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2. Electromagnetism and

2. Electromagnetism and. electromagnetic induction. 2.1 Magnetic flux 2.2 Flux density 2.3 mmf 2.4 Field strength (H) & Permeability 2.5 B/H curves 2.6 Faraday’s Law 2.7 Faraday‘s law of Electromagnetic Induction 2.8 Lenz’s Law

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2. Electromagnetism and

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  1. 2. Electromagnetism and electromagnetic induction • 2.1 Magnetic flux • 2.2 Flux density • 2.3 mmf • 2.4 Field strength (H) & Permeability • 2.5 B/H curves • 2.6 Faraday’s Law • 2.7 Faraday‘s law of Electromagnetic Induction • 2.8 Lenz’s Law • 2.9 Fleming’s Left-hand and Right-hand Rules • 2.10 Self-inductance • 2.11 Energy stored

  2. 2.1 Magneticflux • Magnetic field is invisible-observed by using paper and iron fillings.

  3. Line of magnetic flux (lines of magnetic force). • North-seeking pole or N-pole points towards north. • South-seeking pole or S-pole points towards south.

  4. Magnetic field patterns : Unlike poles-Attraction Like poles-Repulsion

  5. Characteristicof lines of magnetic flux • Line of magnetic flux - direction, closed loop • Shorten lines of magnetic flux (unlike poles attract each other) • Parallel in same direction repel one another (like poles repel each other)

  6. Magnetic Flux • Number of lines of magnetic force coming out or entering into a magnetic surface. • Symbol:  Unit: weber (Wb)

  7. 2.2 Flux density • magnetic flux entering unit area normally (at right angle) is the magnetic flux density • Symbol: B Unit: tesla (T) • Flux density B= /A tesla (T)

  8. 2.3 mmf (magnetomotive force) • The force - produces the magnetic field • F = I*N ampere turn • Symbol: F Unit : AT

  9. 2.4 Field strength (H) & Permeability • Field Strength • magnetic field intensity - m.m.f. per unit length of the magnetic circuit. • Magnetic intensity = F/l = IN/l • Symbol: H Unit: ampere per metre • known as magnetizing force OR magnetic field strength

  10. Magnetic field is concentrated in the iron rod. • Brass rod has little effect on the magnetic field.

  11. Iron has a lower “magnetic resistance” than that of the surrounding air path. • Magnetic resistance of the brass has much the same value of the surround air. • Every material has particular value of conductivity known as magnetic permeability. • Flux Density = absolute permeability x magnetic field intensity • B = H • Symbol:  Unit: T/(A/m) OR H/m

  12. Permeability of Free Space • magnetic space constant - permeability of vacuum or of air. • symbol: o Unit: T/(A/m) Or H/mm • value: 4xx10-7 H/m • Relative and Absolute Permeability • Absolute permeability = o x relative permeability  = o x r • where r = relative permeability • symbol: r unit: none

  13. Ferromagnetic materials • r1 • e.g. iron, steel, nickel and cobalt • Paramagnetic materials • r 1 • i.e. material become weakly magnetized in the direction of the magnetising field. • e.g. aluminium, chromium

  14. Diamagnetic materials • r 1 • e.g. gold, silver.

  15. magnetic saturation - all domains aligned with the magnetic field. B Flux density Knee point –further increase of H causes very little change in magnetic field. Flux density increases rapidly due to alignment of a large number of magnetic domains Magnetic field intensity H Fig. 6 2.5 B/H curves • Magnetization characteristics of soft magnetic materials

  16. 2.6 Faraday’s Law

  17. an e.m.f. is induced in the conductor whenever it cuts line of magnetic flux. • Conductor move upwards or downwards - induced e.m.f. is zero. • Conductor’s velocity will affect the magnitude of the induced e.m.f.

  18. 2.7 Faraday’s law of Electromagnetic Induction • Magnetic field linking with the conductor changes - e.m.f. is induced. • Induced e.m.f.’s magnitude rate of change of the magnetic flux linking with the conductor.

  19. 2.8 Lenz’s Law • The e.m.f. induced - circulate current in a direction opposes the change of magnetic flux inducing the e.m.f.

  20. 2.9 Fleming’s left and Right- hand Rules Fleming’s right-hand rule

  21. Fleming’s left-hand rule

  22. 2.10 Self-inductance • Conductor carrying current is surrounded by magnetic field. • Conductor current  , magnetic field  . • By Lenz‘s Law,  in flux  e.m.f. to be induced in conductor opposes the change in flux (change in current). • This e.m.f. is Self-induced e.m.f. (self-inductance)

  23. Self-inductance of 1 henry (H) if - e.m.f. of 1 volt (V) is induced when current changes at 1 ampere per second (A/s).

  24. 2.11 Energy Stored • In a circuit of inductance L henrys. • Current increases at a uniform rate from zero to I amperes in t seconds. • Average current in the circuit is I/2 amperes, • Average value of induced e.m.f. is • L*(rate of change of current)=LI/t • The average energy consumed by the inductance is therefore: • W = Eit =

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