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Electrical Technology
September 1
2017 Website: www.electricaledu.com Electrical Engg.(MCQ) Question and Answer for the students of SSC(JE), PSC(JE), BSNL(JE), WBSEDCL, WBSETCL, WBPDCL, CPWD and State and Central Service Examination
1st year 2nd semester
Electrical Technology
Contents
Unit-1(Difference sources of Energy)
i. Conventional sources and Non-conventional sources
ii. Advantages of electrical energy
iii. Uses of electrical energy
iv. Electric charge
v. Electric current
vi. Electric potential
vii. Power: Electrical energy
viii. Resistance
ix. Inductance
x. Capacitance
D.C. Circuits
i. Ohms law
ii. Factors at which the resistance of a conductor depends
iii. Resistivity
iv. Temperature co-efficient of resistance
v. Series combination of resistance
vi. Parallel combination of resistance
vii. Parameter
viii. Circuit
ix. Electrical network
x. Active network
xi. Passive network
xii. Node
xiii. Branch
xiv. Loop
xv. Kirchhoffβs current law
xvi. Kirchhoffβs voltage law
xvii. Sign of e.m.f.
xviii. Sign of IR drops
xix. Equivalent resistance
xx. Branch current
A.C. Circuits
i. Alternating voltage
ii. Wave form of voltage
iii. Wave form of current
iv. Cycle,Time, period, Frequency, Average value, R.M.S. value, Form factor,Peak factor, Phase, Phase
difference
v. Vector Diagram
vi. A.C through pure resistive circuit
vii. A.C. through pure inductive circuit
viii. A.C. through pure capacitive circuit
ix. R-L series circuit
x. R-C series circuit
xi. R-L-C series circuit
xii. Impedance, Reactance, Inductive reactance, Capacitance reactance
xiii. Power factor,Type of power factor
xiv. Power triangle
xv. Impedance table
Unit-II (Electromagnetism)
i. Definition, Magnetic field around a straight current carrying conductor
ii. Right hand gripping rule
iii. Magnetic field around a solenoid
iv. Force on a conductor placed in the magnetic field
v. Force between parallel current carrying conductor
vi. Magneto-motive force (mmf)
vii. Magnetic field intensity
viii. Magnetic flux
ix. Magnetic flux density
x. Permeability
xi. Reluctance
xii. Permeance
xiii. Comparison between magnetic circuit and electric circuit
xiv. Magnetic hysteresis, Hysteresis loop, Hysteresis loss
Electromagnetic induction
i. Faradays law
ii. Lenzβs law
iii. Self inductance and Mutual inductance
iv. Eddy current and Eddy current loss
v. Flemingβs Right hand rule and Flemingβs Left hand rule Electrical Machines
i. Working principle of D.C. generator ii. Type of D.C. Generator iii. Parts of D.C. Generator iv. Working principle of D.C. motor v. Back e.m.f. vi. Types of D.C. motor vii. Application of D.C motor viii. Working principle of transformer ix. E.m.f. equation x. For an ideal transformer
xi. Voltage transformation ratio xii. Rating of transformer xiii. Classification of transformer xiv. Application of transformer
Storage cell i. Difference between primary and secondary cell ii. Name of active material used in lead acid cell iii. Chemical reaction during discharging iv. Chemical reaction during charging v. Indication of full charge battery
Unit-III (Electrical power supply systems) i. Comparison between A.C. and D.C. transmission system ii. Draw the layout of power system or Draw the single line diagram of supply system from generating
station to consumers iii. Idea of Generation, Transmission and Distribution iv. Types of power station or generating station v. Name of thermal power station in West Bengal vi. Hydel power station vii. Some non-conventional sources viii. Single phase system ix. Three phase system x. Star connection xi. Delta connection
Domestic power supply i. Service connection for house wiring ii. Types of house wiring iii. Conductor used iv. Fuse v. Some common name of fuse vi. Earthing, necessity of earthing vii. Type of earthing viii. Connection diagram of two way switch
Measuring and testing of instruments i. Ammeter ii. Voltmeter iii. Circuit diagram for single phase energy meter iv. Meter constant v. Specification of energy meter vi. Multi meter vii. Megger viii. Application of Megger ix. Type of instruments
Unit-1(Difference sources of Energy)
Conventional sources: Traditional sources are known as conventional source. Such as coal, petrol, diesel, nuclear
fuel etc.
Non-conventional sources: Non-traditional sources are known as non-conventional source. Such as solar energy,
wind energy, tidal energy and bio-gas etc.
Advantages of electrical energy:
(i) It is transportable over a long distance.
(ii) It is pollution less.
(iii) It can be converted into any other form of energy.
(iv) It can be stored for future use.
Uses of electrical energy:
(i) Lighting
(ii) Heating
(iii) Electrical motor
(iv) Electroplating
(v) Electronics
(vi) Welding
Basic concepts of electrical quantities
Electric charge: Electric charge is the property that causes to experience a force when close to other charged body.
It is denoted by Q and unit is Coulomb (C). It is two types:
(i) Positive charge
(ii) Negative charge
Electric current: The flow of free electron from higher potential to lower potential is known as current. It is
denoted by I and unit is Ampere (A).
Electric potential: The amount of work done per unit charge is known as electric potential. It is denoted by V and
unit is Volt. i.e. V = π
π
Power: The energy consumed per unit time is known as power. It is denoted by P and unit is Watt (W)
Electrical energy: The energy consumed in a given time is known as electrical energy. It is denoted by E and unit is
kilo-watt-hour (kWh).
Resistance: It is the property of material by virtue of which it opposes the flow of electric current through it. It is
denoted by R and unit is Ohm (Ξ©).
Inductance: It is the property of a material by virtue of which it opposes any change of current passing through it.
It is denoted by L and unit is Henry (H).
Capacitance: It is the capacity of a condenser to store electrical energy in its dielectric medium. It is denoted by C
and unit is farad (F) or micro-farad (Β΅F).
D.C. supply A.C. supply
1. Its polarity cannot change with time Its polarity changes with time
2. It can be stored It cannot be stored
3. Example of DC supply is battery or cell Example of AC supply is generator
4. DC symbol is AC symbol is
Instruments To be measured Unit
Ammeter Current Ampere (A)
Voltmeter Voltage Volt (V)
Wattmeter Power Watt (W), kilo-watt (kW)
Energy meter Energy kWh
Multi meter Current, voltage, resistance --
Parameter Symbol Unit
Resistance (R)
Ohm (Ξ©) or kilo-ohm (kΞ©)
Inductance (L)
Henry (H) or mili-henry (mH)
Capacitance (C)
Farad (F) or micro-farad (Β΅F)
D.C. Circuits
Ohms law: The potential difference (V) between the end of the conductor is directly proportional to the current (I)
flowing through the conductor, keeping the temperature and other physical condition unchanged.
i.e. V Ξ± I
or, V=IR
Where, R is the resistance.
Note: Ohms law is not applicable to the non-linear device such as diode, transistor, SCR, electrolytes, arc lamp,
semi-conductor etc.
Law of resistance: The resistance of a conductor material is directly proportional to the length of the conductor
and inversely proportional to the cross-sectional area of the conductor at constant temperature.
i.e. R Ξ± π
π
or R = πππ
Where, Ο is the specific resistance or resistivity
Factors at which the resistance of a conductor depends:
(i) Length of conductor (RΞ±l)
(ii) Cross-sectional area of the conductor (RΞ±1
π)
(iii) Working temperature: The resistance of pure metals or alloys increases with the rise of temperature.
But in case of semi-conductor, insulator and electrolytes decreases the resistance with the rise of
temperature.
(iv) Nature of current: Due to skin effect the resistance of the conductor is higher in A.C. than that of D.C.
(nearly about 15% to 20%)
Resistivity: The resistivity of a conductor may be defined as the resistance offered by unit length of the conductor
and having unit cross-section of the conductor. Its unit is ohm-meter (Ξ©m)
Temperature co-efficient of resistance: The temperature co-efficient of resistance of a conductor is the change in
resistance per ohm per degree change in temperature counting from 00. It is denoted by alpha (Ξ±).
Let, R0 = resistance of the material at 00
R1 = β β β β π‘10C
R2 = β β β β π‘20C
Then we can write,
R1= R0(1+Ξ±0t)
and
R2= R1[1+Ξ±(t2 β t1)]
Series combination of resistance: Let the three pure resistances R1, R2 and R3 are connected in series across a d.c.
voltage source V.
V = VR1 + VR2 + VR3
= IR1 + IR2 + IR3
= I(R1 + R2 + R3) β¦β¦β¦β¦β¦.(i)
If the equivalent resistance is Req then,
V = IReq β¦β¦β¦β¦β¦β¦..(ii)
Therefore, IReq = I(R1 + R2 + R3)
And
Req = R1 + R2 + R3
Parallel combination of resistance: Let the three resistances R1, R2 and R3 are connected in parallel across a d.c.
voltage V. Let the source current is I and branch current are I1, I2 and I3 respectively.
Since the voltage drop in each branch is same
Then, V = VR1 = VR2 = VR3
And I = I1 + I2 + I3 .................(i)
If the equivalent resistance is Req
Then, V = IReq β¦β¦β¦β¦β¦β¦(ii)
Or π
π ππ =
π
π 1 +
π
π 2 +
π
π 3
Or
1
π ππ =
1
π 1 +
1
π 2 +
1
π 3
Parameter: The different elements of an electrical circuit such as resistance, inductance and capacitance are
known as parameter.
Circuit: It is the conducting path through which the electric current flows or tends to flow.
Electrical network: A combination of different elements such as resistance, inductance and capacitance are
connected in any manner is called an electrical network.
Active network: The network which contains one or more sources of e.m.f. is called Active network.
Passive network: The network which has no source of e.m.f. is called Passive network.
Node: It is the junction point of two or more branches of the network.
Branch: It is the part between two junctions.
Loop: It is the closed path of the circuit.
Kirchhoffβs current law: In any electrical network the algebraic sum of the currents meeting at a point is zero.
Assuming incoming current to be positive (+ve) and outgoing current to be negative (-ve)
Therefore, I1 + (-I2) + (-I3) + (+I4) + (-I5) = 0
Or, I1 + I4 = I2 + I3 + I5
Or, Incoming current = outgoing current
Therefore, β π = 0
Kirchhoffβs voltage law: The algebraic sum of the products of current and resistance plus the algebraic sum of
e.m.f. in any closed network is zero.
β I0R0 + β e. m. f = 0
Sign of e.m.f.:
A rise in potential should be +ve
A fall in potential should be -ve
As we go from the βve terminal of a battery to its +ve terminal, there is a rise in potential. Hence the sign should
be +ve. Similarly if we go from the +ve terminal to βve terminal, there is a fall in potential. And sign should be βve.
Sign of IR drops: If we go through a resistor in the same direction as the current, then there is a fall in potential
and sign will be βve.
If we go through a resistor in the opposite direction to the current then there is a rise in potential and sign will be
+ve.
Applying KVL,
-I1R1 β I2R2 β E2 + I3R3 + E1 β I4R4 = 0
Or, - I1R1 β I2R2 + I3R3 β I4R4 = E2 β E1
Equivalent resistance:
1
π ππ =
1
π 1 +
1
π 2
Req = π 1π 2
π 1+π 2
Branch current:
I1 = IΓπ 2
π 1+π 2 and I2 = IΓ
π 1
π 1+π 2
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