Steady State AC Circuits

Question 1

Why use ac over dc

Answer 1

easier to generate and easier to change from one voltage to another

Question 2

voltage at any time =

Answer 2

v(t) = Vp * sin(angular frequency * time)

Question 3

What is Vp

Answer 3

Peak voltage

Question 4

Symbol for angular frequency

Answer 4

omega

Question 5

How to calculate angular frequency

Answer 5

2Pifrequency

Question 6

frequency units

Answer 6

Hz

Question 7

Relationship between frequency and time period

Answer 7

f=1/T

Question 8

How to calculate root mean square voltage

Answer 8

Vrms = Vp / sqrt(2)

Question 9

Mains supply in the UK

Answer 9

50Hz 230Vrms 325Vp

Question 10

How to calculate current at any time in a circuit

Answer 10

i(t) = Ip*sin(angular frequency *t)

Question 11

What is the reactance of a capacitor

Answer 11

1/ j2PifC, the j here is not necessary when calculating magnitude will have units of ohms

Question 12

What is the phase difference between the current and the voltage waveform across a capacitor

Answer 12

the voltage lagging the current by 90 degrees, or the current leads the voltage by 90 degrees

Question 13

What is the phase difference between the current and the voltage waveform across a resistor

Answer 13

Nothing they are in phase with each other, however the magnitude will be scaled by the magnitude of the resistance

Question 14

What is the inductive reactance

Answer 14

2PifLj, j is there as a directional thing does not give the magnitude of the reactance

Question 15

What is the phase difference between the current and the voltage waveform across a inductor

Answer 15

The current lags the voltage by 90 degrees or the voltage leads the current by 90 degress

Question 16

What is the acronym to work out the phase difference

Answer 16

CIVIL, In a capacitor the current leads the voltage

in an inductor the voltage leads the current

Question 17

When drawing phasor diagrams what is the usual position

Answer 17

choose the zero degree point for one of the phasors one along x axis

Question 18

How do we normally express a phasor in elecrical engineering

Answer 18

Z

Question 19

What is the symbol for capacative reactance

Answer 19

Xc in ohms

Question 20

What is the symbol for inductive reactance

Answer 20

XL in ohms

Question 21

What is the symbol of Impedance

Answer 21

Z

Question 22

What is the update form of ohms law

Answer 22

v=iZ

Question 23

When looking at series circuits what do you normally draw everything relative to

Answer 23

the current as it is constant for all

Question 24

What will the angle between the capactior voltage and current always be

Answer 24

the voltage across the capactior will always be behind by 90 degrees or pi/2

Question 25

What will the angle between the inductor voltage and current always be

Answer 25

the voltage across the inductor will always be ahead by 90 degrees or pi/2

Question 26

Why does the resultant voltage and current not stay at constant phase difference for different circuits

Answer 26

as the values of the inductors and capacitors will change therefore their voltages will change, making the phase angle between the two of them vary

Question 27

What are you generally looking for in ac circuit questions

Answer 27

will often be asked for the phase angle between the total voltage and current

Question 28

in a inductor and resistor circuit whats the relationship to the supply voltage

Answer 28

vs = sqrt( vr^2 + vL^2) = square root of the resistors voltage squared plus the inductors voltage squared

Question 29

at low frequencies what happens in an inductor plus resistor circuit and at high freq

Answer 29

low freq => omega => zero and magnitude of the impedance => R therefore phase angle between voltage and current => 0 high freq => omega => infinity and magnitude of the impedance => omega*L therefore phase angle between voltage and current => 90

Question 30

at low frequencies what happens in an capacitor plus resistor circuit and at high freq

Answer 30

low freq => omega => zero and magnitude of the impedance => 1/omega*C => infinity therefore phase angle between voltage and current => -90 high freq => omega => infinity and magnitude of the impedance => R therefore phase angle between voltage and current => 0

Question 31

difference between resistance and impedance

Answer 31

impedance has both phase and magnitude where as resistance only has magnitude

Question 32

What is impedance

Answer 32

the measure of the opposition that a circuit presents to a current when a voltage is applied

Question 33

What are the real parts and imaginary parts of impedance called

Answer 33

real is resistance and the imaginary part is reactance

Question 34

What does the reactance part of impedance do

Answer 34

it causes a phase shift between the voltage and current, it is created due the capacitance or inductance

Question 35

What do you normally calculate everything relative to in a parallel circuit

Answer 35

The voltage as it is constant for all

Question 36

What is the resonant frequency

Answer 36

it is the frequency at which the circuit appears purely restive as the capacitor and inductor cancel each other out

Question 37

what are components required in a resonant circuit

Answer 37

inductor, capacitor, resistor

Question 38

what happens at resonance property wise for series

Answer 38

magnitude of vL=vC v=vr impedance = R phase difference = 0 and impedance will be at a minimum therefore current will be at a maximum

Question 39

What is the Q factor

Answer 39

it is the magnification factor at resonance in series it is the voltage in parallel it is the current

Question 40

How do you calculate the Q factor in series

Answer 40

magnitude of the inductor voltage (or capacitor voltage)/ the resistor voltage (1/R)*sqrt(L/C)

Question 41

What is the equation of resonance for a series and parallel circuit

Answer 41

1/(2*Pi*sqrt(L*C))

Question 42

difference between resonance for a parallel and a series circuit

Answer 42

in series the voltages are 180 degrees out of phase and of equal in magnitude and in parallel the current is 180 degrees out of phase and equal magnitude

Question 43

what happens at resonance property wise for parallel

Answer 43

magnitude of iL=iC i=ir impedance = R phase difference = 0 and impedance will be at a maximum therefore current will be at a minimum

Question 44

What is the issue with having all three components in parallel and what is the solution

Answer 44

draw very large currents therefore the resistor is normally put in series

Question 45

How do you calculate the Q factor in parallel

Answer 45

magnitude of the inductor current (or capacitor current)/ the resistor current R*sqrt(L/C)