Download Electrical & Electronic Principles

Electrical and Electronic
Principles
BTEC National Diploma
O
P7, P8, P9, D1
Magnetism Assessment Criteria
P7. describe the characteristics of a magnetic field.
P8. describe the relationship between flux density (B)
& field strength (H).
P9. describe the principles & applications of
electromagnetic induction.
D1. analyse the operation and the effects of varying
component parameters of a power supply circuit that
includes a transformer, diodes and capacitors.
Know the principles and properties of
magnetism: content
Magnetic field:
• Magnetic field patterns eg
flux, flux density (B),
magnetomotive force (mmf)
and field strength (H),
permeability, B/H curves
and loops;
• Ferromagnetic materials;
reluctance; magnetic
screening; hysteresis
Electromagnetic induction:
• Principles eg induced
electromotive force (emf),
eddy currents, self and
mutual inductance;
• Applications (electric
motor/generator eg series
and shunt motor/generator;
transformer eg primary and
secondary current and
voltage ratios);
• Application of Faraday’s and
Lenz’s laws
Using iron filings to show magnetic field lines
These images show that
magnetism and electricity
are linked
Wire carrying a
DC current
Bar magnet
A solenoid is a coil
in the form of a
cylinder:
Current-carrying
solenoid
(notice magnetic
field pattern
similar to that
for bar magnet)
Using plotting compasses to show
magnetic field direction
Magnetic poles
• An electric dipole is a paired
arrangement of a positive (+)
electric charge and a negative (–)
one. They are equal and opposite.
• A magnetic dipole is a paired
north (N) and south (S) pole
arrangement. An atom is a tiny
magnetic dipole.
• If a bar magnet is cut in half, it is
not the case that one half has
only the north pole and the
other half has only the south.
• Instead, each piece has its own
pair of north and south poles.
• Whereas a single electric charge
can exist on its own, a single
magnetic pole on its own (a socalled magnetic monopole) has
never been observed and can
never be created from normal
matter (though some theories in
physics predict it does exist).
Man-made permanent magnets
• Naturally occurring ferromagnets were used in first experiments.
• Man-made products – based on a mixture of naturally occurring
magnetic elements or compounds.
• Magnets often manufactured by sintering (a sort of ‘baking’).
• Some common man-made magnets in table below:
Magnet type
Composition
Neodymium
Neodymium, iron, boron
SamCo
Samarium, cobalt (+ iron, copper)
Alnico
Aluminium, nickel, cobalt
Sr-ferrite
Strontium oxide, iron(II) oxide
Ferrimagnetism
• Almost every item of electronic equipment produced today contains some
ferrimagnetic material: loudspeakers, motors, deflection yokes, interference
suppressors, antenna rods, proximity sensors, recording heads, transformers and
inductors are frequently based on ferrites.
• Ferrimagnets possess permeability to rival most ferromagnets but their eddy
current losses are far lower because of the material's greater electrical resistivity.
Also it is practicable to fabricate different shapes by pressing or extruding - both
low cost techniques.
• Ferrimagnetic materials are usually oxides of iron combined with one or more of
the transition metals such as manganese, nickel or zinc. Permanent ferrimagnets
often include barium.
• The raw material is turned into a powder which is then fired in a kiln or sintered.
Magnetic field lines
At any point where two magnetic fields are
acting and a compass needle does not
point in any particular direction, then
there is no resultant field at the point.
Such a point is called a neutral point or a
null point. (See ‘np’ on bottom diagram.)
Strength of magnetic field around a
bar magnet
www.coolmagnetman.com
Strength of magnetic field around a
bar magnet's north pole: close-up
www.coolmagnetman.com
Magnetic field lines at north pole of
bar magnet
www.coolmagnetman.com
Two mutually attracting horseshoe magnets
Can you identify a neutral point?
Magnetic flux and flux density
Around the magnet there is a magnetic field which we think of as corresponding to a
‘flow of magnetic energy’ from the north pole to the south pole. We call this ‘flow’
magnetic flux (Φ) and the units are Webers (Wb). The diagram shows that there is as
much flux flowing ‘from the north pole’ as there is ‘flowing into the south pole’.
However, the amount of magnetic
flux flowing through a given area
will change from one point to
another. At position X there is a
greater number of field lines
passing through the loop than
there is when the same loop is at A.
The amount of flux passing through a unit area (1 m2) at right angles to the field lines is
called the magnetic flux density (B) at that point.
B is measured in Tesla (T) where 1 T = 1 Wbm-2
Magnetic flux density formula
𝑓𝑙𝑢𝑥 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝐵 =
𝑓𝑙𝑢𝑥 Φ
𝑎𝑟𝑒𝑎 𝐴 𝑡ℎ𝑟𝑜𝑢𝑔ℎ 𝑤ℎ𝑖𝑐ℎ 𝑓𝑙𝑢𝑥 𝑝𝑎𝑠𝑠𝑒𝑠
𝐵=
Φ
𝐴
Φ = BA
If we now use a coil of N
turns instead of just one
single loop, as shown in
position Z, the effect of the
flux through the N turns is N
times that through the
single loop.
(The quantity NΦ is called
the flux linkage for the coil
at that point – not required
for the BTEC Diploma.)
WORKED EXAMPLE: flux and flux density
The flux flowing through a horse-shoe
magnet is 0.16 Wb.
The cross sectional area of the gap is
200 mm2.
Calculate the magnetic flux density in the
gap.
SOLUTION
Φ = 0.16 Wb
A = 200 x 10-6 m2.
So B = Φ/A = 0.16/200 x 10-6 = 800 T
Wilhelm Eduard Weber
(1804-91)
• Important role in electrical
science.
• The unit of magnetic flux weber (Wb) - is named after
him.
Nikola Tesla (1856–1943)
• Serbian American inventor,
electrical engineer, mechanical
engineer, physicist, and futurist
• Best known for his contributions
to the design of the modern AC
electricity supply system
• Made a lot of money from his
patents and lived for most of his
life in New York hotels. Spent a lot
of income financing own projects
-eventually declared bankrupt.
• Regarded as a bit of a "mad
scientist.“
• The unit of magnetic flux density
– tesla (T) – named after him.
Magnetic field round a currentcarrying solenoid
Adapted from the Penguin IB physics guide
Magnetic field round a
current-carrying solenoid
This graphic has been created mathematically by computer
The LHC and liquid helium
Top left: Large Hadron
Collider (LHC) beam pipe
Top right: Liquid helium and
liquid nitrogen are both
pumped in to different parts
of the cyromodules
Bottom left: liquid helium in
an open container
Superconducting magnets at the LHC, CERN
The Compact Muon
Solenoid (CMS - left) is one
of the Large Hadron
Collider's massive particle
detectors.
The Solenoid is a
cryomagnet, i.e. an
electromagnet that
operates at extremely low
temperatures.
Cryomagnets are also used for the Large Hadron Collider itself (right).
The main magnets operate at around 8 tesla and a temperature of
̶ 271.3°C (1.9 K), colder than the temperature of outer space (2.7K).
At these very low temperatures, the wire is superconducting, i.e. its electrical resistance is exactly zero.
This means it can conduct much larger electric currents than ordinary wire, creating intense magnetic
fields. Because no energy is dissipated as heat in the windings, they can be cheaper to operate.
Cross-section of LHC beam pipes, containing a vacuum as empty as interplanetary space
Measuring magnetic fields: the flux density meter
(this one uses a Hall probe)
The Hall probe consists of a slice of semiconducting material with a small current
passing through it. When it is placed in the magnetic field a p.d. that is directly
proportional to the magnetic flux density is produced across the slice at right angles
to the current direction.
 A flux density meter is
sometimes called a Tesla
meter.
 The Hall probe is only
suitable for measuring
steady magnetic fields.
Types of magnetism and the periodic table
 Paramagnetic materials create a magnetic field in alignment with an externally
applied magnetic field. They are weakly attracted to a magnet. [Due to orbital electron motion]
 Diamagnetic materials create a magnetic field in opposition to an externally applied
magnetic field. There are weakly repelled by a magnet.
[Due to unpaired electron spins]
 Ferromagnetic materials are strongly attracted to a magnet. Iron, nickel and cobalt
are ferromagnetic. It is these your BTEC course is most interested in.
[Due to magnetic domains]
This periodic table
shows magnetic
properties of ELEMENTS,
not minerals, alloys or
compounds.
If interested, look up:
• Antiferromagnetism
(due to neighbouring ions equal
& opposite dipole moments)
•
Ferrimagnetism
(due to neighbouring ions
UNequal & opposite dipole
moments)
Paramagnetism & diamagnetism
Oxygen is paramagnetic and so
is attracted to a magnet.
See
https://www.youtube.com/watch?v=KcGEev8qulA
Diamagnetic forces
acting upon the
water within its body
levitating a live frog.
The frog is inside a
special solenoid that
generates an
extremely powerful
magnetic field (16 T).
Nijmegen High Field Magnet Laboratory.
Pyrolytic carbon, which
is highly diamagnetic,
levitating over
permanent magnets
Ferromagnetism
Iron, nickel, cobalt (and some of the rare earth elements) exhibit a behaviour called
ferromagnetism because iron (Latin: ferrum) is the most common and dramatic example.
• Ferromagnetism is a
very strong form of
magnetisation.
• This is due to the
existence of magnetic
domains in
ferromagnetic
materials.
Unmagnetised
ferromagnetic material:
magnetic domains are
unaligned
Magnetised
ferromagnetic material:
magnetic domains are
aligned
You may like to look up paramagnetism, diamagnetism,
ferromagnetism, ferrimagnetism and antiferromagnetism.
Effect of matter on
applied magnetic field
For ferromagnetic
matter, this effect is
more extreme.
Magnetic flux density B, magnetic field strength
H and permeability μ.
When a magnetic field is applied to a material, the resulting overall magnetic flux
density B within the material has two components, arising from:
1. The original applied field
2. An extra induced field resulting from the effect of the applied field on the atoms of
the material (the material itself has become magnetised – even if only minutely –
owing to the effect of the applied field and has produced a field of its own)
A common formula to express this situation is
B = μH
Where B is the overall magnetic flux density, H is the magnetic (or applied) field
strength and μ is the permeability of the material, measured in henry per metre (Hm-1).
The permeability μ is a measure of the extent to which the material enhances the
existing applied field. It is measured in amps per metre (Am-1)
The permeability is composed of two components: μ = μ0 μr
Where μ0 is the permeability of free space (4π × 10-7 Hm-1) and μr is the relative
permeability of the substance (no units).
Relative permeability (μr) values for some materials
μr for a vacuum = 1 exactly, by definition
◙ Paramagnetic (μr > 1)
Platinum
1.000265
Aluminium
1.000022
Air
1.00000043
Wood
1.0000004
◙ Diamagnetic (μr < 1)
Bismuth
0.999834
Water
0.999992
Copper
0.999994
Sapphire
0.9999998
For paramagnetic &
diamagnetic materials,
μr is very close to 1.
◙ Ferromagnetic
Metglas
Iron (annealed)
Mumetal
Permalloy
Rhometal
Steel
Nickel
Cobalt
(μr >> 1)
1,000,000
to 350,000
to 100,000
to 25,000
to 5000
to 800
to 600
to 250
◙ Ferrimagnetic
Ferrite (Ni-Zn)
(μr >> 1)
to 640
A stack of ferrite magnets
Here, ferrite means
a chemical
compound of
ceramic materials
with iron(II) oxide as
its main constituent.
It was invented in
Japan in 1930.
(Ferrite also has
other meanings.)
Magnetisation in different materials
These are often called B-H curves.
Note: the B axis here is in tesla,
whereas for the paramagnetic &
diamagnetic graphs it is in millitesla.
Magnified B-H curve for a
ferromagnetic material
(These ‘steps’ are called Barkhausen jumps - not required for BTEC Diploma! They
occur because of the magnetic domain structure of ferromagnetic materials.)
Typical hysteresis loop
(Greek hystérēsis =
‘lagging behind’)
Magnetic domains and hysteresis
Magnetically hard and soft materials
Magnetic memory
(permanent magnet)
Transformer core
(temporary magnet)
Incremental permeability
The permeability of a material, as
already discussed, is given by
𝐵
𝝁=
𝐻
So at point P on the curve (see
diagram), μ = 6.7 Hm-1
The incremental permeability is
given by the gradient of the
curve at P:
𝝁𝒊𝒏𝒄 =
𝛿𝐵
𝛿𝐻
So at P, μinc = 1.3 Hm-1
Quite often, books confuse readers by
alluding to both B/H and δB/δH as the
‘permeability’, whereas they can have very
different values!
Shielding
Electromagnetic or magnetic shielding is the practice of isolating electrical
equipment from the 'outside world‘.
• Electromagnetic shielding is used against relatively high frequency
electromagnetic fields. It is made from conductive or magnetic
materials. A conductive enclosure used to block electrostatic fields is
known as a Faraday cage. Such shielding is also used in cables to isolate
wires from the environment.
• Magnetic shielding is used against static or slowly varying magnetic
fields. Shields made of high magnetic permeability metal alloys can be
used, such as sheets of Permalloy (80% iron, 20% nickel) and Mu-Metal
(77% nickel, 16% iron plus a little copper and chromium or
molybdenum). These materials don't block the magnetic field, as is the
case with electric shielding, but rather draw the field into themselves.
Magnetic shields often consist of several enclosures one inside the
other.
How magnets are made
There are four main ways to magnetize a magnetisable object or substance:
1. bringing the substance near a magnet;
2. using electric current;
3. stroking the substance with a magnet; and
4. striking a blow to the substance while it is in a magnetic field.
A permanent magnet can be made by stroking a magnetic substance with either the N
or the S pole of a magnet. Stroking lines up the domains in the material.
A piece of iron can be magnetized by holding it parallel to a compass needle (along the
lines of force in the earth's field) and hitting the piece of iron with a hammer. The
blow will overcome the resistance of the domains to movement, and they will line up
parallel to the earth's field.
To demagnetize an object, a strong magnetic field is used. In one method, the
magnetic field is made to fluctuate very rapidly. In another method, the magnetized
object is placed so that a line drawn between its poles would be at right angles to the
field. The object is then tapped or hit until its domains are no longer lined up
magnetically.
Strengths of some magnetic fields
Source
Magnetic
flux density
(tesla)
Magnetically shielded room
10-14
Interstellar space
10-10
Earth's magnetic field (UK)
5×10-5
Small bar magnet
0.01
Sunspot
0.2
Neodymium magnet
1
Big electromagnet; big
transformer; speaker coil
1-2.4
Superconducting magnet
1-40
Regular neutron star
107
Magnetar
108-1011
A neodymium magnet
(developed in 1982) is
• the most widely used type of
rare-earth magnet
• made from an alloy of
neodymium, iron and boron
• the strongest type of
permanent magnet
commercially available
• used in applications that
require strong permanent Neodymium
can easily
magnets, such as motors in magnets
lift thousands of
cordless tools, hard disk
times their own
weight – such as
drives and magnetic
these steel spheres
fasteners.
There are 17 ‘rare earth’ metals in the periodic table. They are actually not rare in themselves, but are scattered far
and wide rather than being concentrated in easily found minerals. It is the minerals that are rare.
Magnetomotive force & reluctance
Magnetomotive force (mmf) is what ‘causes’ there to be a magnetic flux in a magnetic circuit.
The mmf ℱ is defined as
ℱ = NI
where “N” is the number of turns of wire in the coil and “I” is the current in the coil. The unit for
mmf is ampere-turns (A·t).
Example: calculate the mmf for a coil with 2000 turns and a 5 mA current.
Answer: ℱ = N × I = 2000 × 5 × 10-3 = 10 A·t
For a magnetic circuit we have
ℱ = ΦS
See table below for comparison of magnetic scenario with electrical scenario.
Magnetic circuit
Electrical circuit
ℱ = ΦS
ε = IR
where
ℱ is the mmf
Φ is the magnetic flux
S is the reluctance of the material
through which the flux ‘passes’.
where
ε is the emf
I is the current
R is the total circuit resistance
Electromagnetism
F = Bil
ε = Bl v
Electromagnetic induction
worked example
Worked example. A plane of wingspan 30 m flies through a vertical field of strength 5
x 10-4 T. Calculate the emf induced across its wing tips if its velocity is 150 ms-1.
ε = Bl v = 5x10-4 x 30 x 150 = 2.25V
Electromagnetic
Induction
A galvanometer is a
type of very
sensitive ammeter
used to detect tiny
currents.
(They were the
original ammeters)
Principles linking
magnetism and electricity:
• Every electric current has a magnetic field
surrounding it.
• Alternating currents have fluctuating magnetic fields.
• A fluctuating magnetic fields produces an emf which
causes a current to flow in conductors lying within
the fields. This is known as electromagnetic
induction.
Electromagnetic induction applications
Electromagnetic induction is the principle that
makes possible devices such as:
• electrical generators, transformers and certain
kinds of motor
• rechargeable electric toothbrushes and
wireless communication devices
• rice cookers.
Ways that EMFs
are generated
EMF
ε
In accordance
with Faraday’s
Law
𝒅𝜱
𝜺 = −𝑵
Generated
electrochemically etc
Batteries
Induced using
external
magnetic field
Photoelectric /
thermoelectric /
junction / etc
devices
Varying
magnetic field
(produced by AC.)
No motion
Inductors
Transformers
(self induction)
(mutual induction)
𝒅𝑰
𝜺 = −𝑳
𝒅𝒕
𝑽𝒑 𝑵𝒑
=
𝑽𝒔 𝑵𝒔
𝒅𝒕
e.g.
ε = Bl v
Constant
magnetic field +
conductor. One
or both moving
Electricity
generators
𝑽 = 𝑽𝟎 𝒔𝒊𝒏𝝎𝒕
Faraday’s law of electromagnetic induction
“The emf induced is equal to the rate of change of
magnetic flux linkage or the rate of flux cutting.”
𝑑Φ
ε = −𝑁
𝑑𝑡
… where
𝛆 = induced emf,
𝚽 = magnetic flux,
𝑵 = number of turns, 𝒕 = time
The general equation above simplifies to
ε = Bl v
for the motional emf
induced in a straight
conductor of length l ,
both positioned and
moving (at a velocity v)
at right angles to a
uniform magnetic field of
density B. See diagram.
LENZ’S LAW: “An
induced electric
current flows in a
direction such that
the current opposes
the change that
induced it.” Hence
the ‘ ̶ ‘ sign in the
Faraday equation.
Eddy currents
A kayaker can use river eddies. On the
downstream side of every rock that breaks the
surface of a river, you will find an eddy large
enough for the front of your kayak to sit in
while you have a rest and admire the view.
Eddyhopping is where a white water kayaker
sprints upstream from one eddy to another.
This 93 mile wide deep
underwater eddy was spotted
off the coast of South Africa
by satellite.
Electrical eddy currents
Mutual and self induction
• A changing magnetic flux induces an emf in a
conductor. General term for this:
𝑑Φ
ε = −𝑁
electromagnetic induction.
𝑑𝑡
Faraday’s Law
• If the source of the changing magnetic flux is itself a
current-carrying conductor, this it termed mutual
induction. The quantity of induction is called the
mutual inductance 𝑴 of the two circuits.
• A conductor carrying a changing current induces an
emf in itself (sometimes called a back emf). This is
termed self induction, and the amount of this is
called the self inductance (or just the inductance) 𝑳.
An inductor is an electrical component
𝑑𝐼
ε = −𝐿
that is used in some AC circuits.
𝑑𝑡
• [ It can be shown that 𝑀 = 𝑘 𝐿1 𝐿2 , where 𝑘 is called the
coupling coefficient. ]
Typical
values:
μH
mH
Unit of inductance:
the henry (H)
Mutual induction
(switch being closed in the primary circuit)
Does the galvanometer’s pointer remain deflected to the right?
Which way will it go if S is now opened?
Mutual induction
(AC in the primary circuit)
How Induction Cooktops Work
http://home.howstuffworks.com/induction-cooktops3.htm
Diagram of simple inductor
Examples of Inductors
More on inductors
An inductor is somewhat like a capacitor. They both store electromagnetic energy.
They both oppose changes in a circuit.
• A capacitor likes to maintain a constant voltage.
It stores this energy in an electric field.
Its reactance decreases with frequency.
• An inductor likes to maintain a constant current.
It stores this energy in a magnetic field.
Its reactance increases with frequency.
[NOTE: reactance means a capacitor’s or inductor’s “resistance” to AC.]
Because of this “constant current“ feature, when current through an inductor is
increased or decreased, the inductor "resists" this change by producing a voltage
between its leads in opposing polarity to the change.
Inductors when combined with capacitors become useful when you want to make
filters that let only chosen frequencies through (e.g. In radio tuner circuits and
speaker crossovers.) The capacitor blocks off low frequencies, the inductor blocks
off high frequencies.
Inductor circuit symbols
The transformer
A transformer steps up or steps down an AC voltage.
𝑉𝑝 𝑁𝑝
=
= 𝑡𝑢𝑟𝑛𝑠 𝑟𝑎𝑡𝑖𝑜
𝑉𝑠 𝑁𝑠
Core laminations
A symbol for a transformer
US (and original UK)
symbol for a resistor
Core losses
(iron losses)
Hysteresis
losses
Eddy current
losses
 Core losses are
sometimes called
no-load losses
Winding losses
(copper losses)
(I2R losses)
 Winding losses
are sometimes
called load losses
Stray losses
(flux leakage)
 Stray losses are
relatively small
 In addition to the above, there is a very small
amount of mechanical loss due to vibrations,
which result in an audible transformer hum
Transformer losses
Flux leakage (stray losses) in a
transformer
LEAKAGE
LEAKAGE
LEAKAGE
LEAKAGE
A simple AC electric generator
AC generator (continued)
1
2
3
4
EMF induced in a coil rotating in a magnetic field
The ‘motor effect’
F = Bil
where F is the force on a conductor of length l carrying a current i
and perpendicular to a magnetic field of flux density B.
Worked example. Calculate the force on a power cable of length 100 m carrying a
current of 200 A at place where the Earth's magnetic field is 10-5 T and is
perpendicular to the cable.
The cable will experience a force given by F = Bil = 10-5x200x100 = 0.2 N
The ‘catapult effect,
Used in ‘motor
effect’ situations.
Using Fleming’s LHR
The homopolar motor
With the motor effect or generator effect, we have
three ‘vectors’:
1. The magnetic field
2. The electric current
3. The motion of (i.e. thrust on) the object.
In diagrams, two of these are likely to lie within the
plane of the page. The third is likely to go into or
come out of the page.
If it goes into the page, the direction is denoted by a
cross ‘×’ inside a small circle. If it comes out of the
page, its direction is denoted by a dot ‘•’ inside a
small circle. (These represent an arrow going into or
coming out of the page.)
https://www.youtube.com/watch?v=xbCN3EnYfWU
In the diagram to the left, the magnetic field B and
the current I lie within the plane of the paper. The
direction of motion of the wire is out of the page on
the left hand side, and into the page on the right.
Homopolar machines
(they hardly ever used due to inefficiency)
◄ The first superconducting electric motor, made in 1966
by NEI for the MOD - a homopolar machine containing
no iron and rated at 50 horsepower (hp).
1 hp = 746 watts.
The term horsepower was adopted in the late 18th
century by James Watt to compare the output of steam
engines with that of draft horses.
◄ The powerful NPT301 turbojet was designed
for use primarily in Remotely Piloted Vehicle
(RPV) applications. The nose bullet housed a
homopolar alternator.
RPVs are more often called UAVs (Unmanned
Aerial Vehicles) or drones these days.
NPT went bust in 1990 due to competition from overseas
companies.
YOU WON’T BE TESTED ON THIS
Brake horsepower (bhp) is the measure of an engine's
horsepower before the loss in power caused by the gearbox,
alternator, differential, water pump, and other auxiliary
components such as power steering pump & muffled exhaust.
Basic electric motor
‘Catapult effect’
on a coil in a magnetic field
Commercial motors
A commercial motor is different in several ways from our simple model.
It uses:
carbon brushes for good electrical contact with the commutator and
also so that when the brushes wear away, they can easily be
replaced. Carbon brushes do not wear away as quickly as metal
brushes.
a multi-section commutator - two sections for each of several
rotating coils wound in different planes. Although only one of these
coils carries a current at any one time, having a lot of them makes
the rotation far smoother.
field coils rather than a permanent magnet. These coils become
magnetised when a current is passed through them. Field coils give
a stronger, more easily shaped magnetic field than permanent
magnets.
Field windings
A commercial motor is different in several
ways from our simple model. It uses:
• carbon brushes for good electrical contact
with the commutator and also so that
when the brushes wear away, they can
easily be replaced. Carbon brushes do not
wear away as quickly as metal brushes.
• a multi-section commutator - two
sections for each of several rotating coils
wound in different planes. Although only
one of these coils carries a current at any
one time, having a lot of them makes the
rotation far smoother.
• field coils rather than a permanent
magnet. These coils become magnetised
when a current is passed through them.
Field coils give a stronger, more easily
shaped magnetic field than permanent
magnets.
Appendix
1.
2.
3.
4.
5.
6.
7.
Magnetism Formulae
AC Motor
Alternative names for B and H
History of magnet strengths
BBC Learning Zone 1
BBC Learning Zone 2
Building a tunnel
Some magnetism formulae
• 𝐵=
ℱ
Φ
𝐴
• ℛ =
Φ
or
or
ℱ = Φℛ
Φ = BA
• F = Bil
• ε = Bl v
• B = μH
or
μ=
•
𝐵
•
𝐻
μ𝑖𝑛𝑐𝑟𝑒𝑚𝑒𝑛𝑡𝑎𝑙
δ𝐵
=
δ𝐻
𝑉𝑝
𝑉𝑠
=
𝑁𝑝
𝑁𝑠
=𝑎
Example of AC motor developed locally
According to the Green Motorsport website …
This water-cooled 48 volt high frequency
AC motor is capable of pulling 650 amps
peak. It delivers its power in a very
different way from the conventional DC
motor. Its high performance capability is
obtained by means of a water-cooling
system and highly efficient windings. The
water cooling jacket is totally seamless.
Woking
(opposite McClarens)
“Environmentally conscious
motorsport”
The GMS M1 motor is brushless and
totally sealed from the elements, making
it durable and robust. This makes it
suitable for almost any application,
from electric cars to water craft. The
technology will be proven in motorsport,
the most demanding environment known.
Alternative names for B and H
Alternative names for B
•Magnetic flux density
•Magnetic induction
•Magnetic field
Alternative names for H
•Magnetic field intensity
•Magnetic field strength
•Magnetic field
•Magnetizing field
How the strength of magnets has
increased over the years
BBC Learning Zone (1)
www.bbc.co.uk/learningzone/clips/
TRP
reference
code
Clip
number
BBC title
Brief overview of the topic
How wind energy produces electricity
LZ1
6616
Engineers at a wind farm in Wales explain: choice of site, transportation of turbines to the site,
the farm’s construction, production of electricity for the national grid, and positive and
negative aspects of wind energy.
A solar power plant in Spain is producing enough power for thousands of homes
LZ2
LZ3
6617
6618
Engineers explain how hundreds of mirrors are used to reflect sunlight to a receiver on a
central tower. There, water is heated to create steam, which drives a turbine and generates
electricity. A second system using parabolic reflectors is shown, together with new ways to
store heat to increase the useful output from the power plant.
How electricity can be produced by nuclear fusion, and arguments for and against its use
Engineers at JET in Oxfordshire explain their research into fusing hydrogen isotopes to create
energy to produce electricity. The aim is to allow them to get closer to being able to design and
build a commercial fusion power plant. Positive and negative aspects of harnessing fusion
energy are considered.
How does an electric shaver work, and how is it made?
LZ4
6619
Engineers at Braun explain how an electric razor works, and the innovations incorporated into
the latest shaver designs. The different stages of manufacture – from design to mass
production – are shown and discussed.
BBC Learning Zone (2)
www.bbc.co.uk/learningzone/clips/
How does a loudspeaker work, and how is it made?
LZ5
6620 Engineers explain how a loudspeaker is made from a number of components assembled into an
enclosure, and the technical basis on which it operates. Its operation is demonstrated and a postproduction testing procedure described.
How does a hover lawnmower work?
LZ6
6621 A design engineer at Flymo explains and demonstrates the principle of operation and the safety tests
that a mower must pass. Cut-away sections through the mower allow the internal components to be
seen.
The world’s longest, deepest tunnel
LZ7
6622 Swiss engineers describe the design and construction of the Gotthard Base tunnel. They explain using
an electronic system to correctly align the tunnel and recycling excavated rubble into concrete for its
lining.
LZ8
6623 Engineers describe the design and construction of a device that can accelerate electrons to almost the
speed of light in order to produce x-rays that can see deep inside metals and other substances.
The Synchrotron: the world’s biggest microscope
The use of the Synchrotron, the world’s biggest microscope
LZ9
An engineer from Rolls-Royce explains how the materials which go into the manufacture of aero
6624 engines can be made stronger and lighter if more is known about their internal structures. To do this
the engineers use x-rays from the Synchrotron to look deep into metals. Components are subjected to
forces, and the stresses and deformations within them investigated.
Building a tunnel for high-speed trains
… er … the link www.bbc.co.uk/learningzone/clips/6622.html
has got very little to do with this unit except for
the electronic system used to align the tunnel,
but it’s quite interesting and it’s a BTECrecommended video clip, so I suppose I might
as well show it …
Swiss engineers explain the need for the Gotthard Base Tunnel to
reduce the amount of traffic on the roads. The long, flat rail tunnel
through the Alps will allow both passenger trains and shuttles carrying
lorries to cross the Alps using far less energy. The tunnel is being built
in sections and electronic systems are used to ensure the sections
meet up to within 25 cm. The engineers explain how they have
developed a way to use the rubble from the excavations in the
concrete used to build the tunnel.
Measuring magnetic fields: the search coil
The search coil method can be used to measure both constant and varying
fields. Typical characteristics: 1000 turns; ½ cm diameter.
Measuring varying magnetic fields. An e.m.f is induced in it which is directly
proportional to the flux density. This e.m.f. is conveniently displayed as a
vertical line on an oscilloscope whose time-base is switched off.
Measuring steady magnetic fields. The search, connected to a ballistic
galvanometer, is placed in the field and held still, then removed quickly. The
maximum galvanometer deflection is proportional to the field strength.
End