Fundamentals of Electrical Engineering

Electromagnetism

Paramjeet Singh Jamwal
B.Tech, EEE, College Of Engineering Roorkee
  • Anybody which possesses the power of attaracting piececs of iron called a magnet. The property by virtue of which this attaraction takes place is called magnetism. The iron ore like magnetite, lode stone are called natural magnets.
  • Artificial magnets are made artificially from iron, steel or alloy material. They can be prepared either by rubbing iron bars with a magnet OR By passing an electric current through the wire wound round of iron piece.
  • Artificial magnets are electromagnets and the phenonmenon is called electromagnetism. Magnets used in electrical machines and equipments are of artificial type and generally are horse shoe or U shaped.
  • Permanent magnets are prepared from hardened steel and certain alloys of Nickel and Cobalt. Alniko is a widely used alloy for making permanent magnets. Alniko extensively used in electrical instruments, earphones, loud speaker, telephone receivers, small dc motors etc.
  • Temporary magnets are prepared from soft iron or nickel. Temporary magnets are of more importance than permanent magnets. Temporary magnets have wide applications in the field of electrical generators, motors, relays etc.
  • The total number of lines of force in the magnetic field is called the magnetic flux (Ф in Weber). The concept of these flux lines are purely imagenary and was introduced by Faraday. It is a pictorical method of representing the distribution and density of a magnetic field.
  • Magnetic flux density is defined as the magnetic flux per unit area of surface at right angles to the magnetic field. Magnetic flux density is also known as Magnetic Induction (B, Wb/m2 or Tesla).
  • Measure of the degree to which the lines of force of magnetizing field can penetrate of permeate the medium is called the absolute permeability of the medium (µ). The permeability of all non-magnetic materials including air is represented by µ = 4π x 10^-7 H/m.
  • At any point in a magnetic field, field strength or field intensity H is the force maintaining the magnetic flux  and producing a particular value of flux density B at that point. Hence the field intensity H is the cause and the flux density B is the effect.
  • Ampere’s circuit law obtain the relationship between current and field intensity H.
H = NI / 2πr
  • Biot-Savart’s Law
dH = (1/4π).(I dl Sinθ/r^2)
  • Magnetic field due to an infinite linear conductor
H = I / 2πr
  • Field Strength due to circular loop
Bz = µIR^2 / 2(R^2+z^2)^3/2
  • At the center of the loop z = 0
B = µI/2R
  • A cylindrical coil closely wound with a large number of turns of insulated wire is called solenoid. The magnetic filed produced by the solenoid resemble more or less that of bar magnet.
  • Flux Density at the center of solenoid
Bc = µNI/(4R^2+l^2)^1/2
  • Flux density at the one end of solenoid
Be = µNI/2(R^2+l^2)^1/2
  • For any elemental current element of length dl, the force experienced is given by
dF = BIdl Sinθ
  • Electrical motors producing mechanical power, works basically on Flemming’s left hand principle. As such, the above relationship is very useful in the study electrical engineering.
Basic Electrical Engineering | V N Mittle & Arvind Mittal
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