1 2. Electromagnetism and b b 2.1 Magnetic flux b b 2.2 Flux density b b 2.3 mmf b b 2.4 Field...
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Transcript of 1 2. Electromagnetism and b b 2.1 Magnetic flux b b 2.2 Flux density b b 2.3 mmf b b 2.4 Field...
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2. Electromagnetism and
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
electromagnetic induction
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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.
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Characteristic of 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)
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Magnetic Flux
Number of lines of magnetic force coming out or entering into a magnetic surface.
Symbol: Unit: weber (Wb)
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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)
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2.3 mmf (magnetomotive force)
The force - produces the magnetic field F = I*N ampere turn Symbol: FUnit : AT
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2.4 Field strength (H) &
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
Permeability
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• Magnetic field is concentrated in the iron rod.• Brass rod has little effect on the magnetic field.
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• 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
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Permeability of Free Space • magnetic space constant - permeability of
vacuum or of air.
• symbol: o Unit: T/(A/m) Or H/mm
• value: 4xx10-7 H/m Relative and Absolute Permeability
• Absolute permeability = o x relative permeability = o x r
• where r = relative permeability
• symbol: r unit: none
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Ferromagnetic materialsr1
• 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
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2.5 B/H curves Magnetization characteristics of soft
magnetic materials
Flux density increases rapidly due to alignment of a large number of magnetic domains
Knee point –further increase of H causes very little change in magnetic field.
magnetic saturation - all domains aligned with the magnetic field.
Magnetic field intensity H
B F
lux density
Fig. 6
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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.
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2.7 Faraday’s law of
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.
Electromagnetic Induction
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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.
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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)
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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).
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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 =