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EEE218 > Steady State AC Circuits > Flashcards

Steady State AC Circuits Flashcards

1
Q

Why use ac over dc

A

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

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2
Q

voltage at any time =

A

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

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3
Q

What is Vp

A

Peak voltage

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4
Q

Symbol for angular frequency

A

omega

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5
Q

How to calculate angular frequency

A

2Pifrequency

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6
Q

frequency units

A

Hz

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7
Q

Relationship between frequency and time period

A

f=1/T

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8
Q

How to calculate root mean square voltage

A

Vrms = Vp / sqrt(2)

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9
Q

Mains supply in the UK

A

50Hz 230Vrms 325Vp

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10
Q

How to calculate current at any time in a circuit

A

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

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11
Q

What is the reactance of a capacitor

A

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

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12
Q

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

A

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

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13
Q

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

A

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

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14
Q

What is the inductive reactance

A

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

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15
Q

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

A

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

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16
Q

What is the acronym to work out the phase difference

A

CIVIL, In a capacitor the current leads the voltage

in an inductor the voltage leads the current

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17
Q

When drawing phasor diagrams what is the usual position

A

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

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18
Q

How do we normally express a phasor in elecrical engineering

A

Z

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19
Q

What is the symbol for capacative reactance

A

Xc in ohms

20
Q

What is the symbol for inductive reactance

A

XL in ohms

21
Q

What is the symbol of Impedance

A

Z

22
Q

What is the update form of ohms law

A

v=iZ

23
Q

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

A

the current as it is constant for all

24
Q

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

A

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

25
Q

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

A

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

26
Q

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

A

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

27
Q

What are you generally looking for in ac circuit questions

A

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

28
Q

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

A

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

29
Q

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

A

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

30
Q

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

A

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

31
Q

difference between resistance and impedance

A

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

32
Q

What is impedance

A

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

33
Q

What are the real parts and imaginary parts of impedance called

A

real is resistance and the imaginary part is reactance

34
Q

What does the reactance part of impedance do

A

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

35
Q

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

A

The voltage as it is constant for all

36
Q

What is the resonant frequency

A

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

37
Q

what are components required in a resonant circuit

A

inductor, capacitor, resistor

38
Q

what happens at resonance property wise for series

A

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

39
Q

What is the Q factor

A

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

40
Q

How do you calculate the Q factor in series

A

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

41
Q

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

A

1/(2Pisqrt(L*C))

42
Q

difference between resonance for a parallel and a series circuit

A

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

43
Q

what happens at resonance property wise for parallel

A

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

44
Q

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

A

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

45
Q

How do you calculate the Q factor in parallel

A

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

EEE218 (7 decks)

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