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Electrical FE Prep > Electrical > Flashcards

Electrical Flashcards

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1
Q

What is a Coulomb?

A

A large group of electrons. Simply a unit of charge.

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2
Q

What is an Amp equivalent to?

A

A coulomb per second.

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3
Q

Explain the concept of current.

A

The flow of charge from one point to another per unit time.

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4
Q

Explain the concept of voltage.

A

How much energy a unit of charge has. It’s the potential energy difference between two points. It causes current to flow.

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5
Q

What is a volt equivalent to?

A

A joule per coulomb.

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6
Q

Explain the difference between energy and power.

A

Energy is the ability to do work. It allows us to do stuff. Power is the rate that energy is used. Like how miles traveled and mph, energy is the usage but power is the rate.

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7
Q

What is watt equivalent to?

A

Joule per second.

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8
Q

Explain the concept of electrical resistance.

A

It’s the opposition to flow of current. Like rocks in a flowing river. How much depends on the material or equipment.

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9
Q

What is Ohm’s Law?

A

V = I x R. It relates voltage, current, and resistance.

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10
Q

Explain Kirchoff’s Current Law.

A

The sum of current in a point must equal the sum of current out.

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11
Q

Explain Kirchoff’s Voltage Law.

A

Sum of voltage produced in a loop is equal to sum of voltage consumed in a loop. Voltage around a loop equals 0.

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12
Q

Explain series vs parallel in a circuit.

A

Series is a single loop, and parallel is multiple loops.

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13
Q

How do you add resistors in series?

A

Sum all of the resistors.

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14
Q

How do you add resistors in parallel?

A

1 / (1/R1 + 1/R2)

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15
Q

What is voltage division?

A

Finding the voltage of a certain element. Multiply whole V by ratio of element R to whole R.

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16
Q

What is current division?

A

Finding the current thru a certain leg of a circuit. Multiply I times ratio of opposite R over R total.

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17
Q

How to correctly connect an ammeter in a circuit?

A

Must become part of the current flow. Do not connect it across a voltage source or load.

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18
Q

How to correctly connect a voltmeter to a circuit?

A

Across voltage sources or loads. Very durable and high resistance.

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19
Q

Explain wattmeter usage.

A

Uses a current coil and pressure coil to measure current and voltage. It then calculates power in watts.

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20
Q

Explain the two wattmeter method.

A

Two wattmeters are necessary to measure three phase power.

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21
Q

How to correctly connect a wattmeter.

A

Current coil in series. Pressure coil in parallel.

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22
Q

What is magnetomotive force (MMF) and its units?

A

Essentially the voltage of electromagnetism. It’s the driving force. Units of Amp-Turns.

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23
Q

How is magnetomotive force determined?

A

Force = Number if turns in coil x current

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24
Q

What is reluctance and its units?

A

It’s the opposition to electromagnetism. Units of Amp-Turns per Weber.

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25
What is magnetic flux and its units.
It’s the measure of how much of the magnetic field is passing thru the material. Units of Webers.
26
What is the electromagnetism equivalent of Ohm’ Law?
Rowland’s Law: F = Flux x Reluctance
27
What is magnetic field intensity?
How much MMF per unit length. Simply MMF / m. Denoted by “H”.
28
What is flux density and the units?
It’s how much magnetic flux there is in a unit of area. Simply Flux per Area. Denoted by “B”. Units of Tesla aka webers per square meter.
29
What is magnetic permeability?
Its Magnetic Field Intensity per Flux Density. B / H.
30
What’s the right hand rule in electromagnetism?
Determines the direction of current flow and magnetic field. Imagine grabbing the wire: thumb is direction of current and fingers are flow of magnetic field.
31
What is Faradays Law?
The change in magnetic flux can create voltage, which causes current to flow. Essentially, moving magnetic fields create electricity. V = delta flux / delta time.
32
What is Lenz’s Law?
The resistance to a changing magnetic field from the current in a wire’s own magnetic field. The loop tries to resist change in the original field.
33
What do capacitors do?
They smooth out current by storing voltage. They can help to balance power factor.
34
What is the unit of capacitance?
Farads
35
How do you add capacitors?
Use the opposite methods of adding resistors in series and parallel.
36
What is the equation for energy stored in a capacitor?
J = (1/2) x C x V^2
37
How do you calculate a time constant in capacitors?
Tau = R x C
38
What is the significance of a time constant in capacitors?
It takes 5 TC (aka Tau) to charge or discharge a capacitor.
39
What does an inductor do?
Stores energy in a magnetic field using current.
40
How do you add inductors?
Same as resistors and opposite to capacitors.
41
What is the equation for energy stored in an inductor?
J = (1/2) x L x I^2
42
What is the unit for inductance?
Henrys
43
How long does it take for an inductor to build up a maximum magnetic field?
5 Tau
44
How do you calculate time constants for inductors?
TC = L/R
45
What is a Hertz?
Unit of Frequency. It’s how many full cycles occur in one second.
46
Why is 3 Phase Power used?
It fills the gaps of single phase, so there’s more consistent power.
47
What is RMS in electrical and how is it calculated?
Root mean square. Like a rough average, but aka the DC equivalent of AC. Peak / sqrt of 2.
48
What is the peak of a sin wave?
The amplitude or max
49
What is angular frequency?
The frequency used in sin equations and measured in radians per second. Found by 2 x pi x f.
50
How are period and frequency related?
The are inverses. 1/T =f
51
What does the phase angle do in a sin equation?
Shift the wave left or right.
52
What is standard rectangular form?
a + jb
53
Explain rectangular form.
First term is horizontal or real. Second term is vertical or imaginary.
54
Explain phasor form.
M is magnitudes and theta is the angle.
55
How do you calculate rectangular from phasor form?
M x cos theta for horizontal and M x sin theta for vertical.
56
How do you calculate phasor form from rectangular?
Magnitude is the sqrt of the sum of squares. Theta is inverse tan of b/a.
57
What is apparent power?
The total power seen, measured in kVA.
58
What is real power?
The power that does useful work. Measured in kW.
59
What is reactive power?
Useless power. Measured in kVAr.
60
What is the equation for apparent power in rectangular form?
S = P + jQ
61
What is power factor?
The ratio of real to apparent power. It tells us how much value you’re getting for what you pay for. Also known as cos theta.
62
63
Explain how capacitors and inductors affect power factor?
Capacitors hold back voltage, inductors hold back current. They both take some of the energy needed to do work. Use ELI the ICE man.
64
What is reactance?
It’s inductors or capacitors acting like resistors.
65
What is the formula for inductive reactance?
XL = 2 x pi x f x L or omega x L
66
What is the formula for capacitive reactance?
XC = 1 / (2 x pi x f x C) or 1 / (omega x C)
67
What are the units for reactance XL and XC?
Ohms
68
What is impedence?
Total opposition to flow and change to the circuit. Z = R + XL + XC
69
How do you calculate Voltage across an inductor or capacitor?
Same as Ohms Law. V = I x XC or XL for the corresponding piece.
70
How do you know if a circuit is at resonant frequency?
XC = XL
71
What is the resonant frequency formula?
Fr = 1 / (2pi x sqrt of L x C)
72
How far apart are phases in 3 phase power?
120 degrees
73
How many cable do delta and wye connections use?
Delta uses 3. Wye uses 4 because it contains a neutral.
74
What is the difference between phase and line voltage?
Phase is between line and neutral. Line is between 2 lines.
75
What does a balanced electrical system mean?
Each phase has the same power factor and impedance. Current thru each phase is equal.
76
How can you tell if an electrical system is unbalanced?
It will have either unequal currents or impedances.
77
What is the motor efficiency formula?
Eff. = power in / power out
78
What is slip in an induction motor?
Ratio of difference between actual speed and synchronous speeds.

Electrical FE Prep (1 decks)

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