Unit 1 - Principles of Electromagentism and Electromechanical Conversion Flashcards
What is magnetic flux?
The quanitity of magnetic field
What is the symbol of magnetic flux?
ϕ
What is the unit of magnetic flux?
Weber (Wb)
What are 3 equations for magnetic flux?
> ϕ = BA
> ϕ = μrμ0HA
> ϕ = μrμ0A × N ∙ I / 2πr
What is the units for magneitc field intensity?
A/m
What is the symbol of magnetic flux intensity?
H
How can we determine the direction of the magnetic field?
Right-hand grip rule
> Thumb = Current
> Fingers = Direction of the magnetic field
What is the equation to calculate the magnetic field intensity?
H = N ∙ I / 2πr
> N = Number of conductors
> I = Current
> r = Distance from the conductor
What is the flux density?
The total flux passing through a unit of area normal to field lines
What is the symbol of flux density?
B
What are the 3 equations for magnetic flux density?
> B = μrμ0H
> B = ϕ / A
> B = μrμ0 × N ∙ I / 2πr
What is μ0? What is its value and unit?
The permiability of free space. μ0 = 4π × 10-7 H/m
What is μr? What is it based on and what are rough range of values?
Reactive permiability based on the type of material in between the conductor and the area.
2000 < μr < 80,000
What is Lorentz force?
This is a force exerted between two conductors each producing a magnetic field due to the interaction of the magnetic fields
What is the units of Lorentz force?
Newtons (N)
How can we determine the direction of Lorentz force?
Left hand rule
> Thumb (Motion / thrust)
> First finger (Field)
> Second finger (Current)
What is the equation for Lorentz force? What is required for this to be accurate?
F = NABFIAl
> NA = number of conudctors in group A
> BF = Magnetic flux density from group F
> IA = Current through a single conductor in group A
> l = length of conductor that experiences the force This is only accurate when all components (current, field and force) are perpendicular to each other
What is the equation of Lorentz force borken down when all components are perpendicular? How is it simplified?
F = μrμ0 × NANFl / 2πr × IAIF
Simplified because μ0 = 4π × 10-7 H/m
F = μr × 2 × 10-7 × NANFl / r × IAIF
What is flux linkage?
When there are N number of turns in a coil, ψ = Nϕ is considered as the total flux or the flux linkage
What is the symbol of flux linkage?
ψ
What is electromotice force?
This is the induce voltage on a coil of wire from a magnetic field
What is the symbol of electromotive force?
e or e.m.f
What is the unit of electromotive force?
Voltage (V)
What is faradays induction law?
Faraday’s induction law states that the electromotive force (e.m.f) induced across the coil linked to the magnetic flux induced.
What is the equation for EMF?
e = dψ/dt
e = N dϕ/dt
What is induced current?
This is the current that is induced in a conductor by a magnetic field.
What is the unit of induced current?
Amps (A)
What is the symbol of induced current?
I
What are the 2 equations of induced current?
> IA = ψ/L
> IA = Nϕ/L
What is the process of motional electromotive force?
The conductor (A) moves though a magnetic field with a change in distance of dx in a change of time dt at a velocity of v.
There is a change in flux and so EMF will be induced
[Picture 1]
What is the equation for faradays law?
e = N dϕ/dt
What are some (3) equations for the process of motional electromotive force?
e = NABFl × dx/dt
e = NABF × dA/dt
e = NABFlv
How can the speed of the conductor through the magnetic field be calculated?
v = dx/dt
How can the change in the area when a conductor passes through a magnetic field be calculated?
l × dx = dA
What is the key assumption made when we talk about the process of motional electromotive force?
Only applied when the motion is perpendicular to the current which is perpendicular to the field
How are atoms within a metal affected by the magnetic field?
> In each domain within a metal, atoms establish themselves a flux with a certain direction
> When there are no external magnetic fields the directions of these fluxes are random
> When a magnetic field is applied they alligh with the field
> When the magnetic field increases then the magnetic flux density increases
What is the relationship between the magnetic field intensity and magnetic flux tensity?
> Initially the relationship is linear. B increases with H > When all domains are alligned with the field B does not increase with H (Flux saturation)
[Picture 2]
What is the hysteresis relationship between the magnetic field intensity and magnetic flux tensity?
> After being saturated, when the magnetic field is reduced it does not return to its original state so requires some reverse magnetic field to return it back to its original state.
> With enough reverse magnetic field then all domains will be saturdated in the reverse time
What are eddy currents?
Within a metal core, when magnetic field is applied, an EMF is induced. The current flows around the core and this is called eddy currents.
What is the issue with eddy currents?
They produces losses
How can eddy currents be reduced?
The metal is sheet laminated instead of using a single core because this reduces the eddy currents because the sheets are thinner and smaller so the resistance is higher and the induced current will be smaller.
This reduces losses
