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Engineering science > Electrical Term 1 > Flashcards

Electrical Term 1 Flashcards

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1
Q

What is positive power dissipation?

A

Component that consumes energy eg lightbulb/ areoplane

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2
Q

What are independent sources?

A

Constant supply of voltage/ current

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3
Q

What are independent sources represented by?

A

Cirlces

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4
Q

What are dependent sources represented by?

A

Diamonds

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5
Q

What do the first three/ four bands on a resistor tell you?

A

Numbers for resistance value - last of these bands is the resistance multiplier

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6
Q

What is the last band on a resistor tell you?

A

Tolerance - actual value will vary from nominal - due to manufacturing- represents max deviation

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7
Q

How do inductors store energy?

A

As a magnetic field

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8
Q

Forward biaising

A

Current flowing

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9
Q

Reverse biasing

A

Current restricted

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10
Q

Which way does current flow in a resistor?

A

From triangle base to line

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11
Q

What are transistors constructed from?

A

Semi conductor materials

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12
Q

What are the three transistor terminals called?

A

The collector, The base, the emitter

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13
Q

What is a transistor like?

A

A tap - how much current can flow

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14
Q

What is the saturated mode of a transistor?

A

When it is operated completely on or off

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15
Q

What is a node in a circuit?

A

Where two or more branches connect

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16
Q

What is an independent loop?

A

At least one branch high is not part of another independent loop

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17
Q

What is a mesh?

A

A loop that doesn’t house loops inside of it

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18
Q

What is the theorem of network typology?

A

b= l + n - 1

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19
Q

What is an open circuit modelled by?

A

Very high resistance

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20
Q

What resistance is there in a short circuit?

A

No resistance

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21
Q

Why might you use multiple resistors rather than one?

A

less expensive, lower concentration of dissipated heat,

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22
Q

What are planar circuits?

A

Can be drawn on a flat surface with no branches crossing each other

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23
Q

What is superposition in relation to circuits?

A

Find voltages/ currents as if the sources act alone - then sum these

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24
Q

In superposition what is a voltage source replaced with?

A

Internal resistance modelled as a resistor
(or short circuit if ideal)

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25
Q

In superposition what is a current source replaced with?

A

an open circuit

26
Q

What are the two types of equivalent circuits?

A

Thenenin and Norton

27
Q

What is in Thevenin’s Equivalent Circuit?

A

an open circuit voltage and a resistor in series modelling equivalent resistance

28
Q

What is in Norton’s equivalent circuit?

A

a current source and equivalent resistor in parallel

29
Q

What is the goal of impedance matching?

A

to deliver the maximum power from the cell to the load - has nothing to do with efficiency

30
Q

When is impedance matching generally used?

A

When the load is a signal rather than power transmission (eg audio engineering)

31
Q

What is the maximum power transfer theorem?

A

The maximum power is transferred when the load resistance equals the Thevenin or Norton resistance of the power network

32
Q

What two parts of time response for an AC circuit are there?

A

transient response and steady state response

33
Q

What is an LTI system?

A

time-invariant, it doesn’t matter when the change is applied the output will be the same

34
Q

What two important properties do LTI systems satisfy?

A

Homogeneity and Superposition

35
Q

What does homogeneity refer to in the context of AC circuits?

A

if we increase input (eg multiply by a constant) the output will change by the same amount

36
Q

What does superposition refer to in the context of AC circuits?

A

output = sum of inputs

37
Q

If the input to a LTI system is a sinewave the output will be a sinewave with the same…?

A

frequency

38
Q

As well as resistance AC circuits have?

A

Reactance

39
Q

Reactive elements… energy

A

store and release

40
Q

What is impedance of an AC circuit?

A

combination of resistance and reactance

41
Q

What is reactance?

A

A property that opposes a change in current

42
Q

What phase shift between voltage and current occurs when reactance is present?

A

90 degree

43
Q

With capacitive reactance the current waveform /// the voltage by 90 degrees

A

leads

44
Q

With inductive reactance the current waveform /// the voltage by 90 degrees

A

lags

45
Q

How do you work out impedance?

A

Z=R+j(Xl-Xc)

46
Q

For an impedance with both inductance and capacitance at a low frequency what is the impedance like?

A

Large and leading as the capacitance term (1/cw) dominates

47
Q

For an impedance with both inductance and capacitance at a high frequency what is the impedance like?

A

large and lagging (impedance term (Lw) dominates

48
Q

What is the minimum impedance (with both inductance and capacitance)?

A

R (resistance) and Lw & 1/Cw cancel) - hence the angle is zero

49
Q

What is it called when impedance is a minimum (and equal to R)?

A

Resonance

50
Q

What is reactive power due to?

A

Due to the movement of energy within a circuit (between reactive components) - the energy doesnt leave the circuit

51
Q

What is the name of power dissipated by a resistor?

A

active/real power

52
Q

What is reactive power measured in?

A

VAr (Volt amperes reactive)

53
Q

The power angle and impedance angle are the same when…

A

there is a single resistive load - all the components are in series

54
Q

What is the reverse piezo electric effect?

A

a voltage is applied to a material which produces a force

55
Q

What are some applications of the piezo electric effect?

A

spark ignitors, microphones

56
Q
A
57
Q

What is the difference between a wound field machine and a permanent magnet machine?

A

The stator has its own coils - you can confíela the strength of the magnetic field

58
Q

What are some disadvantages of a wound field machine compared to a permanent magnet machiene?

A

Field has to have its own power supply, losses are associated with the resistance of the field coils

59
Q

What two things are used to deal with the fact the armature coils are mounted to a rotating structure?

A

Commutator and brushes

60
Q

Where does the energy conversion take place in a DC brushed machiene?

A

In the air gap between the rotor and stator

61
Q

Why would an air gap in a DC brushed machine need to be small?

A

So it doesn’t effect the amount of magnetic flux ( air has a very low relative permeability)

62
Q

Why would an air gap need to be large enough?

A

Accommodates manufacturing tolerances (doesn’t cause the rotor scraping and jamming on the Moro out)

Engineering science (2 decks)

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