ENB205 Lecture Electrical and Computer Engineering
2
Magnetic Field Intensity (H): Magnetic Flux Density (B): (Wb/m )
(At/m)
Proportional to move charge (I) Magnetic field is defined in terms of Magnetic Flux Density (B), which
For a wire: at a distance r away loosely translates to how close the field lines are drawn. Magnetic
field lines:
Always complete loops (no start or end like electric field
For a coil: Of length l
lines)
Always go from North to South poles
Never intersect each other.
Permeability
A measure of how easily something is magnetised.
-7
Air is 0 = 4π*10 , everything else is measured
relative to free space.
Magnetic Flux Density
The magnetic flux density (In a wire)
created by the moving charge is
proportional to the current in the wire.
B= Magnetic Flux Density (T)
I=Current in Wire
μ0=Magnetic Permeability of free space
-7
(4π*10 T/m)
μr=Relative Magnetic Permeability
r=Radius from centre of wire
1 Week 1 Tuesday, 27 August 2013 ENB205 Lecture Electrical and Computer Engineering
Magnetic flux density (in a coil) can be
increased by ‘coiling’ conductors create
a solenoid.
N=Number of turns
I=Current
L=Length
Magnetic Flux Density (Loop)
applicable only at the centre of the
loop, and only for a single loop or wire.
Flux
Flux (Φ, wb Wei-ber) – If an imaginary surface is drawn,
which the flux lines intersect perpendicular to, the total
flux can be calculated.
∫
If the flux density inside this area is uniform, the total
flux can be simplified to be:
Faradays Law
If the imaginary surface is bounded by a conductor, then
as the flux changes in the area, a voltage is induced.
Lenz’s Law
The current induced in a closed loop due to changing magnetic flux, is such that is opposes the change in flux. If the
flux is decreasing, current will flow in the direction which is constructive to the magnetic field. If the flux is
increasing, current will flow in the direction which is destructive the magnetic field.
2 Week 1 Tuesday, 27 August 2013 ENB205 Lecture Electrical and Computer Engineering
Flux Linkage
For more than one loop, the induced voltage is
summative such that (EMF= e = Electro magnetic field)
The multiplication of turns and flux is called the ‘flux
linkage’(λ) *Wb.t wei-ber-turn]
Induced voltage may be expressed in flux linkage ->
Absolute e =|e| = [V volts]
Voltage induced on a moving conductor
Voltage is also induced in a conductor as it moves
through a magnetic field.
B = Magnetic field density (measured in T – Teslas)
V = Velocity of conductor
L = Length of conductor in magnetic field
α= angle between direction of field and direction of
conductor movement
Magnetic Dipoles
The electron spin, creates a dipole, having both North
and South poles. One pole cannot exist without the
other. A net magnetic field is created when the un-
paired electrons in a material are spinning in the same
plane and direction.
In a strong magn

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