Download 150Lecture 7 Magnetism/Electromagnetism Lecture Notes Page

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.
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.
P.175
AS WITH ELECTRIC CHARGES, LIKE MAGNETIC POLES REPEL EACH
OTHER, UNLIKE MAGNETIC POLES ATTRACT EACH OTHER
MAGNETIC FORCE DECREASES AT A INVERSE SQ. RATE 1/R²
P.175
EACH TIME A MAGNET IS BROKEN A NEW PAIR OF POLES IS CREATED.
MAGNETIC FLUX LINES ARE IN CONTINOUS LOOPS.
FLUX CAN BE ENTIRELY CONTAINED WITHIN A
MAGNET AND THUS HAVE NO POLES. F.7.6 ,P-176
ELIMINATING THE GAP,ELIMINATES THE POLES
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
Electromagnetic wave HD
http://www.youtube.com/watch?v=4CtnUETLIFs
.Motion of particles in magnetic and electric fields .
http://www.youtube.com/watch?v=Gkfwc4Uzgss
P.177
+
P.177
Direction of flux is found by the RIGHT hand rule.
Field strength is directly proportional to the amount of current flowing
thru 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.
ATTRACH EACH OTHER
REPEL 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;
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
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.
PERMANENT MAGNETS
P.180
DOMAINS REMAIN ALIGNED.
EXAMPLES:
Fe WITH 0.8% CARBON
ALLOYS LIKE ALNICO (COMPOUND OF Fe COBALT ,NICKEL, Al, AND Cu)
ALSO CERTAIN CERAMIC MATERIALS
HOW TO MAKE A PERMANENT MAGNETS
TEMPORARY MAGNETS
LOOSE MAGNETISM AFTER MAGNETIC FIELS IS REMOVED, DOMAINS BECOME RANDOM
EXAMPLES: PURE Fe
FERRITE: MAGNETIC MATERIAL THAT IS NOT A CONDUCTOR.
P.180
SILICON STEEL
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
How to demagnetize a screwdriver
http://www.youtube.com/watch?v=OODRqLwV5uI
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.
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
HIGH RELUCTANCE PATH
LOW 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.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.
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
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/ METER
OR MMF/CIRCUIT LENGTH.
YOU TUBE:Magnetic Field and Flux Animation
http://www.youtube.com/watch?v=pB7oZNBIqqc
FLUX DENSITY(B) FLUX /UNIT CROSS SECTIONAL AREA
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.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
http://www.k-grayengineeringeducation.com/blog/index.php/2009/02/2/engineering-education-today-inhistory-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
RELAYS P.192
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
USE THE ATTRACTION BETWEEN A ENERGIZED COIL AND A Fe ARMATURE
TO OPEN/CLOSE ELECTRICAL CONTACTS. THIS CAN ALLOW THE REMOTE CONTROL
OF 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 lowvoltage 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 OF
A CURRENT CARRYING SEMICONDUCTOR WHEN A MAGNETIC FIELD PASSES
THRU IT.
HALL EFFECT ANIMATION
http://www.youtube.com/watch?v=_ATDraCQtpQ
http://www.youtube.com/watch?v=fmZJqhzVXc4
HALL EFFECT VOLTAGE DEPENDS ON:
1 TYPE OF MATERIAL
2 MAGNITUDE OF CURRENT
3 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.