Electrical and Electronic Principles

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: 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 • 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

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 showmagnetic 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:

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 www.coolmagnetman.com

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

12. Magnetic field lines at north pole of bar magnet www.coolmagnetman.com

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 Xthere 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 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 = 0.16 Wb A= 200 x 10-6m2. So B = Φ/A = 0.16/200 x 10-6 = 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 acurrent-carrying solenoid This graphic has been created mathematically by computer

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 MuonSolenoid (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 LHCbeam 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 • Paramagneticmaterials create a magnetic field in alignment with an externally applied magnetic field. They are weakly attracted to a magnet. [Due to orbital electron motion] • Diamagneticmaterials 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 Diamagneticforces 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). Pyrolyticcarbon, 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 onapplied 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 A stack of ferrite magnets Here, ferritemeans 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.) For paramagnetic & diamagnetic materials, μris 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 permeabilityof a material, as already discussed, is given by So at point P on the curve (see diagram), μ = 6.7 Hm-1 The incremental permeability is given by the gradientof the curve at P: 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 shieldingis the practice of isolating electrical equipment from the 'outside world‘. • Electromagnetic shieldingis used against relatively high frequency electromagneticfields. 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. 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 mmfℱ 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.

42. Electromagnetism ε= Blv F = Bil

43. Electromagnetic inductionworked 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= Blv = 5x10-4 x 30 x 150 = 2.25V

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

45. Principles linkingmagnetism and electricity: • Every electric current has a magnetic field surrounding it. • Alternating currents have fluctuating magnetic fields. • A fluctuatingmagnetic 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 EMFsare generated In accordance with Faraday’s Law e.g.ε = Blv Photoelectric / thermoelectric / junction / etc devices Inductors (self induction) Transformers (mutual induction) Electricity generators

48. Faraday’s law of electromagnetic induction 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. “The emf induced is equal to the rate of change of magnetic flux linkage or the rate of flux cutting.” … where = induced emf, = magnetic flux, = number of turns, = time The general equation above simplifies to ε = Blv 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