FOWLER CHAPTER 7LECTURE 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.

THIS MAGNETIC FORCE IS REFERRED TO AS A MAGNETIC FIELD. THE FIELD EXTENDS OUT FROM THE MAGNETIC MATERIAL IN ALL DIRECTIONS.

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.

AS WITH ELECTRIC CHARGES, LIKE MAGNETIC POLES REPEL EACH OTHER, UNLIKE MAGNETIC POLES ATTRACT EACH OTHER

P.175

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

EACH TIME A MAGNET IS BROKEN A NEW PAIR OF POLES IS CREATED.

MAGNETIC FLUX LINES ARE IN CONTINOUS LOOPS.

P.175

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

ELIMINATING THE GAP,ELIMINATES THE POLES

ELECTROMAGNETISM

A ELECTRIC CURRENT CREATES A MAGNETIC FIELD WHICH IS PERPENDICULAR TO THE CURRENT FLOW

P.177

Magnetic field around a straight wire http://www.youtube.com/watch?v=Z927bDX1l04&feature=fvwp&NR=1

.Motion of particles in magnetic and electric fields . http://www.youtube.com/watch?v=Gkfwc4Uzgss

http://www.youtube.com/watch?v=4CtnUETLIFsElectromagnetic wave HD

P.177

+

P.177

Direction of flux is found by the RIGHT hand rule.

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

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.

REPEL EACH OTHER

ATTRACH EACH OTHER

COILS

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

MAGNETIC MATERIALS

ARE ATTRACTED TO MAGNETIC FIELDS.EXAMPLES OF FERROMAGNETIC MATERIALS;IRONIRON COMPOUNDSALLOYS OF IRON OR STEEL

NONMAGNETIC MATERIALS:( NO ATTRACTION) METALS SUCH AS COPPER, BRASS, AL,SILVER ZINC, TIN.FLUX TRAVELS THRU NONMAGNETIC MATERIALS.

P.179

THEORY OF MAGNETISM P180ELECTRONS 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.

PERMANENT MAGNETSDOMAINS REMAIN ALIGNED. EXAMPLES:Fe WITH 0.8% CARBONALLOYS LIKE ALNICO (COMPOUND OF Fe COBALT ,NICKEL, Al, AND Cu)ALSO CERTAIN CERAMIC MATERIALS

P.180

HOW TO MAKE A PERMANENT MAGNETS

TEMPORARY MAGNETSLOOSE MAGNETISM AFTER MAGNETIC FIELS IS REMOVED, DOMAINS BECOME RANDOMEXAMPLES: PURE Fe FERRITE: MAGNETIC MATERIAL THAT IS NOT A CONDUCTOR. SILICON STEEL

P.180

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.

DEMAGNETIZING P.182

CAN BE DONE TO A PERMANENT MAGNET BY 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

http://www.youtube.com/watch?v=OODRqLwV5uIHow to demagnetize a screwdriver

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.

DEMAGNETIZING A MAGNET

RESIDUAL MAGNETISM P.183IS 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.183OPPOSITION TO MAGNETIC FLUX. DEPENDS ON SIZE AND THE MATERIAL OF THE OBJECT.AIR, NONMAGNETIC MATERIALS HAVE HIGH RELUCTANCEMAGNETIC FLUX LINES TAKES THE LOWEST AND SHORTEST RELUCTANCE PATH.

HIGH RELUCTANCE PATH

LOW RELUCTANCE PATH

HIGH RELUCTANCE PATHLOW RELUCTANCE PATH

MAGNETIC SHIELDS P.184

MAKE USE OF THE TENDENCY OF FLUX TO DISTORT AND FOLLOW THE PATH OF LOWEST RELUCTANCE.

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.

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.

SIMPLE DC GENERATOR P.185

PATRS NEEDED: MAGNET COIL OF WIRE COMMUTATOR BRUSH } MAKES CONTACT TO THE

ROTATING LOOPOF WIRE

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)

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

SIMPLE TRANSFORMER P.186WHEN 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) ACURRENT WILL BE INDUCED IN THIS WIRE.THESE CHANGES ARE PROPORTIONAL TO THE AMOUNT OF FLUX CHANGE AND THE RATEOF FLUX CHANGE.

THE INDUCED VOLTAGE FROM A TRANSFORMER DEPENDS ON THE NUMBER OF WINDINGS IN BOTH THE 1º AND 2º COILS.

1º : PRIMARY2º : SECONDARY

YOU TUBE : Transformer Animation http://www.youtube.com/watch?v=VucsoEhB0NA

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

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

WEBER: BASE UNIT OF MAGNETIC FLUX(Φ)

MAGNETIC FIELD STRENGTH (H):DEFINED AS AMPERE-TURN/ METEROR MMF/CIRCUIT LENGTH.

FLUX DENSITY(B) FLUX /UNIT CROSS SECTIONAL AREA

YOU TUBE:Magnetic Field and Flux Animation http://www.youtube.com/watch?v=pB7oZNBIqqc

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

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.

LOW PERMEABILITY (u):

MEDIUM PERMEABILITY (u):

HIGH PERMEABILITY (u):

DC MOTORS P.191IS 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 POLEFIELD POLE

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/

SOLENOIDS P.192

AN ELECTROMAGNETIC DEVICE THAT ALLOWS A ELECTRIC CIRCUIT TO CONTROL A MECHANCIAL DEVICE.( VIA A PLUNGER)INCREASING THE MMF OR DECREASING THE RELUCTANCE WILL INCEASES THE PULL.

ONE CURRENT CARRYING WIRE LOOP AND ITS ASSOCIATED MAGNETIC FIELD.

MULTILOOPED WIRE AND ITS MAGNETIC FIELD.

1. Valve Body 4. Coil / Solenoid   7. Plunger 

2. Inlet Port 5. Coil Windings 8. Spring

3. Outlet Port   6. Lead Wires 9. Orifice

INTERNAL CONSTRUCTION OF A SOLENOID

EXAMPLE OF AN INDUSTRIAL SOLENOID

VARIOUS TYPES OF SOLENOIDS

HOW DOOR CHIMES WORK

A relay is an electrical switch that can be opened or closed by an electrical signal. It is an electrically controlled switch. People use them when they want a small amount of electricity to control a bigger amount of electricity. The same thing can be done with transistors, but transistors can't handle the amount of current that relays can.

RELAYS P.192

RELAYS P.192

USE THE ATTRACTION BETWEEN A ENERGIZED COIL AND A Fe ARMATURETO OPEN/CLOSE ELECTRICAL CONTACTS. THIS CAN ALLOW THE REMOTE CONTROLOF MOTORS, LIGHTS, ETC.RELAYS CAN OPERATE ON AC OR DC, RELAYS ARE NOT INTERCHANGEABLE.

Relays are used as switches. These are different from manual switches where we push a button to turn them on and off. Relays are electromagnetic device, and very useful in scenarios where we want to control a high voltage device through a low-voltage circuit.For example, assume that you have designed a small water pump controller which works on 12V DC. When water level on the overhead tank goes low, the controller outputs 5v on the output pin indicating that the pump should be turned on now. Similarly, when the tank is about to overflow, the circuit outputs 0v on output pin indicating the pump to turn off. Now in this case how will you control a 110V AC pump motor? The answer is very simple. Use relay as show in the diagram below.

VARIOUS RELAY SYMBOLS

CIRCUIT TO CONTROL TRAFFIC LIGHTS USING RELAYS

(a) .Contacts that have not turned a circuit on and off.

(b) After many on and off operations, these contacts show severe wear from arcing.

Fig. 7-35 Condition of relay contacts..

MK-S-series Power Relays with DC Switching Models That Can Switch 220 VDC, 10 A

RELAY DESIGNED FOR USE WITH A PLC (PROGRAMABLE LOGIC CONTROLLER)

HALL EFFECT P.194 THE PRODUCTION OF VOLTAGE ON OPPOSITE SIDES OFA CURRENT CARRYING SEMICONDUCTOR WHEN A MAGNETIC FIELD PASSESTHRU IT.

http://www.youtube.com/watch?v=_ATDraCQtpQHALL EFFECT ANIMATION

http://www.youtube.com/watch?v=fmZJqhzVXc4

HALL EFFECT VOLTAGE DEPENDS ON:1 TYPE OF MATERIAL2 MAGNITUDE OF CURRENT3 AMOUNT OF FLUX

THIS VOLTAGE CAN BE USED AS A CONTROL DEVICE FOR MANY APPLICATIONS.

Here the Hall effect device is used as an ammeter in a automobile. The current sensor consists of a ferrite core placed around the battery leads, with a Hall effect device positioned in the air-gap. A magnetic flux is induced in the ferrite Core when ever current flows through the leads and this flux passes through the Hall effect device which generates a proportional output voltage.