← MES Jr. Grade
Past Questions — 2012
A dynamometer type wattmeter responds to the
A)
average value of active power
B)
average value of reactive power
C)
peak value of active power
D)
peak value of reactive power
A dynamometer type wattmeter measures true power in AC circuits. Its deflecting torque is proportional to the instantaneous product of current and voltage, and the average over a cycle gives the active (real) power. It does not respond to reactive power, and it is not a peak-reading instrument. Hence the correct answer is the average value of active power.
Maxwell’s inductance-capacitance bridge is used for measurement of inductance of
A)
low Q coils
B)
medium Q coils
C)
high Q coils
D)
low and medium Q coils
Maxwell’s inductance-capacitance bridge is mainly used for measuring the inductance of medium-Q coils. It uses a standard capacitor and resistors, and gives good accuracy for coils whose quality factor is neither too low nor too high. Low-Q coils are usually measured by other bridges, while high-Q coils are better suited to bridges like Hay’s bridge.
Capacitive transducers can be used for measurement of liquid level. The principle of operation used in this case is
A)
change of capacitance with change of distance between plates
B)
change of area plates
C)
change of dielectric strength
D)
none of the above
In a capacitive liquid-level transducer, the liquid acts as the dielectric medium between the capacitor plates. As the level of liquid changes, the effective dielectric constant (dielectric property) between the plates changes, which changes the capacitance. Hence the working principle is based on change in dielectric strength/permittivity, not on plate spacing or plate area.
A parallel plate capacitor has a capacitance of 10µF. If the linear dimensions of the plates are doubled and the distance between them is also doubled; the new value of capacitance would be
A)
10µF
B)
20µF
C)
40µF
D)
5µF
For a parallel plate capacitor, C = εA/d. If all linear dimensions of the plates are doubled, the area A becomes 4 times. If the separation d is also doubled, capacitance becomes 4/2 = 2 times the original. So the new capacitance = 2 × 10µF = 20µF. Hence, option (b) is correct.
Electronic voltmeters which use rectifiers employ negative feedback. This is done
A)
to increase the overall gain
B)
to improve stability
C)
to overcome nonlinearity of diodes
D)
none of the above
In electronic voltmeters using rectifiers, negative feedback is applied to reduce the effect of the diode’s nonlinear V-I characteristic. This makes the meter response more linear and accurate, especially at low voltages. It is not mainly to increase gain or just improve stability. Therefore, the correct choice is (c).
The characteristic impedance of an air dielectric co-axial line is 77Ω. If the air is replaced by a dielectric with Єr=2.28, then characteristic impedance will be
A)
33.8Ω
B)
51Ω
C)
116Ω
D)
175Ω
For a coaxial line, characteristic impedance varies inversely with the square root of the dielectric constant: Z0(new) = Z0(air) / √εr. Here, Z0(air) = 77Ω and εr = 2.28. So, Z0 = 77 / √2.28 ≈ 77 / 1.51 ≈ 51Ω. Hence, the correct option is (b).
A current transformer has a phase error of +3°. The phase angle between the primary and secondary current is
A)
3°
B)
177°
C)
180°
D)
183°
In a current transformer, the ideal phase difference between primary and secondary currents is 180° because the secondary current should oppose the primary current. A phase error of +3° means the actual phase angle is 3° more than 180°. Therefore, the phase angle between the primary and secondary current is 180° + 3° = 183°. Hence, option (d) is correct.
The electric field lines and equipotential lines
A)
are parallel to each other
B)
are one and same
C)
cut each other orthogonally
D)
can be inclined to each other at any angle
Electric field lines show the direction of the electric field, while equipotential lines represent points of equal potential. Since the electric field is always perpendicular to the equipotential surface/line, they intersect at right angles. Hence, the correct option is (c). Options (a), (b), and (d) are incorrect because the two are neither parallel nor identical, and their angle is not arbitrary.
Which instrument has the lowest resistance?
A)
Ammeter
B)
Voltmeter
C)
Frequency meter
D)
Megger
An ammeter is connected in series, so it must have very low internal resistance to avoid changing the circuit current. A voltmeter has very high resistance, a frequency meter is not designed for minimum resistance in this sense, and a megger has very high resistance for insulation testing. Therefore, the instrument with the lowest resistance is the ammeter.
Silicon steel is used in the construction of electrical machines because it has
A)
low hysteresis loss
B)
low retentivity
C)
low coercivity
D)
none of these
Silicon steel is used for the magnetic cores of electrical machines because adding silicon reduces the area of the hysteresis loop, thereby lowering hysteresis loss. This improves efficiency in transformers, motors, and generators. While silicon steel also has low coercivity and low retentivity, the standard reason asked in such questions is its low hysteresis loss.
A 5 bit D/A converter has a current output. The digital input is 10100 and the output current corresponding to this is 10mA. The output current for digital input of 11101 is
A)
10.5 mA
B)
14.5 mA
C)
29 mA
D)
none of these
For an ideal 5-bit current-output D/A converter, output current is proportional to the decimal value of the digital input.
Given input 10100 = 20 in decimal produces 10 mA.
So, 1 LSB = 10/20 = 0.5 mA.
Now 11101 = 29 in decimal.
Output current = 29 × 0.5 = 14.5 mA.
Hence, the correct answer is 14.5 mA.
Three types of transducers are compared as regards to their sensitivities. The order in which they exhibit sensitivities (highest to lowest) is
A)
thermistors, RTDs, thermocouples
B)
thermocouples, RTDs, thermistors
C)
RTDS, thermistors, thermocouples
D)
RTDs, thermocouples, thermistors
Sensitivity means change in output per unit change in temperature. Among these temperature transducers, thermistors have the highest sensitivity because their resistance changes very sharply with temperature. RTDs have moderate sensitivity with a more linear but smaller resistance change. Thermocouples have the lowest sensitivity because they produce only a small emf per degree of temperature difference. Therefore, the order from highest to lowest sensitivity is thermistors > RTDs > thermocouples.
The relative permeability of a ferro-magnetic material is
A)
1
B)
less than 1
C)
more than 1000
D)
none of these
Ferromagnetic materials have very high magnetic susceptibility, so their relative permeability is much greater than 1, typically in the hundreds or thousands. Hence the correct choice is 'more than 1000'. Option (a) 1 is for free space/non-magnetic materials, and option (b) less than 1 is for diamagnetic materials.
The capacitance and loss angle of a capacitor can be accurately measured by
A)
Kelvin’s bridge
B)
Anderson’s bridge
C)
Schering bridge
D)
Carey-Foster bridge
The Schering bridge is specifically used for measuring the capacitance and dielectric loss (loss angle) of a capacitor accurately. It is widely used in high-voltage and insulation testing. Kelvin’s bridge is for low resistance measurement, Anderson’s bridge is for inductance measurement, and Carey-Foster bridge is used for comparing nearly equal resistances.
Maxwell’s inductance capacitance bridge is used for measurement of inductance of
A)
low Q coils
B)
medium Q coils
C)
high Q coils
D)
low and medium Q coils
Maxwell’s inductance-capacitance bridge is mainly used for measuring the inductance of medium-Q coils. It is suitable when the coil has moderate quality factor, because the bridge uses a standard capacitor and is convenient for accurate measurement in that range. It is not preferred for low-Q coils or very high-Q coils. Hence, the correct answer is medium Q coils.
For measuring low pressure of the range 10-2 mm of Hg, the apparatus used is
A)
strain gauge
B)
Piezoelectric transducers
C)
Pirani gauge
D)
ionization gauge
Very low pressures in the range of 10^-2 mm of Hg are measured using an ionization gauge. It works on the principle that gas molecules are ionized and the resulting ion current is proportional to the pressure at very low vacuum levels. A Pirani gauge is used for comparatively higher vacuum ranges, while strain gauge and piezoelectric transducers are mainly used for pressure measurement at much higher pressures, not for such low vacuum.
An electromagnetic field is radiated from
A)
a stationary point charge
B)
a capacitor with a DC voltage
C)
a conductor carrying a DC current
D)
an oscillating dipole
Electromagnetic waves are radiated by accelerating charges, not by charges or currents in steady states. An oscillating dipole has charges moving back and forth, so it continuously accelerates charges and emits electromagnetic radiation. A stationary point charge, a capacitor with DC voltage, and a conductor carrying DC current produce static electric/magnetic fields, but do not radiate electromagnetic waves in the ideal steady-state case.
Capacitive transducers can be used for measurement of liquid level. The principle of operation used in this case is
A)
change of capacitance with change of distance between plates
B)
change of area of plates
C)
change of dielectric strength
D)
none of the above
In a capacitive liquid-level transducer, the liquid acts as the dielectric medium between the capacitor plates. As the liquid level changes, the effective dielectric constant (dielectric property) between the plates changes, which changes the capacitance. Hence the operating principle is change of dielectric strength/properties. Option (a) is for varying plate spacing, and (b) is for varying overlapping area, which are not the usual principle in liquid-level measurement.
Maxwell’s divergence equation for the magnetic field is given by
A)
∇ × B = 0
B)
∇ · B = 0
C)
∇ × B = J
D)
∇ · B = ρ
Maxwell’s divergence equation for the magnetic field states that magnetic monopoles do not exist, so the net magnetic flux out of any closed surface is zero. Hence, the divergence of the magnetic flux density B is zero: ∇ · B = 0. Option (a) is incorrect because curl of B is not zero in general; option (c) is Ampère’s law in simplified form; option (d) is Gauss’s law for electricity, not magnetism.
For a loss-less dielectric medium, the phase constant for a travelling wave, β is proportional to
A)
Єr
B)
√ Єr
C)
1/ Єr
D)
1/√ Єr
For a loss-less dielectric medium, the phase constant is
β = ω√(με).
If the medium is non-magnetic, μ ≈ μ0, so β ∝ √ε. Since ε = ε0εr, we get β ∝ √εr. Therefore the correct option is (b). Options (a), (c), and (d) do not match this dependence.
The unit of magnetic field strength is
A)
ampere turns
B)
weber
C)
ampere per metre
D)
weber/metre2
Magnetic field strength (also called magnetizing field intensity, H) is measured in ampere per metre (A/m). Option (a) ampere turns is not a unit of field strength; it is related to magnetomotive force. Option (b) weber is the unit of magnetic flux. Option (d) weber/metre2 is the unit of magnetic flux density (tesla), not field strength.
A wave guide can be considered to be analogous to a
A)
low pass filter
B)
high pass filter
C)
band pass filter
D)
band stop filter
A waveguide allows electromagnetic waves to propagate only above a certain cutoff frequency. Frequencies below cutoff are attenuated and cannot travel through it. This behavior is analogous to a high pass filter, which passes higher frequencies and blocks lower ones. Hence, the correct answer is high pass filter. The other options do not match this cutoff-frequency behavior.
A loss-less transmission line with characteristic impedance of 600Ω is terminated in a purely resistive load of 900Ω. The reflection coefficient is
A)
0.2
B)
0.5
C)
0.667
D)
1.5
For a lossless transmission line, the reflection coefficient at the load is
\[ \Gamma = \frac{Z_L - Z_0}{Z_L + Z_0} \]
Given \(Z_0 = 600\,\Omega\) and \(Z_L = 900\,\Omega\),
\[ \Gamma = \frac{900 - 600}{900 + 600} = \frac{300}{1500} = 0.2 \]
So the correct value is 0.2. However, among the provided options, 0.2 corresponds to option (a).
An A/D converter has a resolution of 20mV. The digital output for an analog input of 2.17V is
A)
01101100
B)
01101101
C)
01101011
D)
none of these
Resolution = 20 mV, so the number of quantization steps for 2.17 V is 2.17 / 0.02 = 108.5. For a standard A/D converter, this is taken as the nearest lower/appropriate integer code 108 or 109 depending on convention; here the options indicate the binary code for 109. Decimal 109 in binary is 01101101. Hence the correct output is 01101101. Option (a) is 108 (01101100), and option (c) is 107 (01101011), so they are not the intended answer.
If an energy meter disc makes 10 revolutions in 100 seconds when a load of 450W is connected to it, the meter constant (in rev/kwh) is
A)
100
B)
500
C)
1600
D)
800
Power = 450 W = 0.45 kW. In 100 s, energy consumed = 0.45 × (100/3600) = 0.0125 kWh. The disc makes 10 revolutions for 0.0125 kWh, so meter constant = 10 / 0.0125 = 800 rev/kWh. Hence, the correct option is (d) 800.