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Chapter 21

Chapter 21. Magnetism. Magnets. Properties Always two poles Opposite poles attract or like poles repel Exert magnetic force, therefore have magnetic field magnetic field is strongest at the pole magnetic field lines: from N to S Certain metals magnetic: iron, nickel, cobalt

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Chapter 21

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  1. Chapter 21 Magnetism

  2. Magnets • Properties • Always two poles • Opposite poles attract or like poles repel • Exert magnetic force, therefore have magnetic field • magnetic field is strongest at the pole • magnetic field lines: from N to S • Certain metals magnetic: iron, nickel, cobalt • Such metals called ferromagnetic • Soft magnetic material: easily magnetized or demagnetized (iron) • Hard magnetic material: cobalt, nickel

  3. Magnetic Fields (B) Magnetic field ( a vector quantity) lines: always from N to S

  4. Magnetic Field of Single Magnet

  5. Two magnets

  6. Earth as a Magnet

  7. Notations Used to Indicate Magnetic Field • From north to south, NS • The tail is the north; the arrow tip is the south • Going into the paper, X • Coming out of the paper,

  8. Magnetic Declination • The difference in degrees between the magnetic pole and the geographical pole

  9. How to become magnetic? A typical domain size: 10-4 cm to 0.1 cm

  10. Electromagnet • Magnetic field from a coil (= solenoid) • Curl the fingers of right hand to the direction of current • Thumb points to N-pole • The strength of magnetic field increases with the number of turns in a coil or inserting a metal rod

  11. Electromagnetism • Hans Oersted Experiment (2 min) • The running current produces magnetic field • Evidence: the moving compass, a magnet • Strength of the magnetic field is inversely proportional to the distance from the wire • The right-hand rule #1 • The thumb points to the direction of current of positive charges • The fingers curl to the direction of magnetic field

  12. Charged Particle in Magnetic Field x means the magnetic field (B) going into the paper. • F: force on the particle, pointing toward the center • v: velocity of the particle (+), tangent to the curve

  13. Circular Motion • Circular Motion Video ( 7 min) • proton in a magnetic field (40 sec)

  14. Magnetic field going into the paper • Velocity of particle (−) tangent to the circle • Magnetic force on the particle pointing to the center

  15. In the same magnetic field, the opposite charges move in opposite directions. When applying the right hand rule #2, point the thumb to the opposite direction for a negative particle.

  16. Indicates that the magnetic field is coming out of paper

  17. Fmagnetic = qvBsinθ • F magnetic = magnetic force(N) • q = charge (Coulombs = C) *1 electron = ‒1.602 x 10-19 C * 1 proton = 1.602 x 10-19 C • v = velocity of charged particle • B = strength of magnetic field (T = tesla = N/A∙m) • A = amperes, the unit of current • θ = angle between B and v • We will assume θ = 90˚ unless indicated otherwise • sin 90˚ = 1

  18. Fmagnetic = qvBsinθ • F magnetic = 0 when? (1) q = (2) v = (3) B = (4) Θ =

  19. Easy way to Remember, F, v & B • Right hand rule #2 • Fingers aligned with the magnetic field lines • Thumb aligned with the velocity of positive charge • For a negative charge, point the thumb to the opposite direction • Magnetic force coming out of palm

  20. Example 21A, Pg 774 A proton moving east at 1.0×105 m/s experiences a force of8.8×10‒19 N upward (away from Earth) due to Earth’s magnetic field. What is the magnitude and direction of Earth’s magnetic field at this location?

  21. Example 2 A beam of electrons travel at 3.0×106 m/s through a uniform magnetic field of 4.0×10-2 T at right angles to the field. How strong is the force acting on each electron? The charge of an electron = ‒1.60×10-19C

  22. If v and B are perpendicular, The charged particle moves in circular path: Fc = Fm *Only the direction, not magnitude, of velocity changes. *Fm = qvB *Fc = mv2/R

  23. Current in a Magnetic Field • What will happen to the wire? • Current in magnetic field (3 min)

  24. Current in Magnetic field

  25. Current placed in magnetic field follows the same right hand rule • Thumb points to the direction of current (of positive charge), i • Open fingers point to the direction of magnetic field, B • Force points out of the palm, F

  26. F = iLBsinө • F = force on a current-carrying wire (N) • i = current (A) • L = length of wire (m) • B = strength of magnetic field (T = tesla = N/A∙m) • ө = angle between wire and magnetic field

  27. A single charge vs. Current in magnetic field • F mag = qvB Fmag = iLB • Use: i = q/t (or q = it), v= L/t

  28. Example 21B, Pg 778 A wire 36 m long carries a current of 22 A from east to west. Find the magnetic force on the wire if the magnetic field of Earth at this location is directed from south to north and has a magnitude of 5.0×10‒5 T.

  29. Example 4 A straight wire carrying a 5.0-A current is in a uniform magnetic field at right angles to the wire. When 0.10 m of the wire is in the field, the force on the wire is 0.20 N. What is the strength of the magnetic field, B?

  30. Two Parallel Conducting Wires

  31. Application of Magnetic Field • electric motors • Video:Beakman Motor Lab (4 min) • speakers • Video:How do speakers work? (3 min) • magnetic storage – How does magnetic storage work? (3 min)

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