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Electromagnetism..

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Electromagnetism ppt according to GTU syllabus.

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Electromagnetism..

  1. 1. GEC DAHOD MECHANICAL DEPT. EVEN-SEM 2017 B.E. 1st YEAR (C) SUBJECT:- E.E.E.(2110005) Topic:- “Electromagnetism” Prepared by:- Biniwale Suraj
  2. 2. Contents:  Introduction  History  Laws of Magnetic Force  Magnetic Field  Direction of Magnetic Field  Magnetic Effect of Electric Current  Electromagnetic Induction  Faraday’s Laws of Electromagnetic Induction  Induced E.M.F.  Magnetic Hysteresis
  3. 3. Introduction  Electromagnetism is the key to the operation of great part of electrical apparatus found in industry as well as home. Thus, today magnetism has attained a place of a pride in electrical engineering.  Without the aid of magnetism, it is impossible to operate such a device as electrical generators, motors transformers etc.  Electricity and magnetism are different facets of Electromagnetism.  Without electromagnetism very few of our modern devices are possible.
  4. 4. History  The origin of electricity is and magnetism sprang from ancient men’s curiosity over the ability of two materials, amber and lodestone to attract other materials.  With the publication of James Clerk Maxwell's 1873 Treatise on Electricity and Magnetism in which the interactions of positive and negative charges were shown to be regulated by one force.  An electric current in a wire creates a circular magnetic field around the wire, its direction depending on that of the current.  In early 19th century faraday discover electromagnetic induction.
  5. 5. Laws of magnetic force  First law: Like poles repel each other and unlike poles attract each other.  Second law: The magnetic forces between two isolated magnetic poles placed in a medium is directly proportional to the product of the pole strength and inversely proportional to square of the distance between them. F α m1 m2 Or F = K m1 m2 d2 d2 Where K is a constant whose value depends upon the surrounding medium. K = 1 Where μ0 = Absolute permeability 4𝝅μ0 μr = Relative permeability
  6. 6. Magnetic Field  The magnets exert their influence in the surroundings, this is called magnet field.  Magnetic field is a quantity that has both direction and magnitude.  The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it.
  7. 7. Direction of Magnetic Field  (1) Right hand-palm rule : “First make the thumb and fingers of the right hand perpendicular to each other and put the thumb along the wire in the direction of the current and fingers point towards the point of observation”.
  8. 8.  (2) Right hand-thumb rule : “If the thumb is along the direction of current, wrapped fingers will show the direction of circular magnetic field lines.” (3) Fleming Left-hand rule : “Hold out your left hand with forefinger, second finger and thumb at right angle to one another. If the fore finger represents the direction of the field and the second finger that of the current, then thumb gives the direction of the force.”
  9. 9.  (4) Right hand rule : “Hold out the right hand with the first finger, second and thumb at right angle to each other. If finger represent the direction of the line of force, the thumb points in the direction of motion or applied force, then second finger point in the direction of the induced current.”
  10. 10. Magnetic Effect of Electric current  When a conductor carries a current, magnetic lines of force are set around the length of the conductor.  Magnetic field produced by the current flowing in the conductor.  Magnetic lines of force in the form of concentric circle around the conductor.  The direction of the line of force depends upon the direction of the current
  11. 11. Electromagnetic Induction  When the magnetic flux linking a conductor changes, an e.m.f. is produced in the conductor. If the conductor forms a closed circuit, a current will flow in it. This phenomenon is known as Electromagnetic induction.
  12. 12. Faraday's Laws of Electromagnetic Induction Faraday's First Law: “Any change in the magnetic field of a coil of wire will cause an emf to be induced in the coil. This emf induced is called induced emf and if the conductor circuit is closed, the current will also circulate through the circuit and this current is called induced current. Method to change magnetic field.” (1)By moving a magnet towards or away from the coil (2)By moving the coil into or out of the magnetic field. (3)By changing the area of a coil placed in the magnetic field (4)By rotating the coil relative to the magnet.
  13. 13. Faraday's Second Law: “It states that the magnitude of emf induced in the coil is equal to the rate of change of flux that linkages with the coil. The flux linkage of the coil is the product of number of turns in the coil and flux associated with the coil.” Consider a magnet approaching towards a coil. Here we consider two instants at time T1 and time T2. Flux linkage with the coil at time, T1 = NΦ1 Wb Flux linkage with the coil at time, T2 = NΦ2 wb Change in flux linkage = N(Φ2 - Φ1) Let this change in flux linkage be, Φ = Φ2 - Φ1 So, the Change in flux linkage = NΦ Now the rate of change of flux linkage = NΦ / t Take derivative on right hand side we will get The rate of change of flux linkage = NdΦ/dt But according to Faraday's law of electromagnetic induction, the rate of change of flux linkage is equal to induced emf. Considering Lenz's Law.
  14. 14. Lenz’s law :  “The direction of current induced in a conductor by a changing magnetic field due to Faraday's law of induction will be such that it will create a field that opposes the change that produced it.” Lenz's law is shown by the negative sign in Faraday's law of induction:
  15. 15. Induced e.m.f.  When a moving charge cuts through the flux lines of a magnetic field, it experiences a force given by F = qv×B. To see what occurs, consider Fig. where the crosses represent a flux field B directed away from the reader. When conducting bar ab is moving to the right with velocity v, then every charge within the bar is moving to the right, cutting past flux lines, with a velocity v. The consequence of this is that a force of F = qv×B is exerted on every charge within the bar where the direction of the vector F is along the bar, directed from b to a. This force amounts to an emf within the bar, tending to produce a current from b to a. If the bar is part of a circuit, or connected to a galvanometer as in Fig. 1 above, that emf will cause a current to flow. If the bar is not part of a circuit, then what will happen is that the free electrons in the bar will all move toward end b, making end b negative and end a positive.
  16. 16.  Emf induced in a moving conductor. The emf induced in a straight conductor of length l moving with velocity v perpendicular to a magnetic field B is 1) E = Blv  where B, l and v are mutually perpendicular. The emf is in volts when B is in webers/m2, l is in meters, and v is in m/sec.  If the velocity vector v makes an angle θ with the direction of the magnetic field, 1) becomes 2) E = Blv sin θ
  17. 17. Magnetic Hysteresis  Magnetic hysteresis is an important phenomenon and refers to the irreversibility of the magnetisation and demagnetisation process. When a material shows a degree of irreversibility it is known as hysteretic. We will now explore the physics behind ferromagnetic hysteresis.  When a demagnetised ferromagnetic material is placed in an applied magnetic field the domain that has a direction closest to that of the applied field grows at the expense of the other domains. Such growth occurs by motion of the domain walls. Initially domain wall motion is reversible, and if the applied field is removed the magnetisation will return to the initial demagnetised state. In this region the magnetisation curve is reversible and therefore does not show hysteresis.  The crystal will contain imperfections, which the domain boundaries encounter during their movement. These imperfections have an associated magnetostatic energy. When a domain wall intersects the crystal imperfection this magnetostatic energy can be eliminated as closure domains form. This pins the domain wall to the imperfection, as it is a local energy minima.
  18. 18.  The applied magnetic field provides the energy to allow the domain wall to move past the crystal imperfection, but the domains of closure cling to the imperfection forming spike-like domains that stretch as the domain wall moves further away. Eventually these spike domains snap off and the domain wall can move freely. As the spike domains snap off there is a discontinuous jump in the boundary leading to a sharp change in the magnetic flux, which can be detected by winding a coil around the specimen connected to a speaker. In doing so, crackling noises are heard corresponding to the spike domains breaking away from the domain walls. This phenomenon is known as the Barkhausen effect.  Eventually all the domain walls will have been eliminated leaving a single domain with its magnetic dipole moment pointing along the easy axis closest to the direction of the applied magnetic field. Further increase in magnetisation can occur by this domain rotating away from the easy direction to an orientation parallel to that of the externally applied field. The magnetisation of the material at this stage is called the saturation magnetisation (see Figure J) . The ease of this final rotation is dependent on the magnetocrystalline energy of the material; some materials require a large field to reach this saturation magnetisation.
  19. 19.  If the external applied field is removed the single domain will rotate back to the easy direction in the crystal. A demagnetising field will be set up due to the single domain, and this field initiates the formation of reverse magnetic domains as these will lower the magnetostatic energy of the sample by reducing the demagnetising field. However the demagnetising field is not strong enough for the domain walls to be able to grow past crystal defects so they can never fully reverse back to their original positions when there is no external applied field. This results in the hysteresis curve as some magnetisation will remain when there is no external applied field. This magnetisation is called the remanent magnetisation, Br. The field required to reduce the magnetisation of the sample to zero is called the coercive field Hc. And the saturation magnetisation Bs is the magnetisation when all the domains are aligned parallel to the external field. These are shown on the schematic below:
  20. 20. Reference :  Electrical Technology Volume 1 (B.L. Theraja, A.K. Theraja)  A. Beiser(1987) Concepts of modern physics (4th ed.) McGraw-Hill (International)  University of Cambridge (TLP library ferromagnetic materials-hysteresis)
  21. 21. THANK YOU!

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