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electrical engineering > electricity > Flashcards

electricity Flashcards

1
Q

In a parallel circuit the element with the least resistance …

A

consumes the most power

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2
Q

what will happen if an ammeter is connected in series with a resistor?

A

Almost all the current will flow through the ammeter

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3
Q

What will happen when a voltmeter is connected in series with a resistor?

A

It will show a 0 volts reading.

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4
Q

Kirchofff’s voltage law can be stated as

A

The sum of the voltage drops around any closed path is 0

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5
Q

What can kirchoff’s current law be stated as?

A

The sum of the current entering a node is equal to the sum of current leaving the node.

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6
Q

If a 40w and 60w light bulb are connected in series to the mains, which will glow brighter?

A

The 40w. It has the higher resistance.

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7
Q

If there was a circuit with 3 bulbs in series and one of them was removed, what would happen to the total power dissipated?

A

It would increase. Circuit resistance has fallen.

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8
Q

If there was a circuit with 3 bulbs in parallel and one of them was removed, what would happen to the total power dissipated?

A

Circuit resistance has increased and the power would fall.

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9
Q

If a 9v power source was in a series circuit with an LED (turn on voltage=2.2v) and a resistor of resistance 680 ohms, what would be the expression for current in the circuit?

A

(9-2.2) / 680

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10
Q

What is a perfect voltage source?

A

The current going through it will change to ensure the pd is always constant.

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11
Q

What is conductance?

A

The inverse of resistance I=V*G. Units=siemens

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12
Q

What do we assume the p.d across a LED to be?

A

The turn on voltage. We assume the I/V graph of an LED to be vertical at the turn on voltage.

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13
Q

If you had a circuit with a R1 and R2, how would you maximise the power in R2?

A

Make R1 and R2 equal to each other. The efficiency will be 50%

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14
Q

If you had a circuit with R1 and R2, how would you maximise the efficiency of power to R2?

A

Make R2 large and R1 small.

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15
Q

True or false. Two waves are out of phase when they have different phases or one is ahead or behind the other.

A

False. They are out of phase when one is ahead or behind the other.

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16
Q

When a capacitor and resistor are in parallel, what will the p.d across them be in relation to each other?

A

They will be the same.

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17
Q

How do you find the reactance of a capacitor?

A

= 1/ (2*pi*f*C)

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18
Q

In a capacitive circuit, what is the relationship between input and output voltage?

A

Output lags input

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19
Q

In a capacitive voltage divider, what happens to output voltage with an increase in frequency?

A

It falls.

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20
Q

In a capacitive circuit, what are the output and input frequencies?

A

The same.

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21
Q

If the capacitance of a capacitor is increased, what happens to the current drawn?

A

It increases.

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22
Q

What is the definition of capacitance?

A

charge = capacitance*voltage

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23
Q

If an LED has a turn on voltage of 1.8v, what will the P.D across it be?

A

1.8v

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24
Q

When are two components in parallel?

A

When they share two terminals.

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25
Q

What is reactance?

A

The rate of storage of energy in the electric field around a capacitor.

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26
Q

In a capacitive circuit, what is current?

A

capacitance * rate of change of voltage.

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27
Q

What are the units for reactance?

A

Ohms

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28
Q

What is the phase relationship between voltage and current in a capacitive circuit?

A

Current is pi/2 ahead of voltage.

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29
Q

What is an exprestion for the voltage at any time in a capacitive circuit?

A

v=Vmax*sin(ωt)

=Vmax*sin(2πft)

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30
Q

What is an exprestion for the current in a capacitive circuit?

A

i=2πfVmaxC*cos(2πf)

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31
Q

What is the relationship between voltage and current in a capacitive circuit?

A

I*Xc=V

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32
Q

When a wheastone bridge is balanced, what is the pd?

A

0

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33
Q

When you turn off an ideal voltage source, what do you replace it with?

A

A short circuit

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34
Q

What is Thevenin’s theorem?

A

A combination of perfect sources and resistances with output terminals A and B can be replaced with a single output resistance.

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35
Q

What are the stages to finding a thevenin equivilent circuit?

A

Find the open circuit voltage across AB

Find the output resistance with all the sources off.

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36
Q

What is Norton’s theorem?

A

A combination of perfect sources and resistances with output terminals A and B can be represented by a single ideal current source in parralel with a single output resistance.

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37
Q

How do you find the Norton equivalent?

A

Find the short circuit equivalent between the output termninals A and B

Find the output resistance with all sources turned off.

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38
Q

For an inductor, what is the phase relationship between voltage and current?

A

Current lags by π/2

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39
Q

In an ideal amplifier, what is the ideal ratio of resistences?

A

Rout<<r style=”font-size: 13.5px;”>load</r>

40
Q

What are the units for gain?

A

Decibels.

41
Q

What is symbol for reactance?

A

X

42
Q

What is the symbol for impedance?

A

Z

43
Q

What is the difference in current though a capacitive reactance and an inductive reactance?

A

They will have the same magnitude but different phase.

44
Q

What happens at the resonant frequency?

A

Inductive reactance and capacitive reactance are equal in magnitude and cancel out.

P.d across L and C are the same

Impedance is same as circuit resistance and it totally real.

Impedance is lowest here so current will be at it’s highest

45
Q

How do you find the energy stored in an inductor?

A

energy=0.5*L*I2

46
Q

If you have a capacitor and inductor in series, how do you find the resontant frequency?

A

freq= 1/(2π(LC)0.5)

At this point, energy will go between the inductor and capacitor,

47
Q

What happens to the net amount of energy oscilatting between a capacitor and inductor with time?

A

It falls due to circuit resistance.

48
Q

what are the features of an op-amp?

A

They have a very high gain

Assume Rin=infinity meaning input power=0, though this isn’t quite the case.

49
Q

What is the advantage of an op-amp?

A

The op-amp does not load the source because of the high input impedance and because of the low output impedance the output voltage of the op-amp is the same as the voltage across the load, giving maximum voltage gain.

50
Q

What does an inverting amplifer look like?

A
51
Q

What is the gain of an inverting amplifer?

A

-Rf /Ri

52
Q

What is the maximum voltage that can leave an amplifer?

A

The voltage being used to power the amplifer. On a graph, the peaks might be clipped flat if the amplified vaule is too big.

53
Q
A

From golden rules, V+=V-=Vin

Using circuit divider, V-=(Vout*Ri) / (Ri+Rf)

Vin=(Vout*Ri) / (Ri+Rf)

Vout/Vin = (Ri+Rf)/Ri

54
Q

What is the virutal ground?

A

V- part of an amplifer. You can assume that with pd is lost before this point.

55
Q

What are the golden rules of amplifers?

A

The output attempts to do whatever is necessary to make the voltage difference between the inputs zero.

The inputs draw no current.

56
Q

What is susceptance?

A

1 / reactance

57
Q

That does the sign of the phase angle of a load impedance mean?

A

+ Voltage leads current ( Resistor and inductor)

-Voltage lags current.( Resistor and capacitor)

58
Q

What is the quality factor (in words)?

A

Ratio of power stored in reactance to power dissipated in resistance.

If Q is large the bandwith is small is is useful

59
Q

What are formulas for the quality factor?

A

Q=ω0L/R

Q=(1/R)*(L/C)^0.5

60
Q

What is the RCL phasor diagram before,during and after resonance?

A
61
Q

What is the phase relationship between Vc and VL?

A

They are always 180 out of phase. In a phasor diagram they are always parallel.

62
Q

How do you find the active power in a resistive AC circuit?

A

P=V*I*cos(φ)

=I2R

V and I are the rms values and φ is the phase angle between the two (phase of the impedance)

This is power dissipated as heat due to resistance (units=W)

63
Q

How do you find the apparent (complex) power?

A

S=Vrms*Irms with units of volt amps.

S=P+jQ

=I2Z (If using V2/Z, use complex conjugat of Z)

If using phasors, S=(I*)(v) (the complex conjugate of I is used. This is done such that a leading current (capacitive load, negative reactance), results in negative reactive power)

Power dissipated in heat and stored in components.

64
Q

What is reactive power?

A

Q

=Irms*Vrms*sin(φ)

=I2X

φ is the phase angle of the impedance(angle between voltage and current)

units=VAr (volts amps reactive)

65
Q

What is admittance?

A

1/impedance

66
Q

What is the power factor?

A

cos(φ)

Where φ is the angle of the impedance(angle between voltage and current)

active power/apparent power

67
Q

How do you find capacitive impedance?

A

1/(j*c*ω)

68
Q

How do you find inductive impedance?

A

i*ω*L

69
Q

How do you find inductive reactance?

A

ω*L

70
Q

With transformers, how is n defined?

A

N2 / N1

71
Q

How does V and I change in a transformer?

A

V2=n*V1

I1=-n*I2 (They are out of phase)

V1I1=V2I2

72
Q

How is the RMS value found from the peak value with a triangular wave?

A

Divide peak by root 3

73
Q

What is the equation linking primary and secondary inpedance?

A

Z1=Z2/n2

74
Q

What is magnitising current?

A

When the second circuit is open, it is the current that magnitisies the core. Total current=magnitising current+current for losses.

75
Q

How is magnitising current found?

A

Im=I0cos(Ø)

where I0 is the drawn current and ø is ωt

76
Q

How is magnetic flux found?

A

flux(ø)

=inductance*current

77
Q

How is the magnetomotive force found?

A

F=ø*R Where R is reluctance

F=NI

78
Q

What is magnetic reluctance?

A

It is analogous to resistance in an electrical circuit, but rather than dissipating electric energy it stores magnetic energy. In likeness to the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance. It is a scalar, extensive quantity, akin to electrical resistance. The unit for magnetic reluctance is inverse henry, H-1

79
Q

When smoothing out a DC singal, how big does the capacitance need to be?

A

As a rule of thumb RC>1.5T

T=1/f

80
Q

In a motor, how do you find the back EMF?

A

ε=Kω

81
Q

What are the equations for torque for an electrical motor?

A

T=KI

T=NIø

T=(k/R)(V1-kω)

82
Q

What is the equation for the the supply voltage in a DC motor?

A

V=IR+ε

83
Q

How do you find the energy stored in a capacitor?

A

0.5*C*V2

84
Q

How might you find the phase angle between a current and voltage?

A

θ=tan-1(X/R)

85
Q

Define inductance

A

Where an electromotive force is induced by a varaition of current in the circuit.

86
Q

Faraday’s law

A

The induceed e.m.f is directley proportional to the rate of change of flux linkage.

87
Q

What is the gain of the inverting amplifer?

A
88
Q

How do you measure a gain in decibels?

A

10*log(output / input)

To have it in bels, divide by 10

89
Q

Name this

A

Xor gate

90
Q

How can you tell if sides on a K map are connected?

A

When 1 bit is flipped between them.

91
Q

When chosing resistors for an amplifer, what should they be?

A

Greater than a few kilo-ohms but less than 100 kilo-ohms.

92
Q

What is the output voltage of the wheatstone bridge?

A
93
Q

When ΔR is added to R1 and subtracted from R3 of a wheatstone bridge, what is the output?

A

VAB=-Vs*ΔR /2R

94
Q

What are the colours for three phase power?

A
95
Q

How do you smooth out DC?

A

Have a capacitor in parralel to the output voltage and a resitance parralel to that.

96
Q

How do you find the power from a three phase power source?

A

Comples power=VI*

Red power=VLNIL

Yellow power =(VLN ∠-1200)(IL∠1200)=VLNIL

Blue power =VLNIL

Thus total power =3 VLNIL

If you sub in VLN you get s=√3VLLIL

97
Q

How do you find the VLL?

A

Line to line voltage=VRN-VYN

Subtract phasors and get VLL=√3VLN∠300

electrical engineering (3 decks)

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