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Magnetic Fields Chapter 29

Magnetic Fields Chapter 29. Permanent Magnets & Magnetic Field Lines The Magnetic Force on Charges. Magnetism. Our most familiar experience of magnetism is through permanent magnets.

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Magnetic Fields Chapter 29

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  1. Magnetic FieldsChapter 29 Permanent Magnets & Magnetic Field Lines The Magnetic Force on Charges

  2. Magnetism • Our most familiar experience of magnetism is through permanent magnets. • These are made of materials which exhibit a property we call “ferromagnetism” - i.e., they can be magnetized. • Depending on how we position two magnets, they will attract or repel, i.e. they exert forces on each other. • Thus, a magnet must have an associated field: a magnetic field. • But we have not been able, so far, to isolate a magnetic monopole (the equivalent of an electric charge). • We describe magnets as having two magnetic poles: North (N) and South (S).

  3. What Do We Know About Permanent Magnets? • They always have two poles. • Like poles repel, opposite poles attract. • i.e. there are magnetic forces and fields! • They also attract un-magnetized ferromagnetic materials. • We can map out the field of a magnet using either a small magnet or small magnetic materials....

  4. N S Field of a Permanent Magnet

  5. N N S S Field of a Permanent Magnet The bar magnet (a magnetic dipole) wants to align with the B-field.

  6. N N S S Field of a Permanent Magnet The south pole of the small bar magnet is attracted towards the north pole of the big magnet. Also, the small bar magnet (a magnetic dipole) wants to align with the B-field. The field attracts and exerts a torque on the small magnet.

  7. N S N S Field of a Permanent Magnet The bar magnet (a magnetic dipole) wants to align with the B-field. The field exerts a torque on the dipole

  8. Magnetism • The origin of magnetism lies in moving electric charges. Moving (or rotating) charges generate magnetic fields. • An electric current generates a magnetic field. • A magnetic field will exert a force on a moving charge. • A magnetic field will exert a force on a conductor that carries an electric current.

  9. What Force Does a Magnetic Field Exert on Charges? • NONE!,If the charge is not moving with respect to the field (or if the charge moves parallel to the field). q

  10. What Force Does a Magnetic Field Exert on Charges? • NONE!,If the charge is not moving with respect to the field (or if the charge moves parallel to the field). q • If the charge is moving, there • is a force on the charge, • perpendicularto both v and B. • F = q vxB q

  11. Force on a Charge in aMagnetic Field • As we saw, force is perpendicular to both v and B. • The force is also largest for v perpendicular to B, smallestfor v parallel to B.

  12. Force on a Charge in aMagnetic Field • As we saw, force is perpendicular to both v and B. • The force is also largest for v perpendicular to B, smallestfor v parallel to B. This can be summarized as:

  13. Force on a Charge in aMagnetic Field • As we saw, force is perpendicular to both v and B. • The force is also largest for v perpendicular to B, smallestfor v parallel to B. This can be summarized as: F or: v q m B

  14. Force on a Charge in aMagnetic Field F v q m B

  15. Units of Magnetic Field As , then,

  16. Units of Magnetic Field As , then, Therefore the units of magnetic field are:

  17. Units of Magnetic Field As , then, Therefore the units of magnetic field are: ...or:

  18. Units of Magnetic Field As , then, Therefore, the units of magnetic field are: ...or: (Note: 1 Tesla = 10,000 Gauss)

  19. The Magnetic Force is Different From the Electric Force. Whereas the electric force acts in the same direction as the field: The magnetic force acts in a direction orthogonal to the field:

  20. The Magnetic Force is Different From the Electric Force. Whereas the electric force acts in the same direction as the field: The magnetic force acts in a direction orthogonal to the field: (Use “Right-Hand” Rule to determine direction of F)

  21. The Magnetic Force is Different From the Electric Force. Whereas the electric force acts in the same direction as the field: The magnetic force acts in a direction orthogonal to the field: (Use “Right-Hand” Rule to determine direction of F) And --- the charge must be moving !!

  22. Trajectory of Charged Particlesin a Magnetic Field (B field points into plane of paper.) B + + + + + + + + + + + + + + + + + + + + v F

  23. Trajectory of Charged Particlesin a Magnetic Field (B field points into plane of paper.) v B B + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + v F F

  24. Trajectory of Charged Particlesin a Magnetic Field (B field pointsinto plane of paper.) v B B + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + v F F Magnetic Force is a centripetal force

  25. s  r  at ar ar  v Rotational Motion  = s / r  s =  r  ds/dt = d/dt r  v =  r  = angle,  = angular speed,  = angular acceleration at = r  tangential acceleration ar = v2 / rradial acceleration The radial acceleration changes the direction of motion, while the tangential acceleration changes the speed. Uniform Circular Motion  = constant  v and ar constant but direction changes KE = ½ mv2 = ½ mw2r2 ar = v2/r = 2 r F = mar = mv2/r = m2r

  26. v B + + + + + + + + + + + + + + + + + + + + F r Radius of Charged ParticleOrbit in a Magnetic Field Centripetal Magnetic Force Force =

  27. Radius of Charged ParticleOrbit in a Magnetic Field Centripetal Magnetic Force Force = v B + + + + + + + + + + + + + + + + + + + + F r

  28. v B + + + + + + + + + + + + + + + + + + + + F r Radius of Charged ParticleOrbit in a Magnetic Field Centripetal Magnetic Force Force =

  29. Radius of a Charged ParticleOrbit in a Magnetic Field Centripetal Magnetic Force Force = v B + + + + + + + + + + + + + + + + + + + + F r Note: as , the magnetic force does no work!

  30. v B + + + + + + + + + + + + + + + + + + + + F r Cyclotron Frequency The time taken to complete one orbit is:

  31. v B + + + + + + + + + + + + + + + + + + + + F r Cyclotron Frequency The time taken to complete one orbit is: Hence the orbit frequency, f

  32. v B + + + + + + + + + + + + + + + + + + + + F r Cyclotron Frequency The time taken to complete one orbit is: Hence the orbit frequency, f - known as the “cyclotron frequency” T = 2/ = 1/ƒ  ƒ = /2

  33. The Electromagnetic Force If a magnetic field and an electric field are simultaneously present, their forces obey the superposition principle and may be added vectorially:

  34. The Electromagnetic Force If a magnetic field and an electric field are simultaneously present, their forces obey the superposition principle and may be added vectorially:

  35. + + + + + + + + + + + + + + + q The Electromagnetic Force If a magnetic field and an electric field are simultaneously present, their forces obey the superposition principle and may be added vectorially:

  36. The Magnetic Force is Different From the Electric Force. Whereas the electric force acts in the same direction as the field: The magnetic force acts in a direction orthogonal to the field: (Use “Right-Hand” Rule to determine direction of F) And --- the charge must be moving !!

  37. The Electromagnetic Force If a magnetic field and an electric field are simultaneously present, their forces obey the superposition principle and must be added vectorially: + + + + + + + + + + + + + + + q

  38. Exercise electron B v v’ • In what direction does the magnetic field point? • Which is bigger, v or v’ ?

  39. Exercise: answer electron B v v’ F • In what direction does the magnetic field point ? • Into the page [F = -e vxB] • Which is bigger, v or v’ ? • v = v’ [B does no work on the electron, Fv]

  40. What is the orbital radius of a charged particle (charge q, mass m) having kinetic energy K, and moving at right angles to a magnetic field B, as shown below?. x x x B x x x K • q m

  41. What is the orbital radius of a charged particle (charge q, mass m) having kinetic energy K, and moving at right angles to a magnetic field B, as shown below?. F = q v x B = m a and a = v2 / r q v B = m v2 / r x x x B x x x q B = m v / r  r q B = m v r r = m v / (q B) K = ½ mv2 • q m r2 = m2 v2 / (q B)2 (1/2m)r2 = ½ m v2 / (q B)2 (1/2m)r2 = K / (q B)2  r = [2mK]1/2 / (q B)

  42. What is the relation between the intensities of the electric and magnetic fields for the particle to move in a straight line ?. x x x B E x x x v • q m

  43. x x x B E x x x v • v = E / B q m FB FE • What is the relation between the intensities of the electric and magnetic fields for the particle to move in a straight line ?. FE = q E and FB = q v B If FE = FB the particle will move following a straight line trajectory q E = q v B

  44. x z y Trajectory of Charged Particlesin a Magnetic Field What if the charged particle has a velocity component along B? unchanged Circular motion in xy plane.

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