41. Define Direct Current. Give examples of DC sources. Draw steady, varying and smooth DCs. Direct current (DC) is the unidirectional flow of electric charge. Direct current is produced by such

sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo

type. Direct current may flow in a conductor such as a wire, but can also flow through semiconductors,

insulators, or even through a vacuum as in electron or ion beams.

42. Define Ohm’s Law (name each component in the relation).

Ohm’s law states that the current I flowing in a circuit is directly proportional to the applied voltage V

and inversely proportional to the resistance R, provided the temperature remains constant. Thus

I=U/R

I – current

V – voltage

R - resistance

43. What is a conductor, but an insulator? Give examples of conductors and insulators.

A conductor is a material having a low resistance which allows electric current to flow in it.

An insulator is a material having a high resistance which does not allow electric current to flow in it.

Examples: Copper , Aluminum , Carbon(graphite), Glass , Mica

44. Define the resistance of a conductor (explain each component in the relation).

The resistance of an electrical conductor depends on four factors:

a. (a) the length of the conductor, b. (b) the cross-sectional area of the conductor, c. (c) the type of material and d. (d) the temperature of the material.

R=ρ*l/S

Rezistivity ρ is the resistance of a unit cube of the material measured between opposite

faces of the cube and measured in ohm meters ( Ω m).

45. Define the temperature coefficient of resistivity.

The temperature coefficient of resistance of a material is the increase in the resistance of a 1 Ω

resistor of that material when it is subjected to a rise of temperature of 1°C.

The symbol used for the temperature coefficient of resistance is α (Greek alpha).

46.Write the dependence of resistance on the temperature (explain each component in the relation).

Rθ=R0(1+ α0θ)

Where

R0 - resistance at 0°C

Rθ - resistance at temperature θ °C

α0 - temperature coefficient of resistance at 0°C

47. Define Kirchhoff’s Current Law.

At any junction in an electric circuit the total current flowing towards that junction is equal to the

total current flowing away from the junction

48. Define Kirchhoff’s Voltage Law.

In any closed loop in a network, the algebraic sum of the voltage drops (i.e., products of current

and resistance) taken around the loop is equal to the resultant e.m.f. acting in that loop.

49. Draw a series DC circuit with three resistances and express the circuit total resistance as a function of the three component resistances.

0k

kI

In a series circuit: (a) the current I is the same in all parts of the circuit; (b) the sum of the

voltages V1 , V2 and V3 is equal to the total applied voltage V:

V=V1+V2+V3

R= (R1+R2+R3)

50. From Ohm’s law:.

Potential divider:

51. Draw a parallel DC circuit with three resistances and express the circuit total resistance as a function of the three component resistances.

IRVIRVIRVIRV 332211321 IRIRIRIR

k

kRRRRR 321

INOUT VRR

RV

21

2

52. Draw a current divider with two branches and express the currents through the branches as a

function of the total current.

Kirchoff’s law

53. Write the expressions of electrical power and energy in DC circuits and precise the measurement

units for each quantity (explain each component in the relations).

W=P*t

54. Define the superposition theorem.

[W]VIP

321

1111

RRRR

0k

kI

In any network made up of linear resistances and containing more than one source of e.m.f., the

resultant current flowing in any branch is the algebraic sum of the currents that would flow in that

branch if each source was considered separately, all other sources being replaced at that time by their

respective internal resistances.

55. Define the Thevenin theorem.

Theorem states that a circuit of voltage and current sources and resistors can be converted into a

Thévenin equivalent, which consists of an ideal voltage source in series with an ideal resistor.

56. Norton’s theorem.

Norton’s theorem states that a circuit of voltage and current sources and resistors can be converted into

a Norton equivalent, which consists of an ideal constant-current source in parallel with an ideal resistor.

57. Maximum power transfer theorem.

The power transferred from a supply source to a load is at its maximum when the resistance of the load

is equal to the internal resistance of the source.

58. Define instantaneous value, peak value and rms value for an AC current.

Instantaneous values are the values of the alternating quantities at any instant of time. They are

represented by small letters, i, υ, e, etc.

A peak-to-peak value of e.m.f is the difference between the maximum and minimum values in a cycle.

The effective value (RMS)of an alternating current is that current which will produce the same heating

effect as an equivalent direct current.

59. Define the expression of a sinusoidal waveform quantity (explain each component in relation).

For general sinusoidal voltage, v=Vm sin(ωt ±φ) there is:

Amplitude or maximum value = Vm

Peak-to-peak value = 2 Vm

Angular velocity = ω rad/s

Periodic time, T= 2π/ω seconds

Frequency, f = ω/2π Hz (since ω = 2πf)

φ angle of lag or lead (compared with v=Vm sin ωt)

60. Draw an RL series circuit and the corresponding phasor diagram. Write the expression of

resulting voltage and impedance.

61.

61. Draw an RLC series circuit and the corresponding phasor diagram. Write the expression of resulting voltage and impedance.

LR jVVV

LjXRZ

62. Define electrical active power in R and RLC type AC circuits. P = VI cosθ Watts (W)

63. Define power triangle (active, reactive and apparent powers) and power factor in AC circuits.

Apparent power: S = VI voltamperes (VA)

Active power (true power):

P = VI cosθ Watts (W)

Reactive power: Q = VI sinθ reactive voltamperes (VAr)

Power factor: cosθ

)( CLR VVjVV

ZZXXjRZ CL )(

22 )( CL XXRZ

64. Draw an RC parallel circuit and the corresponding phasor diagram. Write the expression of resulting voltage and impedance.

65. Draw a star connection of a three-phase load and write the relation between the line and phase

quantities (currents and voltages).

CR jVVV

CjXRZ

Van=Vbn=Vcn=Vph VL=sqrt(3) Vph IL=Iph

66. Draw a delta connection of a three-phase load and write the relation between the line and phase quantities (currents and voltages) in case of delta connections.

`

phL VV phL II 3

67. Write the electrical active power in three-phase systems as a function of line and phase quantities

(currents and voltages).

P = VI cosθ Watts

68. Draw the BH and H characteristics for a ferromagnetic material (explain each quantity).

µ=B/ µ0*H, µ0=4π* H/m

69. Draw the hysteresis loop for a soft and hard ferromagnetic materials (explain each representative

point on the graph).

70. Give some examples of use for solid and liquid dielectrics

cos3cos33 LLphph IVIVP

solid : inorganic (ceramic and glass) materials (used in high-voltage overhead lines as suspension insulators, as bushings on high-voltage transformers and switchgear, etc.) plastic films (as insulation between the capacitors plates, slot insulation

of electrical machines, etc.)

flexible insulating sleeving (used for cables, in electrical machines, transformers, domestic and heating

appliances, light fittings, cable connections, switchgear etc.)

Liquid dielectrics are used as a filling and cooling medium for transformers, capacitors and rheostats, as

an arc-quenching medium in switchgear, as an impregnant of absorbent insulation used mainly in

transformers, switchgear, capacitors and cables etc.

71. Properties of copper and aluminum as conductor materials and their applications.

Copper has the highest electrical and thermal conductivity of the common industrial metals.

It has good mechanical properties, is easy to solder, is readily available and has high scrap

value. It is widely used in wire form.

Aluminium is less dense and cheaper than copper and its price is not subject to the same

wide fluctuations as copper. World production of aluminium has steadily increased over

recent years to overtake that of copper, which it has replaced in many electrical

applications.

72. What is a semiconductor material?

A semiconductor is able at room temperature to conduct electricity more than an insulator

but less than a conductor.

73. Define the ideal superconducting state and give examples of use for superconducting materials.

The ideal superconducting state is characterized by two fundamental properties, which are the

disappearance of resistance when the temperature is reduced to a critical value, and the

expulsion of any magnetic flux in the material when the critical temperature (TC) is reached.

The superconducting materials are used to produce high magnetic fields for: magnetic resonance

imaging, high-energy physics research, dc motors and generators, levitated trains, transformers, cables,

ac switches, etc.

74. Name the measurement instruments used to measure the following electric quantities: current,

voltage, resistance, power, energy.

Current: Ammeter

Voltage: Voltmeter

Rezistance: Ohmmeter

Power: Wattmeter

Energy: Energy meter

75. What are power electronics used for?

Power Electronics (PE) are

used to convert (i.e to process

and control) the flow of electric

power by supplying voltages

and currents in a form that is

optimally suited for user loads.

76. What is a diode? Draw the symbol and the I-V characteristic of a diode. The semiconductor diode is a

device that will conduct current in one direction only.

77. What is a thyristor? Draw the symbol and the I-V characteristic of a thyristor. The thyristor is a solid-state

semiconductor device with four

layers of alternating n and p-type

materials and three terminals.

78. What is a power transistor? Draw the symbol and the I-V characteristic of a BJ Transistor. A transistor is a semiconductor

device used to amplify and switch

electronic signals.

79. Explain the term switchgear and present the main classes of switchgear. This term covers the switching devices and their combination

with associated control, measuring, protective and regulating

equipment, together with accessories, enclosures and supporting

structures.

80. What are fuses and protection relays? Fuse and protection relays are specialized devices for ensuring the safety of personnel

working with electrical systems and for preventing damage due to various types of

faults.