Electromagnetism

Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electrically charged particles. The electromagnetic force usually exhibits electromagnetic fields such as electric fields, magnetic fields, and light, and is one of the four fundamental interactions (commonly called forces) in nature. The other three fundamental interactions are the strong interaction, the weak interaction, and gravitation.[1] At high energy the weak force and electromagnetic force are unified as a single electroweak force.

1. UNIT-2 ELECTROMAGNETISM

2. Which electrical Appliances Use Magnets? Motors(spinning motors, sewing machine ,fan, water pumps etc. ) Transformers Hair dryers Food mixers Microwaves Refrigerators A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets. Lodestone is naturally available magnet.

3. Magnetism A material is said to be magnetized if it displaces the following properties. When suspended freely it comes to rest in a line running approximately earth’s north and south. The north pole of a magnet is the pole that, when the magnet is freely suspended, points towards the Earth's North Magnetic Pole(actual magnetic south pole of earth) which is located in northern Canada. Earth’s South magnetic pole(actual magnetic north pole earth) is located in Antarctica.

4. 2. It is able to impart magnetism to other magnetic materials Diamagnetic materials are slightly repelled by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Examples: copper, silver, and gold etc. Paramagnetic materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Examples: magnesium, molybdenum, lithium, etc. Ferromagnetic materials exhibit a strong attraction to magnetic fields and are able to retain their magnetic properties after the external field has been removed. Examples: nickel, iron, cobalt etc. 3. It exerts a force on other magnetic material

5. Permanent Magnets and Electromagnets A permanent magnet is an object made from a ferromagnetic material that is magnetized and creates its own persistent magnetic field. An electromagnet is made from a coil of wire which acts as a magnet when an electric current passes through it. Often an electromagnet is wrapped around a core of ferromagnetic material like steel, which enhances the magnetic field produced by the coil.

6. Permanent Magnet vs. Electromagnet Magnetic Properties 1. A permanent magnet’s magnetic properties exist when the magnet is magnetized. An electromagnetic magnet only displays magnetic properties when an electric current is applied to it. 2.The magnets that you have affixed to your refrigerator are permanent magnets, while electromagnets are the principle behind AC motors. Magnetic Strength 1.Permanent magnet strength depends upon the material used in its creation. 2. The strength of an electromagnet can be adjusted by the amount of electric current allowed to flow into it. 3. As a result, the same electromagnet can be adjusted for different strength levels.

7. Loss of Magnetic Properties 1. If a permanent magnet loses its magnetic properties, it will be rendered useless and its magnetic properties can be only recovered by re-magnetizing. 2. An electromagnet loses its magnetic power every time an electric current is removed and becomes magnetic once again when the electric field is introduced Advantages 1. The main advantage of a permanent magnet over an electromagnet is that a permanent magnet does not require a continuous supply of electrical energy to maintain its magnetic field. 2. An electromagnet’s magnetic field can be rapidly manipulated over a wide range by controlling the amount of electric current supplied to the electromagnet.

8. Magnetic effect of an electric current HISTORY: On 21 April 1820, Hans Christian Orsted noticed a compass needle deflected from magnetic north when the electric current from the battery he was using was switched on and off. This deflection convinced him that magnetic fields radiate from all sides of a wire carrying an electric current

9. Cross and dot conventions

10. Right hand thumb rule An electric current passes through a straight wire. Here, the thumb points in the direction of the conventional current (from positive to negative), and the fingers point in the direction of the magnetic lines of flux.

11. Maxwell’s cork screw rule The direction of rotation of a right-handed cork-screw screwed into the conductor in the direction of current flow indicates the direction of the lines of magnetic flux around the conductor.

12. Right hand grip rule An electric current passes through a solenoid, resulting in a magnetic field. When you wrap your right hand around the solenoid with your fingers in the direction of the conventional current, your thumb points in the direction of the magnetic north pole.

13. Nature of magnetic field of long straight conductors The nature of the magnetic field lines around a straight current carrying conductor is concentric circles with centre at the axis of the conductor. The strength of the magnetic field created depends on the current through the conductor. The strength of the field was strongest next to the wire and diminished with distance from the conductor until it could no longer be detected. The direction of the magnetic field was dependent on the direction of the electrical current in the wire.

14. Nature of magnetic field of solenoid A solenoid is a coil wound into a tightly packed helix. A magnetic field develops that flows through the center of the loop or coil along its longitudinal axis. The magnetic field circling each loop of wire combines with the fields from the other loops to produce a concentrated field down the center of the coil. The strength of a coil's magnetic field increases not only with increasing current but also with each loop that is added to the coil. The concentrated magnetic field inside a coil is stronger than the field outside the coil.

15. Nature of magnetic field of toroid A toroidal coil is a group of current loops on a surface of a torus, that have the same current passing through. Because of symmetry the magnetic field strength is constant in the constant distance from the center of the torus. If the radius of the current loop is small compared to the toroid radius, the magnetic field is constant. The magnetic field outside the toroid is negligible.

16. MAGNETIC FIELD/MAGENTIC FLUX DENSITY (B) A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength). It is a vector quantity. The magnetic field is most commonly defined in terms of the Lorentz force it exerts on moving electric charge.

17. A particle of charge q (stationary)in an electric field E experiences a force When a charged particle moves in the vicinity of a current-carrying wire, the force also depends on the velocity of that particle. How to measure the direction and magnitude of the vector B? Take a particle of known charge q . Measure the force on q at rest, to determine E . Then measure the force on the particle when its velocity is v. Find a B that makes the Lorentz force law fit all these results—that is the magnetic field at the place.

18. B- Magnetic flux density (unit: teslas -T) Magnetic flux: The magnetic flux through a surface is the component of the magnetic B field passing through that surface. ;A-area θ is the angle between the surface and magnetic flux lines direction. Unit of - weber ( Wb ) (SI units) maxwell ( Mx ) (CGS) 1 weber =10 8 maxwell or

19. m.m.f Magneto motive force ( m.m.f - θ or F) is the strength of a magnetic field in a coil of wire. Dependent on how much current flows in the turns of coil. the more current, the stronger the magnetic field. the more turns of wire, the more concentrated the lines of force.

20. Reluctance(S/R): Magnetic reluctance, or magnetic resistance is analogous to resistance in an electrical circuit. rather than dissipating electric energy it stores magnetic energy. A magnetic field causes magnetic flux to follow the path of least magnetic reluctance. Reluctance= m.m.f /flux Ampere-turns/ weber (A-t/ Wb ) Unit: inverse henries (H -1 )

21. Permeability( μ ): In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. Unit: henries /meter (H·m −1 ) The permeability constant (μ )/ the magnetic constant/permeability of free space, is a measure of the amount of resistance encountered when forming a magnetic field in a vacuum. Permeability of any material( μ ) is given by H.m -1 μ r – relative permeability of the medium

22. Magnetic Field strength (H): Magnetic field strength at any point within a magnetic field is numerically equal to the force experienced by a N-pole of one weber placed at that point. Unit – N/ Wb , A-t/m Flux density B= μ H Wb /m 2 Flux density developed in vacuum, B = μ H Relative permeability Relative permeability of a material is equal to the ratio of the flux density produced in that material to the flux density produced in vacuum by the same magnetising force.

23. Magnetic circuit Reluctance(R/S)= m.m.f =

24. Series magnetic circuit

25. μ i =relative permeability of iron

26. Series -parallel magnetic circuits

27. Magnetic circuit

28. Force on a current carrying conductor placed in a magnetic field A conductor carrying a current can produce a force on a magnet situated in the vicinity of the conductor. 2. By Newton’s third law of motion, namely that to every force there must be an equal and opposite force, it follows that the magnet must exert an equal force on the conductor.

29. Fleming’s left hand rule The rule can be summarized as follows: Hold the thumb, first finger and second finger of the left hand as shown in figure , whereby they are mutually at right angles. 2. Point the F irst finger in the F ield direction . 3. Point the se C ond finger in the C urrent direction. 4. The thu M b then indicates the direction of the M echanical force exerted by the conductor. Cause Effect This law is Used in motors

30. Force (F) depends on (1) Magnetic flux density (2) Current through the conductor F= force on the conductor ( newtons ) B=magnetic flux density ( teslas ) = length of the conductor (meters) = current through the conductor ( ampers ) θ = angle between and If θ =90 o then,

31. Flemings left hand rule

32. Relation between magnetism and electricity After the discovery that electricity produces a magnetic field, scientist began to search for the converse phenomenon from about 1821. How to convert magnetism into electricity? Michael Faraday succeeded in producing by converting magnetism. In 1831, he formulated basic laws underlying the phenomenon of electromagnetic induction “Faraday’s Laws of electromagnetic induction”

33. Faraday’s experiment Experiment 1 Experiment 2 Fig (a) Fig (b)

34. In 1831, Michael Faraday made the great discovery of electromagnetic induction, namely a method of obtaining an electric current with the aid of magnetic flux. He wound two coils, A and C, on a steel ring R, as in Fig. (a) and found that, when switch S was closed, a deflection was obtained on galvanometer G, and that, when S was opened, G was deflected in the reverse direction. A few weeks later he found that, when a permanent magnet NS was moved relative to a coil C (Fig. b), galvanometer G was deflected in one direction when the magnet was moved towards the coil and in the reverse direction when the magnet was withdrawn.

35. It was this experiment that finally convinced Faraday that an electric current could be produced by the movement of magnetic flux relative to a coil. Faraday also showed that the magnitude of the induced e.m.f . is proportional to the rate at which the magnetic flux passed through the coil is varied. Alternatively, we can say that, when a conductor cuts or is cut by magnetic flux, an e.m.f . is generated in the conductor and the magnitude of the generated e.m.f . is proportional to the rate at which the conductor cuts or is cut by the magnetic flux.

36. Statically and dynamically induced E.M.F. Statically induced e.m.f : Conductors or the coil remains stationary and flux linked with it is changed. Dynamically induced e.m.f : Field is stationary and conductors cut across the field.

37. Faradays laws of electromagnetic induction First law: It states that, whenever the magnetic flux linked with a circuit changes, an e.m.f is always induced in it. (or) Whenever a conductor cuts magnetic flux, an e.m.f is induced in that conductor

38. Second law: When the magnetic flux linking a conductor is changing, an e.m.f is induced whose magnitude is proportional to the rate of change of flux- linkages. N- number of turns in the coil C The flux through the coil Changes from an initial value of Φ 1 webers to the final value of Φ 2 webers in time ‘t’ seconds. Initial flux linkages= NΦ 1 Final flux linkages= NΦ 2 induced e.m.f (e) is, (statically induced e.m.f )

39. Putting the above expression in its differential form,

40. Dynamically induced e.m.f Induced e.m.f (e),

41. What is the direction of the induced e.m.f or current produced by induced e.m.f ? We have 2 methods to find this. Fleming right hand rule Lenz’s Law

42. Fleming’s right hand rule F ield or F lux - F irst finger M otion of the conductor relative to the flux- Thu M b finger Induced E .m.f . - S E cond finger. cause effect This principle is used in generators

43. Lenz’s Law In 1834 Heinrich Lenz, a German physicist, intoduced this law. The direction of an induced e.m.f . is always such that it tends to set up a current opposing the motion or the change of flux responsible for inducing that e.m.f .

44. Inductance Inductance is the property of an electric circuit which opposes any sudden change in current. Although a straight conductor possesses inductance, the property is most marked in a coil

45. Self inductance(L) Method 1 Method 2 Method 3

46. Self and mutual inductance In electromagnetism and electronics, inductance is the property of a conductor by which a change in current in the conductor "induces" (creates) a voltage (electromotive force) in both the conductor itself (self-inductance) and in any nearby conductors (mutual inductance)

47. Method 1 Method 2 Method 3

48. Coefficient of couplings Ranges from(0-1)

49. Energy stored in magnetic field

50. Charging and discharging of inductor and time constant discharging charging

51. A text book of electrical technology (volume 1) by B.L.THERAJA. (S. chand publications) Electrical technology (Electrical fundamentals) volume 1 by SURINDER PAL BALI (Pearson ) Elements of electrical engineering by U.A PATEL (ATUL)