← MES Jr. Grade
Past Questions
A differential transformer is a
A)
variable inductance transducer
B)
variable pressure transducer
C)
constant displacement transducer
D)
constant pressure transducer
A differential transformer, commonly known as an LVDT (Linear Variable Differential Transformer), works on the principle of mutual induction and changes its output voltage with the displacement of the core. Since its inductance varies with position, it is a variable inductance transducer. The other options are incorrect because it is not related to pressure measurement, and it is not a constant displacement or constant pressure device.
Phantom loading for testing of energy meters is used
A)
to isolate the current and potential circuits
B)
to reduce power loss during testing
C)
for meters having low current ratings
D)
to test meters having a large current rating for which loads may not be available in the laboratory
Phantom loading is used when an energy meter has to be tested at high current, but supplying the full load power would cause excessive power loss and require a very large load in the laboratory. In phantom loading, the current coil is supplied with the required current while the pressure coil is supplied separately, so the meter is tested under rated conditions without wasting large power. Hence, it is mainly used for meters having a large current rating for which suitable loads may not be available in the laboratory. Option (b) is a consequence, but the main purpose is as stated in (d).
A PMMC meter has an internal resistance 200Ω and the current required for its full-scale deflection is 50µA. The meter is capable of measuring, on its own, a maximum voltage of
A)
5mV
B)
10mV
C)
5µV
D)
10µV
For a PMMC meter, the maximum voltage it can measure on its own is given by V = I × R. Here, full-scale current I = 50 µA = 50 × 10^-6 A and internal resistance R = 200 Ω. So, V = 50 × 10^-6 × 200 = 0.01 V = 10 mV. Hence, the correct answer is 10 mV. The other options are incorrect because they do not match the product of the given current and resistance.
Two parallel conductors are carrying currents in the same direction. Which of the following statements is correct?
A)
There is a repelling force between the conductors
B)
The force increases when the current is decreased
C)
There is an attracting force between the conductors
D)
The force increases when the distance between the conductors is increased
Two parallel current-carrying conductors exert magnetic forces on each other. If the currents are in the same direction, the magnetic fields interact so that the conductors attract each other. If the currents were in opposite directions, they would repel. Also, the force is proportional to the product of currents and inversely proportional to the distance between them, so options (b) and (d) are incorrect.
“In all cases of electromagnetic induction, an induced voltage will cause a current to flow in a closed circuit in such a direction that the magnetic field which is caused by the current will oppose the change that produces the current” is the original statement of
A)
Lenz’s law
B)
Faraday’s law of magnetic induction
C)
Fleming’s law
D)
Ampere’s law
The statement describes the direction of induced current: it always flows so that the magnetic field produced by it opposes the change in magnetic flux that caused the induction. This is exactly Lenz’s law. Faraday’s law gives the magnitude of induced emf, Fleming’s law is used for direction in motors/generators, and Ampere’s law relates magnetic field to current.
Which of the following transducers is used for transmitting as well as receiving the acoustic energy in an ultrasonic flow meter?
A)
LVDT
B)
RTD
C)
Piezo-electric crystals
D)
Strain gauge
In an ultrasonic flow meter, the same transducer is used to generate ultrasonic waves and to detect the received acoustic energy. Piezo-electric crystals are ideal for this because they convert electrical energy into mechanical vibrations and vice versa. LVDT measures displacement, RTD measures temperature, and strain gauge measures strain, so they are not used for transmitting/receiving ultrasonic energy.
Hydrometer is used to determine
A)
specific gravity of liquids
B)
relative humidity
C)
specific gravity of solids
D)
specific gravity of gases
A hydrometer is an instrument used to measure the specific gravity (relative density) of liquids. It works by floating at different levels depending on the liquid’s density. It is not used for humidity, solids, or gases.
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.
With the increase in cross-sectional area of the conductor, the value of resistance
A)
increases
B)
remains same
C)
decreases
D)
none of these
Resistance of a conductor is given by R = ρL/A, where ρ is resistivity, L is length, and A is cross-sectional area. Since resistance is inversely proportional to area, increasing the cross-sectional area decreases the resistance. Hence, option (c) is correct. Options (a) and (b) are incorrect because resistance does not increase or remain unchanged when area increases.
With the increase of temperature, insulating property of insulators
A)
weakens
B)
strengthens
C)
remains same
D)
none of these
As temperature increases, the insulating resistance of most insulating materials decreases, so their insulating property weakens. Higher temperature increases leakage current and reduces dielectric strength. Hence option (a) is correct. Options (b) and (c) are incorrect because insulation does not improve or remain unchanged with rising temperature.
A wave travelling in x-direction has Ey and Ex components at 45º phase apart. The wave is
A)
linearly polarized
B)
elliptically polarized
C)
circularly polarized
D)
horizontally polarized
For an electromagnetic wave, if two perpendicular electric field components are present with a phase difference other than 0° or 180°, the wave is generally elliptically polarized. Here, Ex and Ey are 45° phase apart, so the tip of the electric field vector traces an ellipse. Linear polarization requires zero phase difference, and circular polarization requires equal amplitudes with a 90° phase difference. Hence, the correct answer is elliptically polarized.
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.
A current measuring instrument can be used to measure voltage by
A)
adding resistance in series
B)
adding resistance in parallel
C)
using larger wire in the coil
D)
using a high resistance shunt
A current-measuring instrument (ammeter) can be converted into a voltmeter by connecting a high resistance in series with it. This series resistance is called a multiplier and limits the current so the instrument can measure voltage safely. Adding resistance in parallel or using a shunt is used for extending current measurement, not voltage measurement.
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.
Which one of the following is an active transducer?
A)
strain gauge
B)
selsyn
C)
photovoltaic cell
D)
photo emissive cell
An active transducer is one that generates its own electrical output without requiring an external power source. A photovoltaic cell converts light directly into electrical voltage/current, so it is an active transducer. A strain gauge is a passive transducer because it needs external excitation. A selsyn is an electromechanical device, not typically classified as an active transducer in this context. A photo emissive cell also generally requires external biasing and is not the standard answer here.
Piezo-electric transducers are
A)
passive transducers
B)
active transducer
C)
inverse transducers
D)
both (b) and (c)
Piezo-electric transducers generate an electrical output directly from applied mechanical stress, so they do not need an external power source. Hence, they are **active transducers**. They also work on the inverse piezoelectric effect in the sense that mechanical energy is converted into electrical energy, so they are often referred to as **inverse transducers**. Therefore, the correct choice is **both (b) and (c)**. Option (a) is wrong because passive transducers require external excitation.
In a certain magnetic circuit, a current of one ampere in the winding produces a flux of 1 weber. If the reluctance of the magnetic circuit is doubled, then for producing the same flux, the current should be
A)
1 A
B)
½ A
C)
2 A
D)
4 A
For a magnetic circuit, flux \(\Phi = \frac{NI}{\mathcal{R}}\), where \(N\) is turns, \(I\) is current, and \(\mathcal{R}\) is reluctance. Here, the same winding is used, so \(N\) is constant. If reluctance is doubled, to keep the flux the same, the magnetomotive force \(NI\) must also double. Therefore the current must double from 1 A to 2 A. Hence the correct answer is (c) 2 A.
When the plate area of a parallel plate capacitor is increased keeping the capacitor voltage constant, the force between the plates
A)
increases
B)
decreases
C)
remains constant
D)
may increase or decrease depending on the metal
For a parallel plate capacitor, the attractive force between the plates is F = \(\tfrac{1}{2}\) \(\varepsilon A E^2\). Since the voltage is kept constant, the electric field is \(E = V/d\), so F = \(\tfrac{1}{2}\) \(\varepsilon A (V/d)^2\). Thus, with voltage constant, the force is directly proportional to the plate area A. Therefore, increasing the plate area increases the force between the plates. The other options are incorrect because the force does not remain constant, decrease, or depend on the metal in this ideal capacitor case.
An inverse transducer converts
A)
electrical energy to any other form of energy
B)
electrical energy to light energy
C)
mechanical displacement into electrical signal
D)
electrical energy to mechanical form
An inverse transducer performs the reverse function of a normal transducer. While a transducer generally converts a non-electrical quantity into an electrical signal, an inverse transducer converts electrical energy into a non-electrical form, commonly mechanical output. Hence, the correct answer is electrical energy to mechanical form. Option (c) describes a normal transducer, not an inverse transducer.
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.
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.
Inside a hollow conducting sphere
A)
electric field is zero
B)
electric field is a non-zero constant
C)
electric field changes with the magnitude of the charge applied to the conductor
D)
electric field changes with distance from the centre of the sphere
For a hollow conducting sphere in electrostatic equilibrium, all excess charge resides on the outer surface. The electric field inside the conducting material and inside the hollow cavity is zero, provided there is no charge placed inside the cavity. Hence the correct option is (a). Options (b), (c), and (d) are incorrect because the field inside does not depend on charge magnitude or distance from the centre; it remains zero everywhere inside.
The modulation index in FM signal depends on
A)
amplitude of the modulating signal
B)
frequency of the modulating signal
C)
both (a) and (b)
D)
none of these
In frequency modulation (FM), the modulation index is defined as β = Δf / fm, where Δf is the frequency deviation and fm is the modulating signal frequency. The frequency deviation Δf is proportional to the amplitude of the modulating signal, so the modulation index depends on the amplitude of the modulating signal as well as the frequency of the modulating signal. Therefore, the correct answer is both (a) and (b).