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Electrical and Electronic Principles

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1 Electrical and Electronic Principles
BTEC National Diploma O P7, P8, P9, D1

2 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.

3 Know the principles and properties of magnetism: content
Magnetic field: Electromagnetic induction: 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 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

4 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)

5 Using plotting compasses to show magnetic field direction

6 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. Whereas a single electric charge can exist on its own, a single magnetic pole on its own (a so-called magnetic monopole) has never been observed and can never be created from normal matter (though some theories in physics predict it does exist). 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.

7 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

8 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.

9 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.)

10 Strength of magnetic field around a bar magnet

11 Strength of magnetic field around a bar magnet's north pole: close-up

12 Magnetic field lines at north pole of bar magnet

13 Two mutually attracting horseshoe magnets
Can you identify a neutral point?

14 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

15 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.)

16 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 = Wb A = 200 x 10-6 m2. So B = Φ/A = 0.16/200 x = 800 T

17 Wilhelm Eduard Weber (1804-91)
Important role in electrical science. The unit of magnetic flux - weber (Wb) - is named after him.

18 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.

19 Magnetic field round a current-carrying solenoid
Adapted from the Penguin IB physics guide

20 Magnetic field round a current-carrying solenoid
This graphic has been created mathematically by computer

21

22 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

23 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.

24 Cross-section of LHC beam pipes, containing a vacuum as empty as interplanetary space

25 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.

26 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)

27 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). Pyrolytic carbon, which is highly diamagnetic, levitating over permanent magnets Nijmegen High Field Magnet Laboratory.

28 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.

29 Effect of matter on applied magnetic field
For ferromagnetic matter, this effect is more extreme.

30 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: The original applied field 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).

31 Relative permeability (μr) values for some materials
μr for a vacuum = 1 exactly, by definition ◙ Paramagnetic (μr > 1) Platinum Aluminium Air Wood ◙ Ferromagnetic (μr >> 1) Metglas 1,000,000 Iron (annealed) to 350,000 Mumetal to 100,000 Permalloy to 25,000 Rhometal to 5000 Steel to 800 Nickel to 600 Cobalt to 250 A stack of ferrite magnets ◙ Diamagnetic (μr < 1) Bismuth Water Copper Sapphire 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.) ◙ Ferrimagnetic (μr >> 1) Ferrite (Ni-Zn) to 640 For paramagnetic & diamagnetic materials, μr is very close to 1.

32 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.

33 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.)

34 Typical hysteresis loop
(Greek hystérēsis  = ‘lagging behind’)

35 Magnetic domains and hysteresis

36 Magnetically hard and soft materials
Magnetic memory (permanent magnet) Transformer core (temporary magnet)

37 Incremental permeability
The permeability of a material, as already discussed, is given by 𝝁= 𝐵 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: 𝝁 𝒊𝒏𝒄 = 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!

38 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.

39 How magnets are made There are four main ways to magnetize a magnetisable object or substance: bringing the substance near a magnet; using electric current; stroking the substance with a magnet; and 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.

40 Strengths of some magnetic fields
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 magnets, such as motors in cordless tools, hard disk drives and magnetic fasteners. 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 Neodymium magnets can easily lift thousands of times their own weight – such as these steel spheres 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.

41 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 = × 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 where ℱ is the mmf Φ is the magnetic flux S is the reluctance of the material through which the flux ‘passes’. ε = IR ε is the emf I is the current R is the total circuit resistance

42 Electromagnetism ε = Bl v F = Bil

43 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 = 5x x x = V

44 Electromagnetic Induction
A galvanometer is a type of very sensitive ammeter used to detect tiny currents. (They were the original ammeters)

45 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.

46 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.

47 Ways that EMFs are generated
Generated electro-chemically etc Batteries Induced using external magnetic field Varying magnetic field (produced by AC.) No motion Constant magnetic field + conductor. One or both moving In accordance with Faraday’s Law 𝜺=−𝑵 𝒅𝜱 𝒅𝒕 e.g. ε = Bl v Photoelectric / thermoelectric / junction / etc devices Inductors (self induction) Transformers (mutual induction) Electricity generators 𝑽 𝒑 𝑽 𝒔 = 𝑵 𝒑 𝑵 𝒔 𝜺=−𝑳 𝒅𝑰 𝒅𝒕 𝑽= 𝑽 𝟎 𝒔𝒊𝒏𝝎𝒕

48 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.” 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. ε=−𝑁 𝑑Φ 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.

49 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.

50 Electrical eddy currents

51 Mutual and self induction
A changing magnetic flux induces an emf in a conductor. General term for this: electromagnetic induction. 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 that is used in some AC circuits. [ It can be shown that 𝑀=𝑘 𝐿 1 𝐿 2 , where 𝑘 is called the coupling coefficient. ] ε=−𝑁 𝑑Φ 𝑑𝑡 Faraday’s Law Typical values: μ H mH ε=−𝐿 𝑑𝐼 𝑑𝑡 Unit of inductance: the henry (H)

52 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?

53 Mutual induction (AC in the primary circuit)

54 How Induction Cooktops Work

55

56 Diagram of simple inductor

57 Examples of Inductors

58 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.

59 Inductor circuit symbols

60 The transformer 𝑉 𝑝 𝑉 𝑠 = 𝑁 𝑝 𝑁 𝑠 =𝑡𝑢𝑟𝑛𝑠 𝑟𝑎𝑡𝑖𝑜
A transformer steps up or steps down an AC voltage. 𝑉 𝑝 𝑉 𝑠 = 𝑁 𝑝 𝑁 𝑠 =𝑡𝑢𝑟𝑛𝑠 𝑟𝑎𝑡𝑖𝑜

61 Core laminations

62 A symbol for a transformer
US (and original UK) symbol for a resistor

63 Transformer losses Winding losses (copper losses) (I2R losses)
Core losses (iron losses) Hysteresis losses Eddy current losses Winding losses (copper losses) (I2R losses) Stray losses (flux leakage) Winding losses are sometimes called load losses 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 Core losses are sometimes called no-load losses

64 Flux leakage (stray losses) in a transformer

65 A simple AC electric generator

66 AC generator (continued)
1 2 3 4

67 EMF induced in a coil rotating in a magnetic field

68 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

69 The ‘catapult effect,

70 Used in ‘motor effect’ situations.

71 Using Fleming’s LHR

72 The homopolar motor With the motor effect or generator effect, we have three ‘vectors’: The magnetic field The electric current 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.) 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. https://www.youtube.com/watch?v=xbCN3EnYfWU

73 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. 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. 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

74 Basic electric motor

75 ‘Catapult effect’ on a coil in a magnetic field

76 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.

77 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.

78 Appendix 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

79 Some magnetism formulae
𝐵= Φ 𝐴 or Φ = BA B = μH or ℛ = ℱ Φ or ℱ = Φℛ F = Bil ε = Bl v 𝑉 𝑝 𝑉 𝑠 = 𝑁 𝑝 𝑁 𝑠 =𝑎 μ= 𝐵 𝐻 μ 𝑖𝑛𝑐𝑟𝑒𝑚𝑒𝑛𝑡𝑎𝑙 = δ𝐵 δ𝐻

80 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. 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.  Woking (opposite McClarens) “Environmentally conscious motorsport”

81 Alternative names for B and H
Alternative names for H Magnetic flux density Magnetic induction Magnetic field Magnetic field intensity Magnetic field strength Magnetizing field

82 How the strength of magnets has increased over the years

83 BBC Learning Zone (1) www.bbc.co.uk/learningzone/clips/
TRP reference code Clip number BBC title Brief overview of the topic LZ1 6616 How wind energy produces electricity 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. LZ2 6617 A solar power plant in Spain is producing enough power for thousands of homes 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. LZ3 6618 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. LZ4 6619 How does an electric shaver work, and how is it made? 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.

84 BBC Learning Zone (2) www.bbc.co.uk/learningzone/clips/
LZ5 6620 How does a loudspeaker work, and how is it made? 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 post- production testing procedure described. LZ6 6621 How does a hover lawnmower work? 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. LZ7 6622 The world’s longest, deepest tunnel 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 The Synchrotron: the world’s biggest microscope 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. LZ9 6624 The use of the Synchrotron, the world’s biggest microscope An engineer from Rolls-Royce explains how the materials which go into the manufacture of aero 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.

85 Building a tunnel for high-speed trains
… er … the link 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 BTEC- recommended 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.

86 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.

87 End


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