Magnets and magnetism


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Magnets, magnetism, Laws of Magnetism, Faraday's Laws, Biot-Savart Law, Ampere's Law, Lenz's Law

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Magnets and magnetism

  1. 1. Basics of Electrical Engineering  Magnets and Magnetism  Laws of Magnetism By Ms. Nishkam Dhiman Assistant Professor -EEE Deptt. Chitkara Institute of Engg. & Technology
  2. 2. Magnet  A magnet is a material or object that produces magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on her ferromagnetic materials, such as iron, and attracts or repels other magnets.
  3. 3. Magnetic Domains  Iron and other ferromagnetic substances, though, are different. Their atomic makeup is such that smaller groups of atoms band together into areas called domains, in which all the electrons have the same magnetic orientation. Inside each of these domains, the electrons are oriented in the same direction. But the iron is not magnetic because these domains themselves are not aligned.  If you put that piece of iron within a magnetic field, though, the domains all begin to point in the same direction. The result? A magnet! A permanent magnet is nothing more than such an object in which all the domains are aligned in the same direction.
  4. 4. Types of Magnets  Permanent magnets: are magnets that retain their magnetism once magnetized. Ferromagnetic material which include iron, nickel, cobalt, some alloys of rare earth metals, and some naturally occurring minerals such as lodestone.  Temporary magnets: are materials magnets that perform like permanent magnets when in the presence of a magnetic field, but lose magnetism when not in a magnetic field. Example : Electromagnets.  Electromagnets: are wound coils of wire that function as magnets when an electrical current is passed through. By adjusting the strength and direction of the current, the strength of the magnet is also altered. Often, the coil is wrapped around a core of "soft" ferromagnetic material such as steel.
  5. 5. Properties of Magnets 1. Magnets attract objects of iron, cobalt and nickel. 2. The force of attraction of a magnet is greater at its poles than in the middle. 3. Like poles of two magnets repel each other. 4. Opposite poles of two magnets attracts each other. 5. If a bar magnet is suspended by a thread and if it is free to rotate, its South Pole will move towards the North Pole of the earth and vice versa.
  6. 6. Magnetic Dipoles  Every magnet is a magnetic dipole.  If a magnetic piece of steel rod is cut into smaller pieces, each piece is a magnet with a N or a S pole.  Therefore a magnet can be said to be made of lots of "tiny" magnets all lined up with their N poles pointing in the same direction. At the ends, the "free" poles of the "tiny" magnets repel each other and fan out so the poles of the magnet are round the ends.  Magnetic Monopole does not exists
  7. 7. Magnetic Field  A magnetic field is the area around a permanent magnet or a wire carrying a current in which a force is experienced.  Also called magnetic flux density  Units : Weber/meter sq or Tesla  The direction of the field at any point should be the direction of the force on a N pole.  The direction is shown by arrows - these point away from N pole towards S pole.
  8. 8. Magnetic Flux  Magnetic flux can be defined as a measure of magnetic field in a certain medium. In simple terms, if the magnetic field had to pass through a certain medium, it will always travel as "flux" lines (flux lines are imaginary, but continuous lines traveling from north pole of a magnet to its south pole). Units are Weber.
  9. 9. Permanent Magnet and Electromagnet  A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. As the name suggests, a permanent magnet is 'permanent'. This means that it always has a magnetic field and will display a magnetic behavior at all times.  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.
  10. 10. Permanent Magnet Vs Electromagnet  1.Properties: Permanent magnet has persistence magnetic field. An electromagnetic magnet only displays magnetic properties when an electric current is applied to it  2. Magnetic Strength: Permanent magnet magnetic strength depends upon the material used in its creation. The strength of an electromagnet can be adjusted by the amount of electric current allowed to flow into it  3. Advantages : 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. However, an electromagnet’s magnetic field can be rapidly manipulated over a wide range by controlling the amount of electric current supplied to the electromagnet.
  11. 11. Laws of Electromagnetism: There are four laws of electromagnetism:  The law of Biot-Savart: magnetic field generated by currents in wires  Ampere's law : the effect of a current on a loop of flux which it threads  Force law : the force on an electron moving through a magnetic field  Faraday's law: the voltage induced in a circuit by magnetic flux cutting it
  12. 12. Biot Savart Law  The Biot–Savart law is used for computing the resultant magnetic field B at position r generated by a steady current I (for example due to a wire): a continual flow of charges which is constant in time and the charge neither accumulates nor depletes at any point.Units Weber/m-sq
  13. 13. Ampere’s Law  Ampere's Law states that for any closed loop path, the sum of the length elements times the magnetic field in the direction of the length element is equal to the permeability times the electric current enclosed in the loop.  Ampere's Law: ΣB|| Δl = μ0 I
  14. 14. Force Law: Force Due to Magnetic Field F = qv B  F = force (vector)  q = charge on the particle (scalar)  v = velocity of the particle relative to field (vector)  B = magnetic field (vector)  The magnitude of a cross is the product of the magnitudes of the vectors times the sine of the angle between them. So, the magnitude of the magnetic force is given by  F = qvBsin where is angle between q v and B vectors
  15. 15. . The direction of the force is given by the right hand rule
  16. 16. Faraday's law 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 toward or away from the coil 2. by moving the coil into or out of the magnetic field. 3. by changing area of a coil placed in the magnetic field 4. by rotating the coil relative to the magnet.
  17. 17. Faraday's Second Law  It states that the magnitude of emf induced in the coil is equal to the rate of change of flux linkages with the coil. The flux linkages of the coil is the product of number of turns in the coil and flux associated with the coil. Flux Φ in Wb = B.A B = magnetic field strength A = area of the coil Generator action (RHR)
  18. 18. LENZ'S LAW  Lenz law states that when an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it.  If the magnetic flux Ф linking a coil increases, the direction of current in the coil will be such that it oppose the increase in flux and hence the induced current will produce its flux in a direction as shown below (using right hand thumb rule).  If magnetic flux Ф linking a coil is decreasing, the flux produced by the current in the coil is such that it aid the main flux and hence the direction of current is as shown below.
  19. 19. Applications of Faradays Laws  Electrical Transformers  Electrical Generators  Induction Cookers  Musical Instruments
  20. 20. Solenoids  Linear solenoids are electromechanical devices which convert electrical energy into a linear mechanical motion used to move an external load a specified distance. It consists of  Cylindrical coil  A steel or iron frame  A steel or iron plunger  A stationary magnetic pole/ travel stop
  21. 21. Working of Solenoid  Current flow through the solenoid coil winding creates a magnetic field which produces an attraction between a movable plunger and a fixed stop.  When electrical power is applied, the solenoid’s plunger and its external load accelerates and moves toward the solenoid’s stop until an impact occurs. The plunger rides inside the core of the coil assembly.  Removal of power from the solenoid eliminates the current flow through the coil , the plunger with external
  22. 22.  APPLICATIONS OF SOLENOIDS  Electronically activated door locks, pneumatic or hydraulic control valves, robotics, automotive engine management, irrigation valves to water the garden and even the "DingDong" door bell has one