Download Magnetic field

Basic Electronics
DC - Magnetism
Copyright © Texas Education Agency, 2014. All rights reserved.
Presentation Overview
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An overview of magnetism basics
Types of magnets and how to create them
Terms and units
The rules and relationships between current, forces, and
fields
Ways of producing artificial magnets
Permeability of magnetic materials
Magnetic properties
Induction
Copyright © Texas Education Agency, 2014. All rights reserved.
Magnet
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A material or object that
produces a magnetic field
An object that will attract
iron, nickel, or cobalt and that
will produce an external
magnetic field
There are two types of magnets: permanent
magnets and electromagnets
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There are two types of permanent magnets: hard magnets that
do not demagnetize, and soft magnets that lose their magnetism
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Magnetism
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A invisible force that attracts or repels
magnetic materials
A property of certain materials (e.g. iron,
nickel, and cobalt) that exerts a mechanical
force on other magnetic materials, and that
can cause induced voltages in conductors
when relative movement is present
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Some materials feel no force at all from magnetism
(these are called non-magnetic substances)
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Picture of Magnetic Field Lines
Using Iron Filings
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Picture of Magnetic Field Lines
Using Iron Filings
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The pattern revealed by the
iron filing vividly illustrates
that something extremely
well-organized exists beyond
the surface of the magnet.
A common way to refer to
magnetic fields is by using the
term “magnetic lines of force.”
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The Basics of Magnetism
Magnets attract specific metals
 Magnets have a north and a south pole
 Like poles repel, unlike poles attract
 Magnetic and electric fields are related
 The same quantity that can turn a metal
into a magnet is the same property that
allows it to be attracted to other magnets
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Magnetic Poles
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One of the two ends of a magnet where
magnetic field lines converge or diverge
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There are two types of poles: north and south
Every magnet always has both poles, meaning
there are only magnetic dipoles (there are no
magnetic monopoles)
Magnetic field lines are said to leave the north
pole and enter the south pole
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What Creates Magnetism?
Electrons have several fundamental properties
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Electrical charge
Magnetic dipole moment
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This dipole moment comes from a more
fundamental property of the electron called
quantum mechanical spin
This means each electron behaves like a tiny magnet
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What Creates Magnetism?
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Magnetic materials have several unpaired
electrons with the same spin
When these tiny magnetic dipoles are aligned
in the same direction, their individual
magnetic fields add together to create a
measurable magnetic field
Electrons usually come in pairs with opposite
spins where the magnetism cancels
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Permanent Magnet
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A type of magnet found in nature
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Usually made out of iron
Other types of magnets include Alnico,
Neodymium, Samarium, and a type of
ceramic, which includes iron oxide as a
composite
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Neodymium magnets are some of the
strongest on Earth
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Electromagnet
Where magnetism is formed by current
flowing through an electric wire
 A magnet that can be turned on and off
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Example of an Electromagnet
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Force of Attraction
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The paper clips are not originally magnetized
The external magnetic field induces or creates a
temporary magnetism in the paper clips
They are then attracted to the electromagnet
because opposite poles attract
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The attraction only lasts as long as the paper clips are near
the magnetic source
The paper clips will turn so the opposite pole end of the
paper clip faces the electromagnet
This same effect happens with a refrigerator door magnet
creating temporary magnetism in the metal door which
then attracts the magnet to the door
Important Terms:
Retentivity
Also called permanence; how long a
magnet retains its magnetism
 Materials that are hard to magnetize
generally retain their magnetism longer
 Relates to the amount of force needed to
align magnetic domains
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Important Terms:
Permeability
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The ability to pass or conduct magnetic field lines
 Used to guide a magnetic field
Indicates the degree to which a material will
become magnetized due to an external magnetic
field
A constant of proportionality between magnetic
induction and magnetic field strength
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Symbol: μ0
Units: Henry per meter (H
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m)
Ferromagnetism
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Ferromagnetic materials are made from iron
and form permanent magnets
Magnetic materials with high values of
permeability that range from 50 to 5000
Responsible for the type of magnetism found
in nature
(NOTE: Iron, cobalt, nickel, alloys that include these materials
[like alnico], some compounds of rare earth metals, and
certain minerals like lodestone are ferromagnetic materials.)
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Units of Magnetism
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Weber (Wb): the SI unit of magnetic flux
kg-m2
per
s2
A(
𝑘𝑔•𝑚2
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One
𝑠 2 𝐴)
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Related to the fact that the magnetic field strength
(H) is different compared to magnetic flux density (B)
Tesla (T): the SI unit for magnetic flux density (B)
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One weber per square meter (𝑊𝑏
𝑘𝑔
or
𝑚2
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𝑠 2 𝐴)
Magnetic Field Direction
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One form of right hand rule shows the
direction of a magnetic field around a
conductor with current I
• Thumb in the direction
of current flow
• Fingers wrap around the
conductor in the direction
of magnetic field lines
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The Direction of Magnetic Force
A magnetic field creates a
force on a conductor with
current flow
 The direction of force
is perpendicular to
the magnetic field
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Another Right-hand Rule
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Pointing the thumb of the right hand in the direction
of the conventional current and the fingers in the
direction of the magnetic field lines B, the force on
the conductor points up from the palm.
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Left-hand Rule
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When current flows in a wire, and an external magnetic field is applied
across that flow, the wire experiences a force perpendicular both to that
field and to the direction of the current flow. A left hand can be held, as
shown in the illustration, so as to represent three mutually orthogonal
axes on the thumb, first finger, and middle finger. Each finger is then
assigned to a quantity (mechanical force, magnetic field, and electric
current). The right and left hand are used for generators and motors
respectively.
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Use of the LHR: Hall Effect
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The charge carriers of a current carrying conductor
placed in a transverse magnetic field experience a
sideways Lorentz force
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This results in a charge separation in a direction
perpendicular to the current and to the magnetic field
The resultant voltage in that direction is proportional
to the applied magnetic field.
The Hall effect is often used to measure the strength
of a magnetic field.
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Electricity and Magnetism
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Current flowing through a wire creates a
magnetic field around that wire
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This is what creates an electromagnet
This principle is used in a motor
A conductor moving through a magnetic
field creates (induces) a voltage in that
conductor
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This principle is used in a generator
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Motors and Generators
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A changing magnetic field creates an electric field
This is used in a generator
The electric field produces a force on an electron
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A changing electric field generates a magnetic field
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If the electron is in a conductor, current will flow
This is known as Faraday's law
This is known as Maxwell's correction to Ampère's law
This is used in a motor
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Transformers and Generators
A changing magnetic field can be created several ways
 A magnetic field expanding or collapsing
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Through relative motion
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This is how a transformer works
Requires AC voltage
This is how a generator works
Relative motion can be created by moving a
conductor through a magnetic filed or by moving a
magnetic field past a conductor
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Both of these require a prime mover
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Motor Action
The interaction of two magnetic fields
produce mechanical forces
 These forces can be used to make a rotor
spin in a motor
 The torque created by the rotor can be
used to perform work
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AC Motors
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Rotating Magnetic fields
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The rotating magnetic field is a key principle in the
operation of alternating-current motors
A permanent magnet in such a field rotates so as
to maintain its alignment with the external field
The rotating magnetic filed is created by AC
voltages that are different in phase being
applied to magnetic poles (created by coils)
that are at different angles to each other
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Ways of Producing Artificial
Magnets
A.
Electrical Coil Method
B.
Stroking Method
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Permeability of
Magnetic Materials
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High permeability
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Iron, steel, nickel, cobalt
Commercially made alloys of iron, nickel, cobalt, and other elements
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Medium permeability
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Aluminum, platinum, manganese, and chromium
Low permeability
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Silicon steel (used in transformers)
Alnico (used in audio speakers)
Bismuth, antimony, copper, and zinc
Rare metals (mercury, gold, and silver)
Nonmagnetic materials (diamagnetic)
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Glass, paper, rubber, wood, and air
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Magnetic Properties
Magnetic lines of force
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Continuous and form complete loops
Never cross each other
Cause like poles (north-north, southsouth) to repel each other
Cause unlike poles (north-south,
south-north) to attract each other
Parallel lines going in the same
direction repel each other
Attract other lines going in the
opposite direction
Exert tension along their lengths,
tending to shorten themselves
Pass through all materials, both
magnetic and nonmagnetic
Always enter or leave magnetic
material at right angles to the surface
Tend to flow in paths of least
opposition
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Magnetic Properties (cont’d.)
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Magnetic field
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Area around magnet through which
force lines flow
Direction of flow is always from north
pole to south pole
Magnetic flux
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Sum total of magnetic field force lines
flowing from north pole to south pole
Symbol for magnetic flux- Greek letter
phi (φ)
Unit of flux- Maxwell; one maxwell
(Mx) equals one line of force
(example: if a magnetic field contains 6
lines of force, the flux of the magnet is
6 maxwells, or φ= 6Mx
Flux density- number of force lines per
given are
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Magnetic Induction
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Method
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Place iron bar in vicinity of permanent magnet
Do not allow iron bar to touch magnet
Effect
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Magnetic field lines of force flow through the iron bar
The iron bar becomes magnetized
Pole polarity is reversed
 End of bar near north pole of magnet becomes south pole of bar
 End of bar near south pole of magnet becomes north pole of bar
The permanent magnet attracts the iron bar (NOTE: this constitutes more action).
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Practical Applications of Induction in
the Electronics Field
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Radio and television transmission and
reception
Transformers
Relays and solenoids
Coils, chokes, and inductors
Audio speakers
Motors and generators
Magnetic memory
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Math That Involves Magnetism
Magnetic field strength around a wire
 B is magnetic field strength in Tesla
 I is current in amps
 D is distance in meters
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Terms and Definitions
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Magnetism- a property of certain materials (e.g. iron, nickel, and cobalt)
that exerts a mechanical force on other magnetic materials, and that can
cause induced voltages in conductors when relative movement is present
Magnet- an object that will attract iron, nickel, or cobalt and that will
produce an external magnetic field
Natural magnet- any material found in the earth that exhibits the
properties of magnetism. Example: The lodestone, which contains
magnetite, a form of iron, and that has been magnetized by the earth’s
magnetic field
Artificial magnet- a device that has been made magnetic by induction
Magnetic induction- where a magnetic field causes an un-magnetized
ferromagnetic substance to become magnetized
Magnetic lines of force- a set of imaginary, curved lines, around a magnet
that indicates the strength and direction of the magnetic field
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Terms and Definitions (cont’d.)
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Magnetic field- the area around a magnet through which the lines of force flow
Permanent magnet- a magnetic device that remains its magnetism after it is
removed from a magnetic field
Electromagnet- a core of soft iron that is temporarily magnetized by sending
current through a coil of wire wound around the core
Permeability- the ability to pass or conduct magnetic field lines (NOTE: some
materials such as iron have high permeability, others such as aluminum have
medium permeability, and others such as silver and gold have low permeability.)
Magnetic poles- one of the two ends of a magnet where magnetic field lines
converge or diverge (NOTE: by convention, the north-seeking pole is marked with
N, or plus, or is colored red.)
Ferromagnetic- magnetic materials with high values of permeability that range
from 50 to 5000 (NOTE: steel, cobalt, nickel, and alnico are ferromagnetic
materials. )
Diamagnetic- non-magnetic materials; these have a permeability of less than one
(NOTE: diamagnetic materials include bismuth, antimony, copper, and zinc.)
Copyright © Texas Education Agency, 2014. All rights reserved.
Presentation Summary
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Terms and definitions
Types of magnets
Ways of producing artificial magnets
Permeability of magnetic materials
Magnetic properties
The use of the Left-hand Rule for conductors and coils
Induction
Practical applications of induction in the electronics field
Copyright © Texas Education Agency, 2014. All rights reserved.