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FOWLER CHAPTER 7 LECTURE 7 MAGNETS AND ELECTROMAGNETISM

FOWLER CHAPTER 7 LECTURE 7 MAGNETS AND ELECTROMAGNETISM. CHAP 7 P-173 MAGNETISM MAGNETISM IS A FORCE THAT ACTS ON CERTAIN MATERIALS. WHAT MATERIALS? ALLOYS OF COPPER,NICKEL, ALUMINIUM, IRON, COBALT.

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FOWLER CHAPTER 7 LECTURE 7 MAGNETS AND ELECTROMAGNETISM

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  1. FOWLER CHAPTER 7LECTURE 7 MAGNETS AND ELECTROMAGNETISM

  2. CHAP 7 P-173 MAGNETISM MAGNETISM IS A FORCE THAT ACTS ON CERTAIN MATERIALS. WHAT MATERIALS? ALLOYS OF COPPER,NICKEL, ALUMINIUM, IRON, COBALT. THIS MAGNETIC FORCE IS REFERRED TO AS A MAGNETIC FIELD. THE FIELD EXTENDS OUT FROM THE MAGNETIC MATERIAL IN ALL DIRECTIONS.

  3. LINES OF FORCE OF A MAGNETIC FIELD ARE KNOW AS MAGNETIC FLUX (Φ) FLUX IS STRONGER WHERE LINES OF FORCE ARE CLOSER. FLUX IS WEAKER WHERE LINES OF FORCE ARE FATHER APART. FLUX IS ALWAYS STRONGEST AT THE END OF A MAGNET. LINES OF FORCE LEAVE THE N POLE AND ENTER THE S POLE.

  4. P.175 AS WITH ELECTRIC CHARGES, LIKE MAGNETIC POLES REPEL EACH OTHER, UNLIKE MAGNETIC POLES ATTRACT EACH OTHER

  5. MAGNETIC FORCE DECREASES AT A INVERSE SQ. RATE 1/R²

  6. EACH TIME A MAGNET IS BROKEN A NEW PAIR OF POLES IS CREATED. P.175 MAGNETIC FLUX LINES ARE IN CONTINOUS LOOPS.

  7. FLUX CAN BE ENTIRELY CONTAINED WITHIN A MAGNET AND THUS HAVE NO POLES. F.7.6 ,P-176 ELIMINATING THE GAP,ELIMINATES THE POLES

  8. P.177 ELECTROMAGNETISM A ELECTRIC CURRENT CREATES A MAGNETIC FIELD WHICH IS PERPENDICULAR TO THE CURRENT FLOW Magnetic field around a straight wire http://www.youtube.com/watch?v=Z927bDX1l04&feature=fvwp&NR=1

  9. http://www.youtube.com/watch?v=4CtnUETLIFs Electromagnetic wave HD .Motion of particles in magnetic and electric fields . http://www.youtube.com/watch?v=Gkfwc4Uzgss

  10. P.177 +

  11. P.177

  12. Direction of flux is found by the RIGHT hand rule.

  13. Field strength is directly proportional to the amount of current flowing thru the conductor. If the current doubles, the field strength doubles.

  14. FORCE BETWEEN CONDUCTORS P.177 TWO PARALLEL CURENT CARRYING CONDUCTORS ATTRACH EACH OTHER, IF THE CURRENTS ARE FLOWING IN THE SAME DIRECTION.( F7-10 P.178) THE FIELD LINES JOIN TOGETHER. THE OPPOSITE OCCURS IF CURRENTS ARE TRAVELING IN THE OPPOSITE DIRECTIONS. ATTRACH EACH OTHER REPEL EACH OTHER

  15. COILS IF A CURRENT CARRYING WIRE IS TIGHTLY WOUND INTO A COIL, A ELECTROMAGNET IS CREATED. P.178,F.7-10

  16. MAGNETIC MATERIALS ARE ATTRACTED TO MAGNETIC FIELDS. EXAMPLES OF FERROMAGNETIC MATERIALS; IRON IRON COMPOUNDS ALLOYS OF IRON OR STEEL NONMAGNETIC MATERIALS:( NO ATTRACTION) METALS SUCH AS COPPER, BRASS, AL, SILVER ZINC, TIN. FLUX TRAVELS THRU NONMAGNETIC MATERIALS. P.179

  17. THEORY OF MAGNETISM P180 ELECTRONS HAVE A MAGNETIC SPIN, WHEN RANDOMLY ARRANGED THE NET SPIN IS ZERO(NO MAGNETIC FIELD). DOMAINS ARE GROUPS OF MOLECULES THAT HAVE MAGNETIC POLES.THESE ARE USUALLY RANDON IN UNMAGNETIZED MATERIALS. F.7-13 P.180 IF AN EXTERNAL MAGNETIC FIELD IS APPLIED, THE DOMAINS WILL LINE UP, FORMING EITHER A TEMPORARY OR A PERMANENT MAGNET.

  18. PERMANENT MAGNETS DOMAINS REMAIN ALIGNED. EXAMPLES: Fe WITH 0.8% CARBON ALLOYS LIKE ALNICO (COMPOUND OF Fe COBALT ,NICKEL, Al, AND Cu) ALSO CERTAIN CERAMIC MATERIALS P.180

  19. HOW TO MAKE A PERMANENT MAGNETS

  20. TEMPORARY MAGNETS LOOSE MAGNETISM AFTER MAGNETIC FIELS IS REMOVED, DOMAINS BECOME RANDOM EXAMPLES: PURE Fe FERRITE: MAGNETIC MATERIAL THAT IS NOT A CONDUCTOR. SILICON STEEL P.180

  21. MAGNETOMOTIVE FORCE( MMF) P.181 IS WHAT CREATES MAGNETIC FIELDS AND FLUX. MMF INCREASES AS THE # OF TURNS OR THE CURRENT IN THE COIL INCREASES. SATURATION: P.182 OCCURS WHEN AN INCREASE IN MMF NO LONGER WILL INCREASE THE MAGNETIC FLUX.

  22. DEMAGNETIZING P.182 CAN BE DONE TO A PERMANENT MAGNETBY HAMMERING IT OR HEATING IT TO A HIGH TEMPERATURE. EASIEST WAY IS TO DO THIS IS WITH A COIL AND A AC SOURCE. F.7-17B How to demagnetize a screwdriver http://www.youtube.com/watch?v=OODRqLwV5uI

  23. DEMAGNETIZING A MAGNET ONE WAY: MAGNET IS SLOWLY MOVED AWAY FROM THE COIL, AND WITH EACH REVERSAL IN POLARITY IT BECOMES WEAKER. THE SOLDERING IRON PICTURED BELOW CAN PERFORM THIS FUNCTION. ANOTHER WAY: SLOWLY REDUCE THR MAGNITUDE OF THE AC UNTIL IT BECOMES ZERO.

  24. RESIDUAL MAGNETISM P.183 IS THE FLUX THAT REMAINS IN A TEMPORARY MAGNET. IDEAL PERMANENT MAGNET WOULD RETAIN ALL ITS FLUX. IDEAL TEMPORARY MAGNET WOULD RETAIN NO FLUX. RELUCTANCE P.183 OPPOSITION TO MAGNETIC FLUX. DEPENDS ON SIZE AND THE MATERIAL OF THE OBJECT. AIR, NONMAGNETIC MATERIALS HAVE HIGH RELUCTANCE MAGNETIC FLUX LINES TAKES THE LOWEST AND SHORTEST RELUCTANCE PATH. HIGH RELUCTANCE PATH LOW RELUCTANCE PATH

  25. LOW RELUCTANCE PATH HIGH RELUCTANCE PATH

  26. MAGNETIC SHIELDS P.184 MAKE USE OF THE TENDENCY OF FLUX TO DISTORT AND FOLLOW THE PATH OF LOWEST RELUCTANCE.

  27. INDUCED VOLTAGE P.185 WE KNOW THAT CURRENT CARRYING CONDUCTORS PRODUCT A MAGNETIC FIELD. CONSIDER THE OPPOSITE. A MAGNETIC FIELD CAN INDUCT A VOLTAGE/CURRENT IN A WIRE. THIS IS THE BASES OF ALL ELECTRIC MOTORS.

  28. Energy for the operation of most electrical equipment depends upon the electrical energy supplied by a generator. A generator is any machine which converts mechanical energy into electrical energy by electromagnetic induction. A generator designed to produce alternating current energy is called an ac generator, or alternator; a generator which produces direct current energy is called a dc generator. Both types operate by inducing an ac voltage in coils by varying the amount and direction of the magnetic flux cutting through the coils.

  29. SIMPLE DC GENERATOR P.185 PATRS NEEDED: MAGNET COIL OF WIRE COMMUTATOR BRUSH } MAKES CONTACT TO THE ROTATING LOOPOF WIRE

  30. AS THE WIRE TURNS, THE COMMUTATOR TURNS WITH IT. WHEN WIRE TURNS IN CW DIRECTION A CURRENT IS INDUCED IN THE WIRE WHICH FLOWS FROM THE COMMUTATOR TO THE BRUSH. THIS CHANGES POLARITY EVERY 90º, SINCE BRUSH CHANGES FROM ONE COMMUTATOR TO THE OTHER. POLARITY STAYS THE SAME AND RESULTS IN A DC OUTPUT. (SEE NEXT SLIDE)

  31. YOU TUBE : Direct Current Electric Motor http://www.youtube.com/watch?v=Xi7o8cMPI0E

  32. SIMPLE TRANSFORMER P.186 WHEN A CURENT CARRYING WIRE IS WRAPED AROUND A Fe CORE ( PRIMARY WINDING) IT WILL INDUCE A MAGNETIC FLUX IN THE Fe CORE. IF A SEPARATE SECOND COIL IS WRAPED AROUND THE CORE (SECONDARY WINDING) A CURRENT WILL BE INDUCED IN THIS WIRE. THESE CHANGES ARE PROPORTIONAL TO THE AMOUNT OF FLUX CHANGE AND THE RATE OF FLUX CHANGE.

  33. THE INDUCED VOLTAGE FROM A TRANSFORMER DEPENDS ON THE NUMBER OF WINDINGS IN BOTH THE 1º AND 2º COILS. 1º : PRIMARY 2º : SECONDARY YOU TUBE : Transformer Animation http://www.youtube.com/watch?v=VucsoEhB0NA

  34. Magnetic Quantities p. 186 Remember MMF is the force that carries flux. Ampere-Turn: The MMF carried by 1 Amp flowing thru 1 turn of a coil. MMF= #of turns( around coil) x current 1 ampere-turn 6 ampere-turn 1 ampere 1 ampere

  35. How many ampere-turns are there for this two coils?

  36. WEBER: BASE UNIT OF MAGNETIC FLUX(Φ) MAGNETIC FIELD STRENGTH(H): DEFINED AS AMPERE-TURN/ METER OR MMF/CIRCUIT LENGTH.

  37. YOU TUBE:Magnetic Field and Flux Animation http://www.youtube.com/watch?v=pB7oZNBIqqc FLUX DENSITY(B) FLUX /UNIT CROSS SECTIONAL AREA

  38. TESLA (T) : FLUX/UNITAREA = 1 WEBER/M²

  39. PERMEABILITY (u): ABILITY OF A MATERIAL TO CONDUCT FLUX. u =B/H=FLUX DENSITY/MAGNETIC FIELD STRENGTH The higher the magnetic permeability, the stronger the magnetic field.

  40. LOW PERMEABILITY (u): MEDIUM PERMEABILITY (u): HIGH PERMEABILITY (u):

  41. DC MOTORS P.191 IS A DC GENERATOR WORKING IN REVERSE. CURRENT FROM THE POWER SOURCE FLOWS THRU THE BRUSHES, COMMUTATORS INTO THE ARMATURE COIL. WHICH PRODUCES A MAGNETIC FIELD IN THE ARMATURE. THE ARMATURE POLES ARE ATTRACHED TO THE FIELD POLES WHICH CAUSES ROTATION OF THE ARMATURE. SINCE THE COMMUTATOR/BRUSHES CHANGE DIRECTION OF CURRENT EVERY 90º, THE ARMATURE SPINS UNITL THE CURRENT IS TURNED OFF. FIELD POLE FIELD POLE

  42. http://www.k-grayengineeringeducation.com/blog/index.php/2009/02/2/engineering-education-today-in-history-blog-thomas-davenport-patents-the-electric-motor-and-electric-railway-2/http://www.k-grayengineeringeducation.com/blog/index.php/2009/02/2/engineering-education-today-in-history-blog-thomas-davenport-patents-the-electric-motor-and-electric-railway-2/

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