7. Electrical Question & Answer (Points) for Tests / Interview

Ø  AC bridge circuits such as wheatstone, Maxwell’s Inductance Bridge etc., are used for the precision measurement of AC resistance, capacitance and inductance.

Ø  Active power is obtained by multiplying kVA by cos φ and its unit is  kW

Ø  As per Kirchhoff’s current law, in any electrical network, the phasor sum of the currents meeting at a junction is zero.

Ø  As per Kirchhoff’s voltage law, the phasor sum of the voltage drops across each of the conductors in any closed path (or mesh) in a network plus the phasor sum of the e.m.fs. connected in that path is zero.

Ø  Maxwell’s L/C bridge is so called because for balance, it uses two opposite impedances in opposite arms.

Ø  Maxwell-Wien bridge is used for measuring inductance.

Ø  Power Factor may be defined as the ratio of true power / apparent power i.e (W/VA)

Ø  Reactive power is known as reactive kVA and is obtained by multiplying kVA by sin φ. It is written by kVAR.

Ø  The P.F of an R-C circuit is between zero and 1.

Ø  The unit of admittance is Siemens.

Ø  The unit of capacitive susceptance is Siemens.

 

6. Electrical Question & Answer (Points) for Tests / Interview

Ø  The reduction of battery capacity at high rates of discharge is primarily due to the rapid formation of lead sulphate on the cell plates.

Ø  The time taken by an alternating quantity to complete one cycle is called its time period (T). (e.g. 50-Hz alternating current has a time period of 1/50 second)

Ø  The number of cycles/second is known as the frequency of the alternating quantity. Its unit is hertz (Hz).

Ø  The maximum value, positive or negative, of an alternating quantity is known as its amplitude.

Ø  The r.m.s. value of an AC is given by that steady (DC) current which when flowing through a given circuit for a given time produces the same heat as produced by the AC when flowing through the same circuit for the same time.

Ø  For the symmetrical sinusoidal current, r.m.s. value is                         0.707 × maximum value of current.

Ø  The r.m.s. value of a complex current wave is equal to the square root of the sum of the squares of the r.m.s. values of its individual components.

Ø  Average value of current is 0.637 × maximum value of current.

Ø  R.M.S. value is always greater than average value except in the case of a rectangular wave when both are equal.

Ø  Form factor is defined as the ratio of r.m.s value / average value           (i.e. 1.11)

Ø  Vectors are a shorthand for the representation of alternating voltages and currents and their use greatly simplifies the problems in AC work.

Ø  Various forms or methods of representing vector quantities are Symbolic Notation, Trigonometrical Form, Exponential Form and Polar Form.

Ø  Power Factor may be defined as the cosine of the angle of lead or lag.

Ø  Power Factor may be defined as the ratio of resistance / impedance (R/Z).

 

5. Electrical Question & Answer (Points) for Tests / Interview

Ø  A moving coil voltmeters measures only DC voltages.

Ø  Bench charge is given to the battery when taken off the vehicle.

Ø  Damping force acts on the moving system of an indicating instrument only when it is moving.

Ø  During the charging and discharging of a nickel iron cell water is neither formed nor absorbed.

Ø  Eddy current is the most efficient form of damping employed in electrical instruments.

Ø  Floating battery systems are widely used for power stations, emergency lighting and telephone exchange installations.

Ø  kWh meter can be classified as recording instrument.

Ø  Moving iron instruments can be used for measuring direct currents and voltages and alternating current and voltages.

Ø  One complete set of positive and negative values of alternating quantity is known as one cycle.

Ø  Permanent magnet moving coil ammeter have uniform scales because they are spring-controlled and their deflecting torque varies directly as current.

Ø  Permanent magnet type meter is suitable for only direct current measurements.

Ø  Sediment accumulates at the bottom of a lead-acid battery consists largely of antimony-lead alloy.

Ø  The efficiency of a nickel-iron cell is less due to its higher internal resistance.

Ø  The hot-wire ammeter reads equally well on DC and / or AC circuits.

Ø  The moving system of an indicating type of electrical instrument is depending upon a deflecting torque, controlling torque and damping torque.

Ø  The reduction of battery capacity at high rates of discharge is primarily due to the rapid formation of lead sulphate on the cell plates.

 

4. Electrical Question & Answer (Points) for Tests / Interview

Ø  The magnetising force at the centre of a circular coil varies directly as the current, varies directly as its radius and varies inversely as its radius.

Ø  The ratio of Ah efficiency to Wh efficiency of a lead-acid cell is always greater than one.

Ø  The resistance offered to the radial leakage (at right angles to the flow of useful current) of current is called insulation resistance of the cable.

Ø  The SI unit of electric intensity is V/m (volt/metre) or N/C (newton/coulomb).

Ø  The SI unit of magnetic flux is weber.

Ø  The unit of absolute permittivity of a medium is farad/metre.

Ø  The unit of capacitance is coulomb/volt i.e. farad

Ø  The various electrical instruments can be divided into absolute instruments and secondary instruments.

Ø  The working principle of thermo-couple ammeter is based on the Seebeck effect.

Ø  Trickle charging of battery is used to balance the open-circuit losses.

Ø  Two heaters, rated at 1000 W, 250 volts each, are connected in series across a 250 Volts 50 Hz A.C. mains. The total power drawn from the supply would be 500 watt.

Ø  Two similar electric charges of 1 C each are placed 1 m apart in air. Force of repulsion between them would be nearly 9 × 109 newton.

Ø  Two types of e.m.f are dynamically induced and statically induced.

Ø  Trickle charging of a storage battery helps to keep it fresh and fully charged.

Ø  Sulphation in a lead-acid battery occurs due to incomplete charging.

Ø  The constant-voltage system of charging a lead-acid cell has the advantage of reduced time of charging and increased cell capacity.

Ø  The capacity of a lead-acid cell does not depend on its rate of charge.

 

3. Electrical Question & Answer (Points) for Tests / Interview

Ø  Maxwell’s loop current method of solving electrical networks utilizes Kirchhoff’s voltage law.

Ø  Megger is a portable instrument used for testing the insulation resistance of a circuit in terms of Mega ohms.

Ø  One farad is the capacitance of a capacitor which requires a charge of one coulomb to establish a potential difference of one volt between its plates.

Ø  One kWh of energy equals nearly 860 kcal.

Ø  Permanent magnets are normally made up of alnico alloys.

Ø  Relative permeability of vacuum is 1.

Ø  Secondary instruments are those, in which the value of electrical quantity to be measured can be determined from the deflection of the instruments, only when they have been pre-calibrated by comparison with an absolute instrument.

Ø  Sources for Generation of Electricity using Conventional methods are Thermal and Hydro-Electric.

Ø  Sources for Generation of Electricity using Non-Conventional methods are wind power, photo-voltaic cells and Fuel cells.

Ø  Statically induced e.m.f is further sub-divided into mutually induced e.m.f. and self-induced e.m.f.

Ø  The algebraic sign of an IR drop is primarily dependent upon the direction of current flow.

Ø  The capacitance of a capacitor is not influenced by plate thickness.

Ø  The capacity of a cell is measured in ampere-hours.

Ø  The damping force in an instruments can be produced by air frictions, eddy currents and fluid friction.

Ø  The direction of the induced current is decided by applying either Fleming’s Right-hand Rule / Flat-hand rule / Lenz’s Law.

Ø  The electric energy required to raise the temperature of a given amount of water is 1000 kWh. If heat losses are 25%, the total heating energy required is 1333 kWh.

2. Electrical Question & Answer (Points) for Tests / Interview

Ø  During the charging of a lead-acid cell its voltage increases.

Ø  Electric flux emanating from an electric charge of + Q coulomb is Q.

Ø  Electromagnetic effect is utilized for wattmeters and watthour meters.

Ø  Electrostatic effect is utilized only for voltmeters only.

Ø  End-cell control system is an economical system developed and used in place of using rheostat in batteries for controlling the voltage.

Ø  For a given line voltage, four heating coils will produce maximum heat when all the four coils connected in parallel.

Ø  For any medium, electric flux density D is related to electric intensity E by the equation D = ε ₀ ε _r E

Ø  If a 220 V heater is used on 110 V supply, heat produced by it will be one-fourth as much.

Ø  If D is the electric flux density, then value of electric intensity in air

is D = ε_o

Ø  If dielectric slab of thickness 5 mm and ε_r = 6 is inserted between the plates of an air capacitor with plate separation of 8 mm, its capacitance is almost doubled.

Ø  If relative permittivity of mica is 5, its absolute permittivity is 5 ε_o.

Ø  In practice, earth is chosen as a place of zero electric potential because it has almost constant potential.

Ø  Induction type instruments are used for AC measurements only. It can be used either as ammeter, voltmeter and wattmeter.

Ø  Inside a charged conducting sphere, the potential and electric intensity is Zero.

Ø  Inside a conducting sphere, potential remains constant.

Ø  Kirchhoff’s current law is applicable to junctions in a network.

Ø  Kirchhoff’s voltage law is concerned with IR drops and battery e.m.f.

Ø  Magnetic effect, Thermal effect, Electrodynamic effect and Electromagnetic effect is utilized for ammeters and voltmeters.

1. Electrical Question & Answer (Points) for Tests / Interview

Ø  A capacitor consists of two conductors separated by an insulator.

Ø  A force of 10,000 N accelerates a body to a velocity 0.1 km/s. Then, the power developed is 1000 kW.

Ø  Absolute instruments are those which give the value of the quantity to be measured, in terms of the constants of the instrument and their deflection only.

Ø  According to Faraday’s Laws of Electromagnetic Induction, an e.m.f. is induced in a conductor whenever it cuts magnetic flux.

Ø  According to Gauss’s theorem, the surface integral of the normal component of electric flux density D over a closed surface containing charge Q is Q.

Ø  A 100 W light bulb burns on an average of 10 hours a day for two weeks. The weekly consumption of energy will be 14 units.

Ø  According to Kirchhoff’s voltage law, the algebraic sum of all IR drops and e.m.f. in any closed loop of a network is always zero.

Ø  Active materials of a lead-acid cell are lead peroxide, dilute sulphuric acid and sponge lead.

Ø  Advantages of Nuclear Generation is that the Quantity of fuel required is small for generating a given amount of electrical energy, compared to other fuels. It is more reliable, cheaper for running cost, and it is efficient.

Ø  An e.m.f and the current is induced in any conductor which is cut across or is cut by a magnetic flux is known as electromagnetic induction.

Ø  Chemical effect is utilized for DC ampere-hour meters.

Ø  Circle diagrams are useful for analysing the operating characteristics of circuits, which, under some conditions, are used in representing transmission lines and AC Electrical machineries.

Ø  Damping force is one which acts on the moving system of the instrument only when it is moving and always opposes its motion.

Ø  Disadvantages of Nuclear Generation is that the Fuel is expensive and not abundantly available everywhere. It has high capital cost and maintenance cost. Disposal of Nuclear waste is a problem.