← MES Jr. Grade
Past Questions — 2015
The transfer function of a continuous time system is described by Y(s)/X(s) = (s^2 + 4s + 3) / (s^3 + 4s^2 + 1). The number of state variables in the state model of the system is
A)
1
B)
2
C)
3
D)
5
For a continuous-time LTI system, the number of state variables in a minimal state-space model equals the order of the transfer function denominator after any pole-zero cancellations. Here, the transfer function is (s^2 + 4s + 3) / (s^3 + 4s^2 + 1). The denominator is a cubic polynomial, so the system is third order. Therefore, it requires 3 state variables. The numerator degree does not determine the number of states, and there is no indication of cancellation that would reduce the order.
Which of these has lowest dielectric strength?
A)
Dry air
B)
Wet air
C)
Cotton
D)
Paper
Dielectric strength is the maximum electric field a material can withstand without breaking down. Among the given options, wet air has the lowest dielectric strength because moisture increases ionization and makes electrical breakdown easier. Dry air has higher dielectric strength than wet air, while cotton and paper are solid insulating materials with comparatively higher dielectric strength than air.
The Fourier transform (FT) of a function x (t) is X (f). The FT of dx(t)/dt will be
A)
dX(f) df
B)
2infX(f)
C)
j2πfX(f)
D)
X(f) Gf
Using the Fourier transform differentiation property, if x(t) ↔ X(f), then d x(t)/dt ↔ j2πf X(f). This is because differentiation in the time domain corresponds to multiplication by j2πf in the frequency domain. Hence option (c) is correct. The other options do not match the standard FT property.
In an intrinsic semiconductor, the number of electrons is equal to the number of holes at which temperature?
A)
0°K
B)
0°c
C)
high temperature
D)
at all temperature
In an intrinsic semiconductor, electron-hole pairs are generated thermally in equal numbers. Therefore, the number of electrons is always equal to the number of holes at any temperature, including room temperature and other temperatures. Option (a) is wrong because at 0 K an intrinsic semiconductor has no free carriers. Options (b) and (c) are not correct because the equality is not limited to only 0°C or high temperature.
In which region are both the collector-base and base-emitter junctions forward-biased?
A)
Active
B)
Cutoff
C)
Saturation
D)
All of these
In a BJT, the saturation region occurs when both the base-emitter junction and the collector-base junction are forward-biased. In the active region, the base-emitter junction is forward-biased but the collector-base junction is reverse-biased. In cutoff, both junctions are reverse-biased. Therefore, the correct answer is Saturation.
Under low level injection assumption, the injected minority carrier current for an extrinsic semiconductor is essentially the
A)
diffusion current
B)
drift current
C)
recombination current
D)
induced current
Under low-level injection in an extrinsic semiconductor, the injected minority carriers create a concentration gradient. Carrier motion due to this gradient is diffusion, so the injected minority carrier current is essentially diffusion current. Drift current is mainly due to an electric field, recombination current is not the transport mechanism here, and induced current is not relevant in this context.
One eV is equal to J.
A)
6.02x107
B)
1.6x10-°
C)
1.6x10-19
D)
1.66x10-4
1 electron volt (eV) is the energy gained by an electron when it moves through a potential difference of 1 volt. Its value in SI units is 1 eV = 1.602 × 10^-19 joule. So the correct option is (c). The other options are not the standard conversion value of eV to joule.
Which of the following is the poorest electrical conductor?
A)
steel
B)
carbon
C)
copper
D)
aluminium
Among the given materials, copper and aluminium are good electrical conductors, carbon conducts electricity better than steel in many forms (especially graphite), while steel has much higher electrical resistivity than the others. Therefore, steel is the poorest electrical conductor of the options.
Thevenin’s theorem cannot be applied to networks that contain elements which are
A)
non-linear
B)
linear
C)
active
D)
passive
Thevenin’s theorem is applicable only to linear bilateral networks. It cannot be directly applied to networks containing non-linear elements because the V-I relationship is not proportional, so a single equivalent Thevenin source and resistance cannot represent the circuit over all operating points. Linear, active, and passive elements can still be part of a Thevenin-equivalent network as long as the overall network is linear.
The property of a material which opposes the production of magnetic flux in it is known as
A)
mmf
B)
reluctance
C)
permeance
D)
permittivity
The property of a magnetic circuit that opposes the establishment of magnetic flux is called reluctance. It is analogous to electrical resistance in a circuit.
- mmf (magnetomotive force) is the driving force that produces magnetic flux, not the opposing property.
- permeance is the ease with which flux is established; it is the inverse of reluctance.
- permittivity relates to electric fields, not magnetic flux.
Therefore, the correct answer is reluctance.
Forbidden band is largest in
A)
insulator
B)
semiconductor
C)
conductor
D)
metal alloy
The forbidden band (band gap) is the energy gap between the valence band and the conduction band. In insulators, this gap is the largest, so electrons cannot easily move to the conduction band. Semiconductors have a smaller gap, while conductors/metals have little to no gap.
When ferromagnetic substance is magnetised, there are small change in dimensions. The phenomenon is called
A)
hysteresis
B)
magnetostiction
C)
diamagnetism
D)
dipolar relaxation
When a ferromagnetic material is magnetised, it undergoes a slight change in shape or dimensions due to the alignment of magnetic domains. This phenomenon is called magnetostriction. Option (b) is the intended answer, though the correct scientific term is usually spelled "magnetostriction". Hysteresis refers to lag in magnetisation, diamagnetism is a weak form of magnetism opposite to the applied field, and dipolar relaxation is related to the reorientation of dipoles in a material.
What causes electromagnetic wave polarisation?
A)
Transverse nature of electromagnetic wave
B)
Longitudinal nature of electromagnetic wave
C)
Refraction
D)
Reflection
Electromagnetic waves are polarised because their electric and magnetic fields oscillate perpendicular to the direction of propagation, i.e., they are transverse waves. Polarisation is a property of transverse waves only. Longitudinal waves cannot be polarised, and refraction or reflection may change the state of polarisation but do not cause it fundamentally.
If the bandwidth of an oscilloscope is given as direct current to 10 MHz, what is the fastest rise time a sine wave can have to be produced accurately by the oscilloscope?
A)
35 ns
B)
10 ns
C)
3.5 ns
D)
0.035 ns
For an oscilloscope, the approximate relationship between bandwidth and rise time is t_r 0.35 / BW. Here BW = 10 MHz, so t_r = 0.35 / (10 00d7 10^6) = 35 d7 10^-9 s = 35 ns. Therefore, the fastest rise time that can be reproduced accurately is 35 ns. The other options do not match the standard bandwidth-rise time relation.
Pure metals generally have
A)
high conductivity and low temperature coefficient
B)
high conductivity and high temperature coefficient
C)
low conductivity and zero temperature coefficient
D)
low conductivity and high temperature coefficient
Pure metals generally have many free electrons, so they conduct electricity well. As temperature increases, lattice vibrations increase and electron scattering becomes greater, so their resistance rises noticeably. Hence pure metals have high conductivity and a high temperature coefficient of resistance. Option (a) is wrong because pure metals do not have a low temperature coefficient; option (c) is wrong because conductivity is not low and the temperature coefficient is not zero; option (d) is wrong because conductivity is not low.
Unit of electric intensity is :
A)
joules/coulomb
B)
newton/coulomb
C)
volt/meter
D)
both (b) and (c)
Electric intensity means electric field intensity. Its SI unit can be written as newton per coulomb (N/C) or equivalently volt per meter (V/m). Since both units are correct and equivalent, the correct option is (d). Option (a) joule/coulomb is the unit of electric potential, not electric intensity.
Which of the following is correct? As frequency increases, the surface resistance of a metal
A)
Decreases
B)
Increases
C)
Remains unchanged
D)
Varies in an unpredictable manner
For a metal, as frequency increases, the skin effect becomes stronger. Current tends to flow only near the surface, reducing the effective conducting area. Hence the surface resistance increases with frequency. Therefore, option (b) is correct. Options (a), (c), and (d) are incorrect because the resistance does not decrease, remain constant, or vary randomly; it rises systematically with frequency.
When the temperature of a magnetic material is raised above the Curie point, it becomes
A)
Diamagnetic
B)
Paramagnetic
C)
Ferromagnetic
D)
Ferrimagnetic
Above the Curie point, a ferromagnetic material loses its spontaneous magnetization because thermal agitation overcomes the alignment of magnetic domains. It no longer remains ferromagnetic and behaves as a paramagnetic substance. Diamagnetic and ferrimagnetic are not the correct general behavior above the Curie temperature.
The current in an RLC series circuit at resonance is
A)
maximum
B)
minimum
C)
infinity
D)
zero
In a series RLC circuit at resonance, the inductive reactance equals the capacitive reactance, so the net reactance becomes zero. Therefore the impedance is minimum and equals only the resistance R. Since current I = V/Z, the current becomes maximum at resonance. Hence, option (a) is correct. The other options are incorrect because the current is not minimum, infinite, or zero in a practical series RLC circuit at resonance.
The errors introduced by an instrument fall in which category?
A)
Systematic errors
B)
Random errors
C)
Gross errors
D)
Environmental errors
Errors introduced by an instrument are generally due to imperfections in the instrument itself, such as calibration error, zero error, or drift. These are classified as systematic errors because they produce a consistent bias in the measurements. Random errors vary unpredictably, gross errors are human mistakes, and environmental errors are caused by external conditions like temperature or humidity.
Schering bridge can be used to measure which one of the following?
A)
Q of a coil
B)
Inductance and its Q-value
C)
Very small resistance
D)
Capacitance and its power factor
A Schering bridge is an AC bridge used primarily for measuring an unknown capacitance and the dielectric loss (often expressed as power factor) of a capacitor. It is not mainly used for measuring inductance or resistance. Therefore, the correct option is (d).
A moving coil galvanometer is made into a dc ammeter by connecting
A)
a low resistance across the meter
B)
a high resistance in series with meter
C)
a pure inductance across the meter
D)
a capacitor in series with the meter
A moving coil galvanometer is converted into a DC ammeter by connecting a low resistance shunt in parallel across the meter. The shunt carries most of the current, while only a small fraction passes through the galvanometer. This allows the instrument to measure larger currents without damaging the coil. A high resistance in series is used to convert a galvanometer into a voltmeter, not an ammeter. A pure inductance or a capacitor is not used for DC current measurement.
Which of the following is a frequency sensitive bridge?
A)
De-sauty bridge
B)
Schering bridge
C)
Wien’s bridge
D)
Maxwell’s bridge
A frequency-sensitive bridge is one whose balance condition depends on frequency. Wien’s bridge is used for frequency measurement and its balance equation includes frequency, so it is frequency sensitive. De-Sauty bridge, Schering bridge, and Maxwell’s bridge are mainly used for measuring capacitance, dielectric loss, and inductance respectively, and are not primarily frequency-sensitive in this sense.
Frequency can be measured by using
A)
Maxwell’s bridge
B)
Schering bridge
C)
Heaviside Campbell bridge
D)
Wien’s bridge
Frequency can be measured using the Heaviside-Campbell bridge, which is specifically used for determining frequency (or frequency-related quantities) in AC bridge circuits. Maxwell’s bridge is used for inductance measurement, Schering bridge for capacitance and dielectric loss, and Wien’s bridge is commonly used for frequency measurement in oscillators, not as the standard bridge answer here in measurements theory. Hence, the correct option is (c) Heaviside Campbell bridge.
A digital voltmeter measures
A)
Peak value
B)
Peak to peak value
C)
Rms value
D)
Average value
A digital voltmeter (DVM) is generally calibrated to display the effective value of AC voltage, i.e., the RMS value. RMS is the standard measure of AC magnitude because it represents the equivalent DC heating effect. Peak, peak-to-peak, and average values are not the usual direct indication of a DVM.