Unit 1 - Principles of Electromagentism and Electromechanical Conversion

Question 1

What is magnetic flux?

Answer 1

The quanitity of magnetic field

Question 2

What is the symbol of magnetic flux?

Answer 2

ϕ

Question 3

What is the unit of magnetic flux?

Answer 3

Weber (Wb)

Question 4

What are 3 equations for magnetic flux?

Answer 4

> ϕ = BA

> ϕ = μ_rμ₀HA

> ϕ = μ_rμ₀A × N ∙ I / 2πr

Question 5

What is the units for magneitc field intensity?

Answer 5

A/m

Question 6

What is the symbol of magnetic flux intensity?

Answer 6

H

Question 7

How can we determine the direction of the magnetic field?

Answer 7

Right-hand grip rule

> Thumb = Current

> Fingers = Direction of the magnetic field

Question 8

What is the equation to calculate the magnetic field intensity?

Answer 8

H = N ∙ I / 2πr

> N = Number of conductors

> I = Current

> r = Distance from the conductor

Question 9

What is the flux density?

Answer 9

The total flux passing through a unit of area normal to field lines

Question 10

What is the symbol of flux density?

Answer 10

B

Question 11

What are the 3 equations for magnetic flux density?

Answer 11

> B = μ_rμ₀H

> B = ϕ / A

> B = μ_rμ₀ × N ∙ I / 2πr

Question 12

What is μ₀? What is its value and unit?

Answer 12

The permiability of free space. μ₀ = 4π × 10^-7 H/m

Question 13

What is μ_r? What is it based on and what are rough range of values?

Answer 13

Reactive permiability based on the type of material in between the conductor and the area.

2000 < μ_r < 80,000

Question 14

What is Lorentz force?

Answer 14

This is a force exerted between two conductors each producing a magnetic field due to the interaction of the magnetic fields

Question 15

What is the units of Lorentz force?

Answer 15

Newtons (N)

Question 16

How can we determine the direction of Lorentz force?

Answer 16

Left hand rule

> Thumb (Motion / thrust)

> First finger (Field)

> Second finger (Current)

Question 17

What is the equation for Lorentz force? What is required for this to be accurate?

Answer 17

F = N_AB_FI_Al

> N_A = number of conudctors in group A

> B_F = Magnetic flux density from group F

> I_A = 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

Question 18

What is the equation of Lorentz force borken down when all components are perpendicular? How is it simplified?

Answer 18

F = μ_rμ₀ × N_AN_Fl / 2πr × I_AI_F

Simplified because μ₀ = 4π × 10^-7 H/m

F = μ_r × 2 × 10^-7 × N_AN_Fl / r × I_AI_F

Question 19

What is flux linkage?

Answer 19

When there are N number of turns in a coil, ψ = Nϕ is considered as the total flux or the flux linkage

Question 20

What is the symbol of flux linkage?

Answer 20

ψ

Question 21

What is electromotice force?

Answer 21

This is the induce voltage on a coil of wire from a magnetic field

Question 22

What is the symbol of electromotive force?

Answer 22

e or e.m.f

Question 23

What is the unit of electromotive force?

Answer 23

Voltage (V)

Question 24

What is faradays induction law?

Answer 24

Faraday’s induction law states that the electromotive force (e.m.f) induced across the coil linked to the magnetic flux induced.

Question 25

What is the equation for EMF?

Answer 25

e = dψ/dt e = N dϕ/dt

Question 26

What is induced current?

Answer 26

This is the current that is induced in a conductor by a magnetic field.

Question 27

What is the unit of induced current?

Answer 27

Amps (A)

Question 28

What is the symbol of induced current?

Answer 28

I

Question 29

What are the 2 equations of induced current?

Answer 29

\> I_A = ψ/L \> I_A = Nϕ/L

Question 30

What is the process of motional electromotive force?

Answer 30

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]

Question 31

What is the equation for faradays law?

Answer 31

e = N dϕ/dt

Question 32

What are some (3) equations for the process of motional electromotive force?

Answer 32

e = N_AB_Fl × dx/dt e = N_AB_F × dA/dt e = N_AB_Flv

Question 33

How can the speed of the conductor through the magnetic field be calculated?

Answer 33

v = dx/dt

Question 34

How can the change in the area when a conductor passes through a magnetic field be calculated?

Answer 34

l × dx = dA

Question 35

What is the key assumption made when we talk about the process of motional electromotive force?

Answer 35

Only applied when the motion is perpendicular to the current which is perpendicular to the field

Question 36

How are atoms within a metal affected by the magnetic field?

Answer 36

\> 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

Question 37

What is the relationship between the magnetic field intensity and magnetic flux tensity?

Answer 37

\> 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]

Question 38

What is the hysteresis relationship between the magnetic field intensity and magnetic flux tensity?

Answer 38

\> 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

Question 39

What are eddy currents?

Answer 39

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.

Question 40

What is the issue with eddy currents?

Answer 40

They produces losses

Question 41

How can eddy currents be reduced?

Answer 41

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