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Magnetic Fields

Magnetic Fields. Tanya Liu. Some Expectations. Participate! Please don’t be scared, I would like to hear everyone’s voice Once again, if you don’t understand me, just raise your hand There will be some short assignments given

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Magnetic Fields

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  1. Magnetic Fields Tanya Liu

  2. Some Expectations • Participate! Please don’t be scared, I would like to hear everyone’s voice • Once again, if you don’t understand me, just raise your hand • There will be some short assignments given • All my slides will be uploaded to Professor Dodero’s site: http://my.liceti.it/prof/dodero/

  3. Some Pronounciations C1 = “C sub one” or “C one”

  4. Some Pronounciations • a + b = “a plus b” • a – b = “a minus b” • a x b = “a times b” • a/b = “a over b” • ab= “a to the b” • a2 = “a to the second/ a squared” • a3, a4 = “a to the third, a to the fourth, etc” • 0.05 = “zero point zero five” • 2x10-5 = “two times ten to the negative fifth”

  5. Introduction to Magnetic Fields We know that charged objects produce electric fields What are sources of magnetic fields? - permanent magnets - current

  6. Introduction to Magnetic Fields: Permanent Magnets A permanent magnet is made from magnetized material that consistently produces its own magnetic field • this is due to something known as ferromagnetism Ferromagnetic materials can either form permanent magnets, or are attracted to magnets

  7. Introduction to Magnetic Fields: Permanent Magnets • A bar magnet is a permanent magnet, and consists of two poles, north and south • The magnetic field lines leave from the north pole and enter the south pole

  8. Introduction to Magnetic Fields: Permanent Magnets • Opposite poles of a magnet attract each other, and like poles repel

  9. Introduction to Magnetic Fields: Permanent Magnets • when you break a magnet in half, you end up with two smaller magnets, both with north and south poles • there are no magnetic monopoles

  10. Introduction to Magnetic Fields: Permanent Magnets • Magnetic Field of a Bar Magnet

  11. How do we define a magnetic field? • In electrostatics, • Since magnetic monopoles don’t exist, however, this changes things a bit

  12. How do we define a magnetic field? • For a single charge q with velocity moving in a magnetic field there is a magnetic force on the charge:

  13. Cross Product Review = perpendicular projection of onto If ,

  14. How do we define a magnetic field? • the SI unit for is the tesla

  15. Concept Question If a moving charge traveling in the positive direction enters a magnetic field pointing in the positive direction, in which direction will the moving charge be deflected? • - • + • - • +

  16. Concept Question Solution Answer: b is correct. =

  17. Magnetic force on a current-carrying wire • we’ve seen that a single charge experiences a magnetic force, • for a current-carrying wire of length l,

  18. Magnetic force on a current-carrying wire A B C is 0 when I=0, so the wire does not bend. I is no longer 0, so now there is an , and the wire bends The direction of current is reversed from B, so the wire bends in the opposite direction

  19. Group Problem A rectangular current loop with width a and length b is placed in a magnetic field, , as shown below. What is the net force felt by the loop, and what is the net torque?

  20. Some Notes • Homework assignment! Due on Friday, Jan 18th

  21. Concept Question A charged particle enters a magnetic field as shown below. What will the resulting path of the particle be? • circle • straight line • helix

  22. Charged particle in a magnetic field What happens to the path of a moving charged particle as it enters a magnetic field? at all times, so the charge follows a circular path ( acts like centripetal force)

  23. Concept Question A charged particle enters a magnetic field as shown below. What will the resulting path of the particle be? • circle • straight line • helix

  24. Charged particle in a magnetic field If the particle has a component of velocity parallel to the magnetic field, it will follow a helical path There is a component of parallel to at all times, so the charge still moves in a circle, it also moves in a path along the direction of

  25. Group Problem For a particle with charge q entering a magnetic field of magnitude with velocity at an angle of θ to the magnetic field, what is the radius of the circular path it travels, and what is the time it takes to complete one circle? θ

  26. Lorentz Force • We know a charged particle in an electric field feels an electric force: • We also know a moving charged particle in a magnetic field feels a magnetic force: • The total force on a moving charged particle in an electric field and a magnetic field feels is the sum of these forces:

  27. Group Problem An electron with charge –e and mass m is emitted from plate C and accelerated towards slit A. There is a potential difference between A and C, where . • What is the speed of the electron after it reaches A? (Use conservation of energy, electrical potential energy ) • As the electron travels through the region after slit A, it passes through a downwards electric field , and a magnetic field into the page. At what velocity will the electron move in a straight line through this region? • Using your answers from part a and b, what is the charge to mass ratio of this electron (what is )?

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