electromagnetism kevin gaughan for bristol myers squibb
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Contents
•Magnets and Ferromagnetism
•Domains Theory
•H, B and µ
•The links between electricity and
Magnetism
–Electromagnets
–Induction
•Applications of Electromagnetism
Magnetism
•Permanent Magnetic
Materials have been
known since antiquity.
•They attract “magnetic
materials”–Iron / Nickel
•They can attract or repel
other magnets
•Every permanent magnet
has two poles (North and
South)
•Like Poles repel, Unlike
Poles Attract
Ferromagnetism
•Permanent magnetic materials are said to be ferromagnetic.
•Ferromagnetic materials become magnetised themselvesin the
presence of an external magnetic field and they retain some of
that magnetismwhen the external field is removed. This is the
cause of permanent magnets.
•Ferromagnetic material interact strongly with magnetic fields and the
induced magnetism causes measureable forces of attraction.
•Ferromagnetic materials include Iron (Latin name Ferrous), Cobalt
and Nickel
Other types of Magnetism
•Paramagnetic materialsbecome magnetised in the
presence of a magnetic field but do not retain that
magnetism when the field is removed (Aluminium,
Tungsten).
•Diamagnetic Materials actually resist an external
magnetic field (Carbon, Copper, W
ater).
•NOTE: Paramagnetismand Diamagnetism are much
muchweaker effects than Ferromagnetism. They do not
produce measurable forces. For our purposes these
materials may be considered to be non-magnetic.
Poles and Flux Lines
Flux Lines are Imaginary lines running from North Pole to South Pole of a
magnet. They show the direction of magnet field. We say Magnetic Flux
flows from North Pole to South pole. The stronger the magnetic field the
higher the flux and the more closely spaced the flux lines.
You can plot the path of the flux lines with a little compass orwith iron filings
but the lines are not physically real –they only represent the direction of flow
of magenticflux.
Unlike Poles Attract
•Like Poles Repel each
other and unlike poles
attract each other.
•One method of visualising
this is to imagine that
magnetic flux always tries
to take the path of least
resistance from a North
Pole to South Pole.
•The magenticflux pulls unlike poles
together in order to shorten the path
but it pushes like poles apart as the
fluxes coming from each magnet bash
against one another.
Did you know that the North Pole is
Really a South Pole?
This is a source of much confusion between Geographers and Physicists.
Geographers even refer to the “Magnetic North Pole”when they mean the
south magnetic pole.
How do you think this confusion came about?Im
age from http://129.128.241.207/carismaweb/content/view/69/1/
Distorting the field
A piece of iron or other ferromagnetic material will distort thelocal
magenticfield. The flux will try to take the path of least resistance
(through the iron). This fact is very useful is you want to control the
path of a magenticfield as in a motor or a transformer.
Domains Theory of
Ferromagnetism
•Ferromagnetic materials have
minute magenticdomains
within their atomic structure.
•When the ferromagnetis
unmagnetisedthese domains
are randomly oriented and
there is no net magnetic field.
•When you apply an external
magnetic field the domains
allignand their fields add up –
increasing the total magnetic
field through the ferromagnetic
material.
Domains (cont)
•Permanent magnetsarise when the
domains are “sticky”they stay allignedand
the material retains a net magnetic field
even after the external field is removed.
Hard magnetic materials retain a large
permanent field. Soft magnetic materials
retain less. Hard magnetic materials make
good permanent magnets.
Demagnetising a Permanent
Magnet
•You can demagnetise a permanent
magnet by
–Hammering
–Heating
–Putting in in an external alternating magnetic
field and slowly withdrawing it again.
•Why do the above methods work?
Ferromagnetic Saturation
•When all the domains are lined up a
Ferromagnetis fully magnetised and is said to
be saturated.
•Increasing the external magnetic field above the
point of saturation can not allignany further
domains so the resulting field will only increase
slowly from then on.
•Magnetic saturation in iron alloys occurs at
around 1.5 Tesla and this is the limiting
magnetic flux density for many magnetic
devices.
H B
•Just like electricity magnetism
has a pressure property (like
voltage) and a flow property
(like current)
•The unit of magenticpressure
is called Magnetic Field
Strength H
and is measured
in Amps/Metre
•The unit of magnetic flow is
called Magnetic Flux Density
B and is measured in Tesla
•(Note –sometimes it is useful to consider
total magnetic flux which is B x Cross
sectional area and is measured in Webers)
Magnetic Permeability µ
•Every material has a magenticpermeability
which represents how easily magnetic flux
flows through it. This is called magnetic
permeabiltiyµ.
•The reference permeability is that of a
vacuum and the permeability of a vacuum is
one of the fundamental scientific constants:
metre
Henries/
10
47
0
−×
=π
µ
Relative Permeabilty
•For any other material we say
0.ur
µµ=
•Where µ
risthe relative permeabililtyof the material.
•All non magnetic materials have relative permeabiltiesvery close to 1.0
They behave very like a vacuum.
•Ferromagnetic materials can have relative permeablitiesof 1000 or more.
This means a ferromagnetis 1000 times better at carrying magnetic flux
than a vacuum.
Mathematical Explanation of µ
B = µH
The bigger µthen the more magnetic flux you will
get for a given magnetic field strength.
For non magnetic materials that simple equation
works. Ferromagnetic materials are very non
linear however and the ratio between B and H
changes.
B H curve of a Ferromagnetic
Material
Its actually even a bit more complicated that this because of ….
Air
Saturation
NB NB
The units of B are
multiplied by 10 in
this graph! For
example Carbon
Steel saturates at
around 1.5 Tesla.
Hysterisis
Copied from bhcurve.com–original source unknown.
Some B remains even after H
is reduced to zero. This is
called Remanenceandis
responsible for permanent
magnetism.
Three fundamental principles of
Electromagnetism
•How to create magnetism from electricity
(Fundamental princplebehind electric motors
and transformers)
•How to create electricity from magnetism
(Fundamental principle behind electric
generators and transformers)
•How a current carrying wire experiences a
force in a magnetic field.
(Used in many electric motors)
Electricty-> Magnetism
•Every wire carrying
current generates a
small magnetic field
The picture comes from
http://www.pbs.org/wgbh/nova/magnetic/reve-drives.html
The Right Hand Grip Rule
•If you imagine gripping
the wire in your right
handwiththumb pointing
in the direction of the
current then your fingers
trace the direction of the
magnetic field.
•Notice how there is no
obvious North or South
Pole –the magnetic field
just goes around in a
circle.
Image from http://sciencecity.oupchina.com.hk/npaw/student/glossary/right_hand_grip_rule.htm
The Solenoid: Its just a coil of wire.
•In practise one wire
produces very little
magnetism so we wrap
many turns of wire into a
coil –often called a
solenoid.
•The resulting coil acts like
a bar magnet with North
and South poles.
•The Magnetic Field in the
middle of the coil is given
by
L
IN
H×
=
Electromagnets
•The current creatsthe H but the magnetic flux B also depends on the
permeability of the material in the middle of the solenoid.
•If we wrap a coild
of wire around an iron core (high permeability) we
get a strong controllable magnet. W
e have just made an
ELECTROMAGNET
•Notice how we can reverse the north and south poles by reversingthe
direction of current.
Magnetism to Electricity
•In order to go in the opposite direction you
need a changing magnetic field.
•A changing magnetic field will induce a
Voltagein a coil of wire.
•A changing magnetic field can mean a a
stationary field which is growing stronger
and or weaker (egtransformers).It can
also mean a constant magnetic field which
is moving (egelectric generators)
Inducing a voltage in a coil of wire
•The voltage
generated in any one
wire is small so you
really need a coil of
wire
•The induced voltage
is given by dt
AB
dN
V)
(.
×=
Force on a current in a Magnetic
Field
•A wire carrying current in
a magnetic field
experiences a force
which is proportional to
the level of current (I), the
Flux Density (B) and the
length of the wire
exposed to the field (L)
Force =IxBxL(Newtons)
•Many electric machines
utilise this principle to
generate mechanical
force from electricity
Force on a current –another view
•Remember that every current carryignwire
generates its own magnetic field.
•The force that the wire experiences in an
external magnetic field is due to the
interaction of the wires magnetic field with
the external magnetic field.
Left Hand Rule
•The force on a current
caryingwire experiences
a force that is
perpendicular to the
current and perpendicular
to the original magnetic
field.
•The Left Hand Rule
allows you to predict the
direction of the force.
A bit of Scientific History
•We have seen that electrictyand magnetism are closely
linked. In fact it is likely that all magnetic fields are
generated by microscopic currents.
•The crowning achievement of 19thcentury science came
about when James Clerk Maxwell produced a unified
theory of electricity and magnetism. This theory even
allowed him to predict the existence of travelling
electromagnetic waves (electromagnetic radiation).
•Electromagnetic radiation include radio waves, micro
waves, x-rays, Ultra violet, Infra Red and even visible
light. Maxwells
equations allowed him to calculate the
speed of light purely from physical constants.
Applications of Magnetism and
Electromagnetism
•The Magnetic Compass
was hugely important to
historical navigation. A
freely rotating magnetised
needle will allignwith the
Earths magenticfield so
that the North Pole of the
needle (usually coloured
red) will point towards
the geographic North
Pole.
Electromagnetic Recording on Disk
and Tape
Magnetic Tape is not very popular any more but most hard disks still use
magnetic recording. The resulting magnetised pattern is read using another
coil to detect the magnetic field.
Solenoid Actuators
Solenoid actuators are very commonly used in automation where linear
movement is required. For example electric control of pneumatic and
hydraulic valves.
Relays
•Relays allow a small
electric current in the coil
to turn on and off a much
larger current in the
contacts circuit.
•Relays provide safety
isolation.There is no
direct contact between
coil and contacts so a low
voltage circuit may safely
control a hazardous
volatgecircuit.
Loudspeakers
•Alternating current in the
coil interacts with the
permanetmagnet to
generate an oscillating
force on the coil.
•The oscillating coil
pushes the air back and
forth and generates
sound waves.
•A reversal of this principle
can be used as a
microphone.
Generators and Motors
ELectricmotors and generators use electromagntismto either
produce a force from electricity or to produce electricity by
electromagnetic induction.
Transformers
Transformers work because the primary winding generatsan
alternating magnetic field in the core which then induces a voltage in
the secondary winding.
Leakage and Fringing
•Most electromagnetic devices
use iron or other ferromagnetic
material to force the magentic
flux to go where it is wanted.
•Some useful flux is still lost. In
the diagram we are using an
iron core to try and focus the
flux through an airgap. Some
flux escapes the core
altogether (a) –This is called
leakage. Also some flux
spreads out at the airgap–this
is called fringing.
•In a typical electric machine up
to one quarter of the flux may
be lost through leakage.
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