2. Chapter 20, 21 Magnetism and Electromagnetism

3. MAGNETISM Arguable the oldest subject in Physics: ancient Greeks (near the City of Magnesia) and Chinese realized certain strange stones attracted iron. Around 1600, William Gilbert proposed that the Earth itself is A gigantic magnet. For a long time, people knew only one source of magnetism from Iron. In 1821, a Danish physicist, Oersted noticed that an electrical wire carrying current made the near-by compass reorient.  First clue of inter-relation between electricity and Magnetism. Ampere, Faraday established the nature of electricity and magnetism (all from their experimental observations).

4. Magnets Poles of a magnet are the ends where objects are most strongly attracted Poles of a magnet are the ends where objects are most strongly attracted Two poles, called north and southTwo poles, called north and south Like poles repel each other and unlike poles attract each other Like poles repel each other and unlike poles attract each other Similar to electric chargesSimilar to electric charges Magnetic poles cannot be isolated Magnetic poles cannot be isolated If a permanent magnetic is cut in half repeatedly, you will still have a north and a south poleIf a permanent magnetic is cut in half repeatedly, you will still have a north and a south pole This differs from electric chargesThis differs from electric charges There is some theoretical basis for monopoles, but none have been detectedThere is some theoretical basis for monopoles, but none have been detected

5. More About Magnetism An unmagnetized piece of iron can be magnetized by stroking it with a magnet An unmagnetized piece of iron can be magnetized by stroking it with a magnet Somewhat like stroking an object to charge an objectSomewhat like stroking an object to charge an object Magnetism can be induced Magnetism can be induced If a piece of iron, for example, is placed near a strong permanent magnet, it will become magnetizedIf a piece of iron, for example, is placed near a strong permanent magnet, it will become magnetized

6. S N S N SN S N S N

7. SNNS Magnets exist in pairs of N-S poles. A theoretical prediction says that it is possible to have magnetic mono-poles but they have not been observed!! Magnetic field cannot be defined as E-field, F E = qE F B = q B B

8. Magnetic Fields A vector quantity A vector quantity Symbolized by B Symbolized by B Direction is given by the direction a north pole of a compass needle points in that location Direction is given by the direction a north pole of a compass needle points in that location Magnetic field lines can be used to show how the field lines, as traced out by a compass, would look Magnetic field lines can be used to show how the field lines, as traced out by a compass, would look

9. Magnetic Field Lines, sketch A compass can be used to show the direction of the magnetic field lines (a) A compass can be used to show the direction of the magnetic field lines (a) A sketch of the magnetic field lines (b) A sketch of the magnetic field lines (b)

10. Magnetic Field Lines, Bar Magnet Iron filings are used to show the pattern of the magnetic field lines Iron filings are used to show the pattern of the magnetic field lines The direction of the field is the direction a north pole would point The direction of the field is the direction a north pole would point

11. Magnetic Field Lines, Unlike Poles Iron filings are used to show the pattern of the magnetic field lines Iron filings are used to show the pattern of the magnetic field lines The direction of the field is the direction a north pole would point The direction of the field is the direction a north pole would point

12. Magnetic Field Lines, Like Poles Iron filings are used to show the pattern of the magnetic field lines Iron filings are used to show the pattern of the magnetic field lines The direction of the field is the direction a north pole would point The direction of the field is the direction a north pole would point Compare to the electric field produced by like chargesCompare to the electric field produced by like charges

13. Magnetic and Electric Fields An electric field surrounds any stationary electric charge An electric field surrounds any stationary electric charge A magnetic field surrounds any moving electric charge A magnetic field surrounds any moving electric charge A magnetic field surrounds any magnetic material A magnetic field surrounds any magnetic material

14. Earth’s Magnetic Field The Earth’s geographic north pole corresponds to a magnetic south pole The Earth’s geographic north pole corresponds to a magnetic south pole The Earth’s geographic south pole corresponds to a magnetic north pole The Earth’s geographic south pole corresponds to a magnetic north pole Strictly speaking, a north pole should be a “north-seeking” pole and a south pole a “south-seeking” poleStrictly speaking, a north pole should be a “north-seeking” pole and a south pole a “south-seeking” pole

15. Earth’s Magnetic Field The Earth’s magnetic field resembles that achieved by burying a huge bar magnet deep in the Earth’s interior The Earth’s magnetic field resembles that achieved by burying a huge bar magnet deep in the Earth’s interior

16. N S In unifrom field, no force only torque!!!

17. I X Right-handed cork-screw rule Magnetic field can make a closed loop (not with E-field).

18. Magnetic Fields In a magnetic field, a current carrying wire experiences a magnetic force In a magnetic field, a current carrying wire experiences a magnetic force This force has a maximum value when the wire is perpendicularly to the magnetic field linesThis force has a maximum value when the wire is perpendicularly to the magnetic field lines This force is zero when the wire is along the field linesThis force is zero when the wire is along the field lines

19. Magnetic Fields, cont One can define a magnetic field in terms of the magnetic force exerted on current carrying wire One can define a magnetic field in terms of the magnetic force exerted on current carrying wire Similar to the way electric fields are definedSimilar to the way electric fields are defined

20. Units of Magnetic Field The SI unit of magnetic field is the Tesla (T) The SI unit of magnetic field is the Tesla (T) Wb is a WeberWb is a Weber

21. I B F F B = ILB Length of the section in B- field Magnetic induction Magnetic flux density Magnetic field (strength) [B] = [F/IL] = Ns/Cm = Tesla *1 Tesla = 10 4 gauss

22. F B = ILB (1) A net force on the loop (2) A net torque on the loop (3) A net force and torque (4) nothing, zip ?Magnetic force on a current carrying loop

23. SN X X

24. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/mothow.html

25. DC Electrical Motor X X X X X X X X X

26. Magnetic Force on Moving Charge The direction of the magnetic force is always perpendicular to both v and B The direction of the magnetic force is always perpendicular to both v and B F = qvB (vB) F = qvB (vB) Force is smaller when v is not perpendicular to B Force is smaller when v is not perpendicular to B F = 0 when v is parallel to B F = 0 when v is parallel to B

27. Right Hand Rule Hold your right hand open Hold your right hand open Place your fingers in the direction of B Place your fingers in the direction of B Place your thumb in the direction of v Place your thumb in the direction of v The direction of the force on a positive charge is directed out of your palm The direction of the force on a positive charge is directed out of your palm If the charge is negative, the force is opposite that determined by the right hand ruleIf the charge is negative, the force is opposite that determined by the right hand rule

28. CRT TV

29. http://www.paikstudios.com/ Nam June Paik

30. XxxxxXxxxxXxxxxXxxxxXxxxxXxxxxXxxxxXxxxxXxxxxXxxxxXxxxxXxxxx q v F F = q v B ┴ Force by a magnetic field on a moving charge is always Perpendicular to the direction of motion. NO WORK DONE BY THE FIELD!!! q v F q v F q v F q v F

31. XxxxxXxxxxXxxxxXxxxxXxxxxXxxxxXxxxxXxxxxXxxxxXxxxx B q m r v F B = q v B Force by B-field This force causes a circular motion. Centrepetal force = F B F c = mv 2 /r = q v B = F B r = mv/qB Since there is no work done by the field, in vacuum, The charge will make a circular motion forever!!!

32. Bubble Chamber

33. ............................................................ B = 0.36 T A univalent ion with mass m Came into the field region with Speed v = 6 x 10 5 m/s. An object with +e charge And mass m kg r = 0.4 m Mass m? Mass spectrometer

34. http://www.geo.mtu.edu/weather/aurora/images/aurora/jan.curtis/index4.html http://science.nasa.gov/headlines/y2002/23sep_auroraseason.htm

36. Magnetic Fields – Long Straight Wire A current-carrying wire produces a magnetic field A current-carrying wire produces a magnetic field The compass needle deflects in directions tangent to the circle The compass needle deflects in directions tangent to the circle The compass needle points in the direction of the magnetic field produced by the currentThe compass needle points in the direction of the magnetic field produced by the current

37. Direction of the Field of a Long Straight Wire Right Hand Rule #2 Right Hand Rule #2 Grasp the wire in your right handGrasp the wire in your right hand Point your thumb in the direction of the currentPoint your thumb in the direction of the current Your fingers will curl in the direction of the fieldYour fingers will curl in the direction of the field

38. Magnitude of the Field of a Long Straight Wire The magnitude of the field at a distance r from a wire carrying a current of I is The magnitude of the field at a distance r from a wire carrying a current of I is µ o = 4  x 10 -7 T m / A µ o = 4  x 10 -7 T m / A µ o is called the permeability of free spaceµ o is called the permeability of free space

39. I xo xo xo xo xo xo xo xo xo

40. http://www.bugman123.com/Physics/Physics.html For infinitely long solenoid B =  o nI n: number of turns/m

41. A solenoid electro-magnet is energized as shown in the figures. Since current flows through wire in the presence of magnetic fields, the solenoid will feel force. Which figure correctly describes the force acting on the solenoid?

43. Induced emf A current can be produced by a changing magnetic field A current can be produced by a changing magnetic field First shown in an experiment by Michael FaradayFirst shown in an experiment by Michael Faraday A primary coil is connected to a battery A primary coil is connected to a battery A secondary coil is connected to an ammeter A secondary coil is connected to an ammeter

44. Faraday’s Experiment The purpose of the secondary circuit is to detect current that might be produced by the magnetic field The purpose of the secondary circuit is to detect current that might be produced by the magnetic field When the switch is closed, the ammeter deflects in one direction and then returns to zero When the switch is closed, the ammeter deflects in one direction and then returns to zero When the switch is opened, the ammeter deflects in the opposite direction and then returns to zero When the switch is opened, the ammeter deflects in the opposite direction and then returns to zero When there is a steady current in the primary circuit, the ammeter reads zero When there is a steady current in the primary circuit, the ammeter reads zero

45. Faraday’s Conclusions An electrical current is produced by a changing magnetic field An electrical current is produced by a changing magnetic field The secondary circuit acts as if a source of emf were connected to it for a short time The secondary circuit acts as if a source of emf were connected to it for a short time It is customary to say that an induced emf is produced in the secondary circuit by the changing magnetic field It is customary to say that an induced emf is produced in the secondary circuit by the changing magnetic field

46. INDUCTION Michael Faraday (1791 – 1867) …it appeared very extraordinary, that as every electric current was accompanied by a corresponding intensity of magnetic action at right angles to the current, good conductors of electricity, when placed within the sphere of this action, should not have any current induced through them, or some sensible effect produced equivalent in force to such a current.

47. Primary CoilSecondary coil G ?

48. Summary of Experimental Findings EMF is induced in the secondary coil Only when the magnetic field through it changes. EMF induced is bigger if the area of coil is bigger. EMF is induced always in the opposite direction of change in magnetic field.  = A B : magnetic flux V ind = (B/t)(B/t) A - = -  /  t Lenz’s Law

49. Magnetic Flux The emf is actually induced by a change in the quantity called the magnetic flux rather than simply by a change in the magnetic field The emf is actually induced by a change in the quantity called the magnetic flux rather than simply by a change in the magnetic field Magnetic flux is proportional to both the strength of the magnetic field passing through the plane of a loop of wire and the area of the loop Magnetic flux is proportional to both the strength of the magnetic field passing through the plane of a loop of wire and the area of the loop

50. B A  = A B ┴ B B // B┴B┴ 0.3 m 0.2 m B = 0.175 T 40 = (0.3 x 0.2)(0.175 sin(40)) = 0.06 x 0.112 = 0.0067 T.m 2

51. V ind = -   /  t V t = -3  /  t Net number of loops 5 Ohm Example 23.2 150 turn loop with a 0.75 cm 2 cross-section Magnetic field: 0 T  0.25 T in 3.6 s What is the induced current in the coil? + - + + - +

52. Faraday’s Law and Electromagnetic Induction The instantaneous emf induced in a circuit equals the time rate of change of magnetic flux through the circuit The instantaneous emf induced in a circuit equals the time rate of change of magnetic flux through the circuit If a circuit contains N tightly wound loops and the flux changes by Φ during a time interval t, the average emf induced is given by Faraday’s Law: If a circuit contains N tightly wound loops and the flux changes by Φ during a time interval t, the average emf induced is given by Faraday’s Law:

53. Faraday’s Law and Lenz’ Law The change in the flux, ΔΦ, can be produced by a change in B, A or θ The change in the flux, ΔΦ, can be produced by a change in B, A or θ Since Φ B = B A cos θSince Φ B = B A cos θ The negative sign in Faraday’s Law is included to indicate the polarity of the induced emf, which is found by Lenz’ Law The negative sign in Faraday’s Law is included to indicate the polarity of the induced emf, which is found by Lenz’ Law The polarity of the induced emf is such that it produces a current whose magnetic field opposes the change in magnetic flux through the loopThe polarity of the induced emf is such that it produces a current whose magnetic field opposes the change in magnetic flux through the loop That is, the induced current tends to maintain the original flux through the circuitThat is, the induced current tends to maintain the original flux through the circuit

54. AC Generator http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motorac.html#c2

55. James Clerk Maxwell Electricity and magnetism were originally thought to be unrelated Electricity and magnetism were originally thought to be unrelated in 1865, James Clerk Maxwell provided a mathematical theory that showed a close relationship between all electric and magnetic phenomena in 1865, James Clerk Maxwell provided a mathematical theory that showed a close relationship between all electric and magnetic phenomena

56. Maxwell’s Starting Points Electric field lines originate on positive charges and terminate on negative charges Electric field lines originate on positive charges and terminate on negative charges Magnetic field lines always form closed loops – they do not begin or end anywhere Magnetic field lines always form closed loops – they do not begin or end anywhere A varying magnetic field induces an emf and hence an electric field (Faraday’s Law) A varying magnetic field induces an emf and hence an electric field (Faraday’s Law) Magnetic fields are generated by moving charges or currents (Ampère’s Law) Magnetic fields are generated by moving charges or currents (Ampère’s Law)

57. Maxwell’s Predictions Maxwell used these starting points and a corresponding mathematical framework to prove that electric and magnetic fields play symmetric roles in nature Maxwell used these starting points and a corresponding mathematical framework to prove that electric and magnetic fields play symmetric roles in nature He hypothesized that a changing electric field would produce a magnetic field He hypothesized that a changing electric field would produce a magnetic field Maxwell calculated the speed of light to be 3x10 8 m/s Maxwell calculated the speed of light to be 3x10 8 m/s He concluded that visible light and all other electromagnetic waves consist of fluctuating electric and magnetic fields, with each varying field inducing the other He concluded that visible light and all other electromagnetic waves consist of fluctuating electric and magnetic fields, with each varying field inducing the other

58. Hertz’s Confirmation of Maxwell’s Predictions Heinrich Hertz was the first to generate and detect electromagnetic waves in a laboratory setting Heinrich Hertz was the first to generate and detect electromagnetic waves in a laboratory setting

59. James Clerk Maxwell’s Equations (1867) Pre-Maxwell E-field comes out from p-charge and terminates at negative charge. B-field cannot do like E-field. No magnetic monopole! Faraday’s law Ampere’s law

60. The velocity of transverse undulations in our hypothetical medium, calculated from the electromagnetic experiments, agrees so exactly with the velocity of light calculated from the optical experiments, that we can scarcely avoid the inference that light consists in the transverse undulation of same medium which is the cause of electric and magnetic Phenomena. 2.9986 x 10 8 m/s

61. Hertz’s Experiment (1887)

62. Hertz’s Experimental Apparatus An induction coil is connected to two large spheres forming a capacitor An induction coil is connected to two large spheres forming a capacitor Oscillations are initiated by short voltage pulses Oscillations are initiated by short voltage pulses The inductor and capacitor form the transmitter The inductor and capacitor form the transmitter

63. Hertz’s Experiment Several meters away from the transmitter is the receiver Several meters away from the transmitter is the receiver This consisted of a single loop of wire connected to two spheresThis consisted of a single loop of wire connected to two spheres It had its own inductance and capacitanceIt had its own inductance and capacitance When the resonance frequencies of the transmitter and receiver matched, energy transfer occurred between them When the resonance frequencies of the transmitter and receiver matched, energy transfer occurred between them

64. Hertz’s Conclusions Hertz hypothesized the energy transfer was in the form of waves Hertz hypothesized the energy transfer was in the form of waves These are now known to be electromagnetic wavesThese are now known to be electromagnetic waves Hertz confirmed Maxwell’s theory by showing the waves existed and had all the properties of light waves Hertz confirmed Maxwell’s theory by showing the waves existed and had all the properties of light waves They had different frequencies and wavelengthsThey had different frequencies and wavelengths