Chapter 16 - Maxwell s Equations

Transcription

1 David J. Starling Penn State Hazleton PHYS 214

2 Gauss s Law relates point charges to the value of the electric field. Φ E = E d A = q enc ɛ 0

3 Gauss s Law relates point charges to the value of the electric field. Φ E = E d A = q enc ɛ 0 We sometimes refer to point charges as electric monopoles. (consider: electric dipole)

4 Gauss s Law relates point charges to the value of the electric field. Φ E = E d A = q enc ɛ 0 We sometimes refer to point charges as electric monopoles. (consider: electric dipole) But what about magnetic flux?

5 Gauss s Law for magnetic fields is much simpler! Φ B = B d A = 0

6 Gauss s Law for magnetic fields is much simpler! Φ B = B d A = 0 There are no magnetic monopoles!

7 Gauss s Law for magnetic fields is much simpler! Φ B = B d A = 0 There are no magnetic monopoles! How is this different from Induction?

8 These two equations make up half of Maxwell s. E d A = q enc ɛ 0 B d A = 0

9 These two equations make up half of Maxwell s. E d A = q enc ɛ 0 B d A = 0 Using the gradient vector ( ) we can differentiate both sides: E = ρ ɛ 0 B = 0

10 We know that a changing mangetic field creates an electric field. E d s = dφ B dt = d B d A dt

11 We know that a changing mangetic field creates an electric field. E d s = dφ B dt = d dt B d A This is Faraday s Law of induction, the third of Maxwell s equations.

12 We know that a changing mangetic field creates an electric field. E d s = dφ B dt = d dt B d A This is Faraday s Law of induction, the third of Maxwell s equations. Differentiated: E = B t

13 We also know how to create a magnetic field from a current: B d s = µ 0 i enc

14 We also know how to create a magnetic field from a current: B d s = µ 0 i enc This is Ampere s Law.

15 We also know how to create a magnetic field from a current: B d s = µ 0 i enc This is Ampere s Law. Unfortunately, it is incomplete!

16 Let s apply Ampere s Law for charging a capacitor with a straight wire. B d s = µ 0 i enc

17 Let s apply Ampere s Law for charging a capacitor with a straight wire. B d s = µ 0 i enc Depending on the surface, the enclosed current is different!

18 Ampere s Law needs to be modified to include electric fields (as well as currents). B d s = µ 0 i enc +???

19 Ampere s Law needs to be modified to include electric fields (as well as currents). B d s = µ 0 i enc +??? Depending on the surface, the enclosed current is different!

20 The magnetic field (LHS) must be the same for each surface! B d s = µ 0 i enc +???

21 The magnetic field (LHS) must be the same for each surface! B d s = µ 0 i enc +??? One surface has a current through it (RHS: µ 0 i enc ) but no electric flux. The other surface has an electric flux ( E A) but no current.

22 Regardless of which surface we use, we should get the same thing. Φ E = E A ( ) q = (Aî) ɛ 0 Aî = q/ɛ 0

23 Regardless of which surface we use, we should get the same thing. Φ E = E A ( ) q = (Aî) ɛ 0 Aî = q/ɛ 0 To equate with current, let s differentiate both sides: dφ E dt µ 0 ɛ 0 dφ E dt = i enc /ɛ 0 = µ 0 i enc

24 Regardless of which surface we use, we should get the same thing. Φ E = E A ( ) q = (Aî) ɛ 0 Aî = q/ɛ 0 To equate with current, let s differentiate both sides: dφ E dt µ 0 ɛ 0 dφ E dt = i enc /ɛ 0 = µ 0 i enc Notice how the LHS is the same as the RHS of Ampere s Law!

25 If we add both terms, this Ampere-Maxwell law is now valid for any situation. B d s = µ 0 i enc + µ 0 ɛ 0 dφ E dt

26 If we add both terms, this Ampere-Maxwell law is now valid for any situation. B d s = µ 0 i enc + µ 0 ɛ 0 dφ E dt Although derived with a capacitor, this is general.

27 The fourth Maxwell s Equation is therefore: ) dφ E B d s = µ 0 (i enc + ɛ 0 dt

28 The fourth Maxwell s Equation is therefore: ) dφ E B d s = µ 0 (i enc + ɛ 0 dt Differentiated: ( ) E B = µ 0 J + ɛ 0 t

29 The fourth Maxwell s Equation is therefore: ) dφ E B d s = µ 0 (i enc + ɛ 0 dt Differentiated: ( ) E B = µ 0 J + ɛ 0 t (note: we use Stoke s Theorem to relate line integrals to surface integrals)

30 dφ We sometimes refer to ɛ E 0 dt current i d,enc. as displacement B d s = µ 0 (i enc + i d,enc )

31 dφ We sometimes refer to ɛ E 0 dt current i d,enc. as displacement B d s = µ 0 (i enc + i d,enc ) Although compact, this form is less helpful. Let s apply our new Law!

32 Magnetic field from charging a capacitor.

33 Magnetic field from charging a capacitor. The displacement current helps us use the right hand rule.

34 To solve for the magnetic field between charging capacitor places, we must use the Ampere-Maxwell law.

35 To solve for the magnetic field between charging capacitor places, we must use the Ampere-Maxwell law. Inside, we look at a loop of fixed (arbitrary) radius r and calculate both sides of the Ampere-Maxwell law.

36 For a fixed radius loop, symmetry allows us to simplify!

37 For a fixed radius loop, symmetry allows us to simplify! ) dφ E B d s = µ 0 (i enc + ɛ 0 dt

38 For a fixed radius loop, symmetry allows us to simplify! ) dφ E B d s = µ 0 (i enc + ɛ 0 dt d(ea int ) B(2πr) = 0 + µ 0 ɛ 0 dt

39 For a fixed radius loop, symmetry allows us to simplify! ) dφ E B d s = µ 0 (i enc + ɛ 0 dt d(ea int ) B(2πr) = 0 + µ 0 ɛ 0 dt B = 1 2πr µ d 0ɛ 0 dt (qa int/ɛ 0 A cap )

40 For a fixed radius loop, symmetry allows us to simplify! ) dφ E B d s = µ 0 (i enc + ɛ 0 dt d(ea int ) B(2πr) = 0 + µ 0 ɛ 0 dt B = 1 2πr µ d 0ɛ 0 dt (qa int/ɛ 0 A cap ) B = µ 0i 2πr πr 2 πr 2 B = µ 0i 2πR 2 r

41 The Earth has its own magnetic field which can be appoximated by a bar magnet at its core.

42 The Earth has its own magnetic field which can be appoximated by a bar magnet at its core. The field at the surface has a declination (from longitude) and an inclination from horizontal.

43 Magnetic fields come from the motion of charges, but also from the intrinsic property of charges called spin.

44 Magnetic fields come from the motion of charges, but also from the intrinsic property of charges called spin. The spin S and magnetic dipole momentum µ s are related: µ s = e m S

45 Experimentally, we find that the z-component of the spin of an electron is quantized.

46 Experimentally, we find that the z-component of the spin of an electron is quantized. S z = ± 1 2 ( = m 2 kg/s is the reduced Planck s constant)

47 Experimentally, we find that the z-component of the spin of an electron is quantized. S z = ± 1 2 ( = m 2 kg/s is the reduced Planck s constant) The dipole moment is therefore: µ s,z = ± e 2m = ±µ B (µ B = J/T is the Bohr magneton

48 Like other dipoles, electrons have potential energy in an external magnetic field.

49 Like other dipoles, electrons have potential energy in an external magnetic field. U s = µ s B ext = µ s,z B ext = µ B B ext (assuming B points in z-direction)

50 As electrons move around nuclei, their motion also causes an orbital magnetic dipole moment.

51 As electrons move around nuclei, their motion also causes an orbital magnetic dipole moment. The orbital angular momentum L orb and magnetic dipole momentum µ orb are related: µ orb = e 2m L orb

52 Experimentally, we find that the z-component of the orbital angular momentum of an electron is quantized.

53 Experimentally, we find that the z-component of the orbital angular momentum of an electron is quantized. L orb,z = m l, for m l = 0, ±1, ±2,...

54 Experimentally, we find that the z-component of the orbital angular momentum of an electron is quantized. L orb,z = m l, for m l = 0, ±1, ±2,... The dipole moment is therefore: µ orb,z = m l e 2m = m lµ B

55 This dipole moment also has energy:

56 This dipole moment also has energy: U orb = µ orb B ext = µ orb,z B ext = m l µ B B ext (assuming B points in z-direction)

57 When electrons and nuclei bond together to form complex materials, they can behave in different ways magnetically.

58 When electrons and nuclei bond together to form complex materials, they can behave in different ways magnetically. Diamagnetism is typically weak and can be found in all materials if not overwhelmed by other effects.

59 Paramagnetic materials have no net magnetic field, but move toward stronger applied magnetic fields (rather than away).

60 Paramagnetic materials have no net magnetic field, but move toward stronger applied magnetic fields (rather than away). When the field is removed, the object reverts to its original non-magnetic state.

61 When a material is exposed to an external magnetic field, only a portion of the magnetic dipoles rotate into alignment.

62 When a material is exposed to an external magnetic field, only a portion of the magnetic dipoles rotate into alignment. The magnetization M = µ total /V is temperature dependent, with M max = Nµ/V.

63 Ferromagnetic materials are similar to paramagnetic materials, but retain their magnetic field.

64 Ferromagnetic materials are similar to paramagnetic materials, but retain their magnetic field. Magnetic domains form, and they persist even after the external field is removed.