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Electricity and Electronics > AC CIRCUITS > Flashcards

AC CIRCUITS Flashcards

(78 cards)

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1
Q

How do electrons behave in AC circuits?

A

Electrons alternate between pushing and pulling, so they move in one direction and then reverse, which gives the illusion that the electrons are flowing randomly through a circuit

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2
Q

Type of motion of AC electricity

A

Periodic motion, similar to that of a pendulum

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3
Q

Why AC currents are preferred?

A

It is easier and cheaper to generate alternation current then direct current since its voltage can be readily altered using transformers.

It’s much more difficult to alter DC to AC voltages

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4
Q

Rectifiers

A

Devices used to convert AC into DC

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5
Q

Transformer

A

An electric device that is used to change voltage in AC electric circuits.

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6
Q

Composition of a simple transformer

A

Consists of two inductive coils, labelled as primary winding and secondary winding.In essence these are two sets of wires wrapped around it for the input and output voltages.

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7
Q

Period/Periodic time

A

The time taken for an alternating quantity to complete one cycle.

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8
Q

Frequency

A

The number of cycles completed in one second

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9
Q

Instantaneous values

A

Values of the alternating quantities at any instant of time.

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10
Q

Peak value

A

The largest value reached in a half cycle

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11
Q

Peak to peak (Vpp)

A

The difference between the maximum and minimum values of a cycle

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12
Q

Average value over a complete cycle

A

Zero

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13
Q

Effective or root mean square (RMS) value of an alternating current (AC) is defined as what?

A

The current which will produce the same effect as an equivalent direct current (DC)

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14
Q

Form factor

A

RMS value/average value

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15
Q

Peak Factor

A

maximum value/RMS value

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16
Q

Why is insulation used

A

To prevent ‘leakage’

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17
Q

What parameter is considered when choosing insulation materials?

A

The peak values

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18
Q

Fuses

A

Are the weak link in a circuit and are used to break the circuit if excessive current is drawn

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19
Q

Parameter used when calculating appropriate fuse size

A

RMS value

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20
Q

Basic properties of passive components in ac circuits

A

Resistance, Inductance and Capacitance

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21
Q

Factors to be considered in Ac circuits

A

-The phase relationship between voltage and current.
-The opposition to current flow in the circuit
-Is the oppositio to the curret flow affected by the frequecy of the supply?

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22
Q

Pure resistance only circuit

A

-Current and voltage are in phase with each other

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23
Q

Relation of frequency and R

A

R is independent of frequency

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24
Q

Phase shift

A

Current and and voltage are out of step with each other

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25
Properties of phasors
-The length of a phasor is proportional to the maximum value of the alternating quantity involved -The projection of a phasor on the vertical axis gives the instantaneous value of the alternating quantity involve
26
Phasor diagram
One in which phasors =, represented by arrows, rotate counter clockwise, with an angular frequency about the origin
27
Pure Inductance only
The current lags the voltage by 90 degrees
28
Why are I and V out of phase in inductive circuits?
This is due to the back emf in the inductor acting in direct opposition to the supply voltage V.
29
What is the back emf dependent of in l circuits
Rate of change of current
30
opposition of current in pure R circuits
Resistance, R
31
oppposition of current in pure L circuits
Inductance Reactance, Xl
32
Relation of frequency and XL
XL is directly proportional to the supply frequency
33
Pure Capacitance Only Circuit
The current leads the voltage by 90
34
Why are I and V out of phase in pure C circuits
The capacitor voltage acts opposite to the supply current I
35
What is current at any instant dependent of
Rate of change of capacitor voltage
36
Opposition of current in pure C circuits
Capacitive Reactance Xc
37
Relation of Xc and f
Capacitive Reactance is inversely proportional to frequency
38
Application of Reactance
Is used to compute amplitude and phase changes of sinusoidal alternating current going through a circuit element
39
CIVIL
In a capacitor (C), the current (I) is ahead of the voltage(V), which is ahead of the current (I) for the inductor (L)
40
Reactance
Opposition to the flow of current
41
RL series circuit
Contains a resistor R and a inductor L connected in series with a voltage supply of v
42
Application of Rl Circuits
-for supplying Dc power to radio-frequency amplifiers where the inductor is used to pass DC bias current and block the RF returning back into the power supply -radio wave transmitters -communication systems -processing of signals -Oscillator circuits -Magnification of Current or Voltage -Filtering Circuits
43
In RLC circuits
The current may lead, lag or be in phase with the voltage depending on the relative values of the inductive and capacitance reactances
44
Reactance of Inductor
Rises with frequency
45
Reactance of capacitor
Falls with frequency increase
46
Inductive RLC circuit
If the current lags the supply voltage of the circuit XL>XC
47
Capacitive RLC circuit
If the current leads the supply voltage of the circuit XC>XL
48
Average power in purely R circuits
P=VI=I^2R=V^2/R
49
Average power is zero in
Purely inductive or purely capacitive
50
Average power in RL,RC,RLC
P=VICos(o) P=I^2R
51
Supply Current (I)
Is broken down in two components: -ICos(o) -ISin(o)
52
ICos(o)
Power component of the supply current
53
ISin(o)
Reactive component
54
True Power
P=VIcos(o)
55
Apparent Power
S=VI
56
Reactive Power
Q=VIsin(o)
57
Power Factor
True Power/Apparent Power Pf=cos(o)=R/Z
58
"Q" or Quality Factor
Is a measure of the "goodness" or quality of a resonant circuit
59
Filters
Are used in Digital Signal Processing and in circuits to help passor amplify certain frequencies while attenuatin (blocking) other frequencies
60
Ideal Filter
Transmits frequencies in its pass-band, without attenuation and without phase shift, while not allowing any signal components in the stop-band to get through
61
When is signal separation required?
When a signal has been contaminated with interference, noise or other signals
62
Signal Restoration
When a signal has been distorted in some way
63
DSP filters
Are applied to signals to select certain frequencies or certain information while rejecting other frequencies or information.
64
Categories of Filters
Passive and active filters
65
Passive Filters
Created using resistors, capacitors and inductors. They have no amplifying elements so there is no signal gain, therefore their output level is always less than the input.
66
Active Filters
Typically use an amplifier component and can add complexity to the signal, such as consuming power and injecting noise into a system.
67
Pass-band
Sometimes called bandwidth, is the frequency range that passes through the filter
68
Stop-band
Range of frequency that the filter attenuates
69
Cut-off frequency
Defines the frequency boundary between the pass-band and the stop-band.
70
Types of Filters
-Low-pass filter -High pass filter -Band pass filter -Notch/Band stop filter
71
Low Pass Filter
Attenuates high frequency energy but passes low frequency energy. Constructed using RC networks
72
Cut-off frequency in low pass filter
Occurs at resonance, where Xc=R
73
High-pass filter
Attenuates low-frequency energy but passes high-frequency energy. Constructed using CR
74
Band-pass filters
Transmits only those signal components within a band around a centre frequency. It rejects all frequencies outside of the range (fh-fl)
75
Application of band-pass filter
Extraction of a specific tone, such as test tone, from adjacent rones or broadband noise
76
Band-stop or notch filter
Transmits all signals except those above fL and below fH.
77
Application of band-stop filter
Medical instrumentation, where high-impedance sensors do pick up the AC mains frequency.
78
What dB does the corner frequency occur?
-3dB

Electricity and Electronics (5 decks)

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