CCW • Force on sides

  20. Problem 7 90° w.r.t. field 80° w.r.t. field Force: toward viewer toward viewer

  21. Magnetron - Microwave ovens • Accelerated radially by radial Electric Field • Spun circularly by crossed magnetic field. • Interacts with resonant cavities around perimeter.

  22. Magnetron - Details • Magnets top and bottom

  23. Magnetron - History Amazon.com Tuxedo Park : A Wall Street Tycoon and the Secret Palace of Science That Changed the Course of World War II Paperback – May 6, 2003 by Jennet Conant  (Author) • The untold story of an eccentric Wall Street tycoon and the circle of scientific geniuses he assembled before World War II to develop the science for radar and the atomic bomb. Together they changed the course of history. • helped defend Britain from the Blitz! • Could now put radar in planes!

  24. Magnetism and Induction Flowchart Magnetic field plus moving charge/current creates force

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