2nd Year Class Test

Question 1

What direction do magnetic fields take?

Answer 1

N to S

Question 2

What is the right hand screw rule for?

Answer 2

shows direction of magnetic field around a coil. Thumb indicated current flow.

Question 3

What is Flux Density?

Answer 3

flux/Cross section Area
B = Φ/A
Measured in Teslas

Question 4

What is flux?

Answer 4

Φ

Measured in Webbers (Wb)

Question 5

What is relative permeability?

Answer 5

ratio of permeability / permeability of free space.

Permeability easured in (Wb/Am)

Question 6

What is permeability of free space?

Answer 6

4pi*10^-7 (Wb/Am)

Question 7

What is reluctance?

Answer 7

(Fancy R)= length / perm*Area (At/Wb)

Question 8

What is MMF?

Answer 8

Magnetomotive Force

Fancy F) = Φ(fancyR
MMF = flux * Reluctance

Question 9

MMF =

Answer 9

N*I

This can be subbed back into the equation relating MMF Reluctance and Flux

Question 10

What is the field intensity?

Answer 10

H = MMF / length

At/m

Question 11

How is Field intensity (magnetising force) and flux density related?

Answer 11

B = perm * H (T)

Question 12

What is hysteresis?

Answer 12

a non linear relationship which follows different paths depending on direction. This is for B and H relationship.

Question 13

What is the left hand Rule?

Answer 13

Thumb = FORCE
First = FIELD
Second = CURRENT

Question 14

What is the equation for the thrust force in a field?

Answer 14

F = BIL

Question 15

What is the equation in a linear generator?

Answer 15

emf=BLv
(v=velocity)

also:
emf = dΦ/dt

Question 16

What is the right hand rule?

Answer 16

Thumb = MOTION
First = FIELD
Second = EMF

Question 17

What is Faraday’s Law?

Answer 17

emf = N* dΦ/dt

Question 18

How does the power output look from a 3 phase system?

Answer 18

total power out is a steady value and does not oscillate.

Question 19

How do you calculate the power from a 3 phase system?

Answer 19

P = 3V(rms)I(rms)

All in phase

Question 20

What are the characteristics of a STAR connection?

Answer 20

I line = I phase
V line = Root3<30 * V phase

neutral point is zero voltage when loads balanced.

Question 21

What are the characteristics of a DELTA connection?

Answer 21

V line = V phase
I line = Root3<-30 * I phase

No neutral point

Question 22

How do you find the power from a connection? (star or delta)

Answer 22

S = root3 * V line(mag) * I line (mag)

Question 23

How do you calculate the power in an AC circuit?

Answer 23

P = V(max)^2/R *cos^2(wt)

always above 0

Question 24

How do you calculate the average power in an AC circuit?

Answer 24

Pav = v(max)^2/2R

Question 25

How do you measure capacitance?

Answer 25

C = Q/V (Farads)

Where Q = charge and V = PD

Question 26

Whats the reactance of a capacitor?

Answer 26

Xc = 1 / wC

V lags I by 90deg

(multiply I by -j for IMPEDANCE)

Question 27

How do you calculate inductance?

Answer 27

L = N^2 / fancyR (Henry)

Fancy R = reluctance N=turns

Question 28

Whats the reactance of an Inductor?

Answer 28

Xl = wL

V leads I by 90 deg

(multiply by j for IMPEDANCE)

Question 29

Whats the impedance of a resistor?

Answer 29

Same as resistance mate.

Question 30

Why are impedances important?

Answer 30

used instead of resistances in AC circuits. V = Z*I

Question 31

How to remember leading and lagging?

Answer 31

CIVIL
Capacitor = I leads V
Inductor = V leads I

Question 32

What are the units for S P and Q respectively?

Answer 32

S=VA
P=W
Q=VAr