Chapter 20, 21

1. Chapter 20, 21 Magnetism and Electromagnetism

2. 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).

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

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

5. S S S S N N N N S N

6. N N S S 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, FE = qE FB = qBB

7. Magnetic Fields • A vector quantity • Symbolized by B • 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

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

9. Magnetic Field Lines, Bar Magnet • 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

10. Magnetic Field Lines, Unlike Poles • 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

11. Magnetic Field Lines, Like Poles • 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 • Compare to the electric field produced by like charges

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

13. Earth’s Magnetic Field • The Earth’s geographic north pole corresponds to a magnetic south 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” pole

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

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

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

17. Magnetic Fields • 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 lines • This force is zero when the wire is along the field lines

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

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

20. Magnetic induction Magnetic flux density Magnetic field (strength) [B] = [F/IL] = Ns/Cm = Tesla *1 Tesla = 104 gauss F Length of the section in B-field B I FB = ILB

21. FB = ILB ?Magnetic force on a current carrying loop • A net force on the loop • A net torque on the loop • A net force and torque • nothing, zip

22. X X S N

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

24. X X X X X X X X X DC Electrical Motor

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

26. Right Hand Rule • Hold your right hand open • Place your fingers in the direction of B • Place your thumb in the direction of v • 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 rule

27. CRT TV

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

29. q q v v v v v q F F F F F q X x x x x X x x x x X x x x x X x x x x X x x x x X x x x x q 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!!!

30. B X x x x x X x x x x X x x x x X x x x x X x x x x r v FB = q v B Force by B-field q m This force causes a circular motion. Centrepetal force = FB Fc = mv2/r = q v B = FB r = mv/qB Since there is no work done by the field, in vacuum, The charge will make a circular motion forever!!!

31. Bubble Chamber

32. An object with +e charge And mass m kg r = 0.4 m B = 0.36 T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A univalent ion with mass m Came into the field region with Speed v = 6 x 105 m/s. Mass m? Mass spectrometer

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

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

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

37. 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 • µo = 4  x 10-7 T m / A • µo is called the permeability of free space

38. x x x x x x x x x o o o o o o o o o I

39. http://www.bugman123.com/Physics/Physics.html For infinitely long solenoid B = monI n: number of turns/m

40. 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?

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

43. Faraday’s Experiment • 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 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

44. Faraday’s Conclusions • 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 • It is customary to say that an induced emf is produced in the secondary circuit by the changing magnetic field

45. 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.

46. G Primary Coil Secondary coil ?

47. Summary of Experimental Findings EMF is induced in the secondary coil Only when the magnetic field through it changes. Lenz’s Law EMF induced is bigger if the area of coil is bigger. EMF is induced always in the opposite direction of change in magnetic field. Vind = - A (dB/dt) = - d/dt  = A B : magnetic flux

48. 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 • 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

49. B = 0.175 T B┴ B 40 0.2 m B// 0.3 m B  = A B┴ A = (0.3 x 0.2)(0.175 sin(40)) = 0.06 x 0.112 = 0.0067 T.m2

50. - + - + + + Net number of loops Vind = - d/dt Vt = -3 d/dt Example 23.2 150 turn loop with a 0.75 cm2 cross-section Magnetic field: 0 T  0.25 T in 3.6 s What is the induced current in the coil? 5 Ohm