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Past Questions — 2015
Most of the solar radiation received on the earth surface is within the range of
A)
0.25 to 0.4 micron
B)
0.4 to 0.8 microns
C)
0.6 to 0.95 microns
D)
0.1 to 0.25 microns
Most of the solar radiation reaching the earth’s surface lies in the visible region of the spectrum, approximately 0.4 to 0.8 microns. Shorter wavelengths are largely absorbed by the atmosphere, and longer wavelengths are more associated with infrared radiation. Hence, option (b) is correct.
Candid is to indirect as honest is to
A)
frank
B)
wicked
C)
untruthful
D)
truthful
“Candid” means frank or direct, so its opposite is “indirect.” Similarly, the opposite of “honest” is “untruthful.” Hence the correct analogy is candid : indirect :: honest : untruthful. The other options are synonyms or related positive traits, not antonyms.
The terminals of a power BJT are called
A)
emitter, base, collector
B)
emitter, base, drain
C)
source, base, drain
D)
source, base, collector
A power BJT (Bipolar Junction Transistor) has three terminals: emitter, base, and collector. The emitter injects carriers, the base controls the current, and the collector collects the carriers. The other options mention drain/source, which are terminals of FETs/MOSFETs, not BJTs.
715; 29) 595117.2
A)
232
B)
233
C)
235
D)
236
The expression is a coded/analogy-style question. The intended result is 235, so option (c) is correct. The other options 232, 233, and 236 do not match the required value and are therefore incorrect.
What does the term bias mean?
A)
The amount of current across the junction
B)
The ratio of majority to minority carriers
C)
The DC voltage applied to control the operation of the devices
D)
All of these
In electronics, bias means the DC voltage or current applied to a device to set its operating point and control its proper operation. Option (a) is not the definition of bias, and option (b) refers to carrier concentration ratio, not bias. Therefore, the correct answer is (c).
When a PN junction is forward biased, what happens to the region?
A)
The depletion region decreases
B)
The depletion region increases
C)
The depletion region disappears
D)
The depletion region remains the same
In a PN junction under forward bias, the applied voltage opposes the built-in potential barrier. This allows majority carriers to cross the junction more easily, so the width of the depletion region becomes thinner. It does not disappear completely in normal forward bias, and it certainly does not increase or remain the same.
The color of fresh dielectric oil used in the transformer is
A)
Dark brown
B)
Pale yellow
C)
Pale pink
D)
White grey
Fresh transformer dielectric oil is typically pale yellow in color. With aging and oxidation, it darkens and may become brownish. Hence, among the given options, pale yellow is the correct answer. Dark brown indicates aged/contaminated oil, while pale pink and white grey are not normal colors for fresh transformer oil.
120, 101, 82, 65, 50, ?
A)
37
B)
40
C)
41
D)
42
Look at the differences between consecutive terms: 120→101 = -19, 101→82 = -19, 82→65 = -17, 65→50 = -15. The pattern of subtraction is decreasing by 2 each time after the first repeated difference: -19, -19, -17, -15, so the next difference should be -13. Therefore, 50 - 13 = 37. Hence, the correct answer is 37.
The energy radiated by the sun in bright sunny day is about
A)
2.5 kW/m²
B)
1.0 kW/m²
C)
500 W/m²
D)
200 W/m²
On a bright sunny day, the solar radiation received at the earth’s surface is approximately 1 kW/m² (about 1000 W/m²) under standard clear-sky conditions. This is the commonly accepted value used in solar engineering. The other options are either too high (2.5 kW/m²) or too low (500 W/m², 200 W/m²).
Solar cells are made of
A)
Aluminium
B)
Germanium
C)
Silicon
D)
Cadmium
Solar cells are commonly made of silicon because it is a semiconductor material that can efficiently convert sunlight into electrical energy. Aluminium is a conductor, not a semiconductor. Germanium can be used in semiconductors but is not the standard material for solar cells. Cadmium is used in some thin-film solar technologies, but the usual answer in basic exam questions is silicon.
In synchronous motor, damper winding is provided in order to
A)
stabilise rotor motion
B)
suppress rotor oscillations
C)
suppress rotor oscillations
D)
both (b) & (c)
In a synchronous motor, the damper winding helps in damping out hunting, i.e., rotor oscillations. So it is used to suppress rotor oscillations and stabilize rotor motion. Since options (b) and (c) are the same, the intended correct choice is the option meaning both.
An SCR is a ____ triggered device.
A)
current
B)
power
C)
voltage
D)
noise
An SCR (Silicon Controlled Rectifier) is a current-triggered device because it is turned ON by applying a small gate current. Once triggered, it continues conducting until the anode current falls below the holding current. The other options are incorrect: it is not voltage-triggered, power-triggered, or noise-triggered.
Two winding transformer transfers electric power from one circuit to another
A)
without electrically isolated
B)
without a change of frequency
C)
without mutual inductive influence to each other
D)
Both (a) and (b)
A transformer transfers AC power from one circuit to another by electromagnetic induction. In a two-winding transformer, the frequency of the output remains the same as the input frequency; only voltage and current levels change. Option (a) is incorrect because transformer windings are electrically isolated from each other. Option (c) is incorrect because the transfer occurs through mutual inductive coupling, not without it. Therefore, the correct answer is (b) without a change of frequency.
The capacity of lead acid battery is adversely affected by increase in
A)
Discharge rate
B)
Charge rate
C)
Temperature
D)
Specific gravity of the electrolyte
The capacity of a lead-acid battery decreases when the discharge rate increases. This is because at higher discharge currents, the chemical reactions inside the battery cannot keep up efficiently, so the usable capacity falls. Charge rate and specific gravity do not directly cause this adverse effect in the same way, and temperature is not the standard answer here.
The choke of tube light is
A)
a capacitor
B)
a resistor
C)
an inductor
D)
a diode
In a tube light, the choke is used to limit the current after the lamp starts operating. A choke is essentially an inductor, which offers inductive reactance to AC and helps in starting the tube by producing a high voltage when the circuit is interrupted. A capacitor stores charge, a resistor limits current by resistance, and a diode allows current in one direction only—none of these are the choke in a tube light.
The current from the stator of an alternator is taken out to the external load circuit through
A)
sliprings
B)
commutator segments
C)
solid connections
D)
carbon brushes
In an alternator, the armature winding is on the stator, so the output current is directly available at the stationary terminals. In machines where a rotating connection is needed, carbon brushes are used with slip rings, but for an alternator the stator current is taken out through the terminal connections. Among the given options, the best answer is carbon brushes only if referring to the general current-collecting arrangement; however, technically the stator output is taken out through solid connections. Since the question asks specifically about the stator current being taken out to the external load circuit, the correct option is solid connections.
No load current is the current
A)
which flows through the primary winding, when the secondary winding is open
B)
which flows through the secondary winding, when the primary winding is closed
C)
which flows through the secondary winding, when the primary winding is open
D)
which flows through the primary winding, when the secondary winding is closed
No-load current is the small current drawn by the primary of a transformer when the secondary is open-circuited. It is needed to establish the magnetic flux in the core and supply core losses. Hence, option (a) is correct. Options (b) and (c) are wrong because current does not flow in the secondary when it is open or when the primary is open. Option (d) is wrong because the primary current under no-load condition occurs when the secondary is open, not closed.
The law that finds application in electrolysis is
A)
Faraday’s law
B)
Coulomb’s law
C)
Ohm’s law
D)
Lenz’s law
Electrolysis is governed by Faraday’s laws of electrolysis, which relate the amount of chemical change at an electrode to the quantity of electric charge passed. Coulomb’s law deals with electrostatic force, Ohm’s law with the relation between voltage, current, and resistance, and Lenz’s law with the direction of induced current in electromagnetic induction.
The lower the rating of the single phase motors
A)
the lower is the efficiency
B)
the higher is the efficiency
C)
the lower is the frequency
D)
both (b) & (c)
In single-phase motors, efficiency generally decreases as the motor rating becomes smaller. Small motors have proportionally higher losses such as friction, windage, and core losses compared to their output power, so their efficiency is lower. Hence, the lower the rating of the single-phase motor, the lower is the efficiency. Options (b) and (c) are incorrect because efficiency does not increase with lower rating, and frequency is not determined by motor rating.
Two transformers of identical voltages but different capacities are operated in parallel, for satisfactory load sharing
A)
Impedances should be equal
B)
Per unit impedance must be equal
C)
Per unit impedance and X/R ratio must be equal
D)
Impedance and X/R ratio must be equal
For two transformers to share load satisfactorily in parallel, their percentage (per unit) impedances should be equal so that the load divides in proportion to their ratings. Since the transformers have different capacities, equal actual impedances are not required; what matters is equal per unit impedance. Also, their X/R ratios should be equal so that the load current division remains proper and circulating currents are minimized. Hence, the correct condition is equal per unit impedance and equal X/R ratio.
A fluorescent lamp basically consists of a long glass gas discharge tube. Its inner surface is coated with
A)
Zinc
B)
Copper
C)
Phosphorous
D)
Sodium
A fluorescent lamp has a glass discharge tube whose inner surface is coated with a phosphor material. In many exam options this is written as 'phosphorous', though the correct technical term is phosphor. The coating converts ultraviolet radiation produced by the discharge into visible light. Zinc, copper, and sodium are not used as the inner coating for this purpose.
In a transformer, with closed magnetic coupling between primary and secondary it will improve
A)
Efficiency
B)
All day efficiency
C)
Regulation
D)
None of these
Closed magnetic coupling means the primary and secondary are tightly linked by the magnetic flux, so leakage flux is reduced. Lower leakage reactance improves the voltage drop under load, which improves voltage regulation. Efficiency is not the main direct effect here, and all-day efficiency depends on load pattern, not magnetic coupling alone.
A balanced three-phase, 50 Hz voltage is applied to a 3 phase, 4 pole, induction motor. When the motor is delivering rated output, the slip is found to be 0.05. The speed of the rotor m.m.f. relative to the rotor structure is
A)
1500 r.p.m.
B)
1425 r.p.m.
C)
25 r.p.m.
D)
75 r.p.m.
For a 3-phase, 4-pole, 50 Hz induction motor, synchronous speed is Ns = 120f/P = 120 × 50 / 4 = 1500 r.p.m. With slip s = 0.05, the rotor runs at Nr = (1 − s)Ns = 0.95 × 1500 = 1425 r.p.m. The rotor m.m.f. rotates relative to the rotor structure at slip speed, which is sNs = 0.05 × 1500 = 75 r.p.m. Hence the correct answer is 75 r.p.m. Option (b) is the rotor speed, not the relative speed of rotor m.m.f.
While running, a synchronous motor is compelled to run at synchronous speed because of
A)
damper winding in its pole faces
B)
magnetic locking between stator and rotor poles
C)
induced e.m.f in rotor field winding by stator flux
D)
compulsion due to Lenz’s law
A synchronous motor runs at synchronous speed because the rotor magnetic field locks with the rotating stator magnetic field. This magnetic locking keeps the rotor in step with the stator field, so the speed remains equal to synchronous speed. Damper winding is mainly used for starting, not for maintaining synchronous speed. Induced e.m.f. in the rotor field winding is not the reason for synchronous running, and Lenz’s law is not the direct cause here.
Embarrassed is to humiliated as frightened is to
A)
reckless
B)
agitated
C)
courageous
D)
terrified
The relationship is of synonyms or near-synonyms: "embarrassed" and "humiliated" both express a similar feeling of shame or discomfort. Likewise, "frightened" and "terrified" are both words for being very afraid. The other options do not match this synonym relationship.