← MES Jr. Grade
Past Questions
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.
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.
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 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.
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).
“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.
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.
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.
The velocity of a travelling electromagnetic wave in free space is given by
A)
µ0ε0
B)
√ µ0ε0
C)
1/ µ0ε0
D)
1/√µ0ε0
For an electromagnetic wave in free space, the speed is given by c = 1/√(μ0ε0). This comes from Maxwell’s equations. Hence the correct option is (d). Options (a), (b), and (c) are dimensionally incorrect for velocity.
An air gap is usually inserted in magnetic circuits to
A)
increase the flux
B)
prevent saturation
C)
increase mmf
D)
none of these
An air gap increases the reluctance of the magnetic circuit, so for a given magnetizing force the flux is reduced and the core is less likely to reach magnetic saturation. Hence, air gaps are commonly inserted to prevent saturation. Option (a) is wrong because an air gap does not increase flux; option (c) is wrong because mmf is not increased by the gap itself; it only requires more mmf to produce the same flux.
Piezo-electric crystals are used for the measurement of
A)
temperature
B)
velocity
C)
sound
D)
none of these
Piezo-electric crystals generate an electrical signal when subjected to mechanical stress and are widely used in microphones, ultrasonic transducers, and sound/vibration measurement. Hence they are used for measurement of sound. Temperature is measured by thermocouples/RTDs, and velocity is not the standard direct use here.
Relative permeability of vacuum is
A)
4π x 10-7/h/m
B)
1 H/m
C)
1
D)
1/4 π
Relative permeability (μr) is defined as the ratio of the permeability of a medium to the permeability of free space: μr = μ/μ0. For vacuum, μ = μ0, so μr = 1. Option (a) is the permeability of free space (μ0 = 4π × 10^-7 H/m), not relative permeability. Option (b) is incorrect because 1 H/m is not the permeability of vacuum. Option (d) is also incorrect.
A uniform plane wave travelling in a perfect dielectric is incident normally on the surface of a perfect conductor. Then
A)
the wave is transmitted into the conductor without attenuation
B)
50% of the incident wave is transmitted and 50% is reflected
C)
a standing wave is set up in the conducting medium
D)
a standing wave is set up in the dielectric
For a perfect conductor, the tangential electric field at the surface must be zero. So when a uniform plane wave in a perfect dielectric is incident normally on it, the wave cannot be transmitted into the conductor. The reflected wave has equal amplitude and opposite phase, and the incident and reflected waves superpose in the dielectric to form a standing wave. Hence, option (d) is correct. Options (a) and (b) are wrong because a perfect conductor does not allow transmission. Option (c) is wrong because the standing wave is formed in the dielectric region, not inside the conductor.
When the load impedance is equal to the characteristic impedance of the transmission lines, then the reflection coefficient and standing wave ratio are respectively
A)
0 and 0
B)
1 and 0
C)
0 and 1
D)
1 and 1
When the load impedance equals the characteristic impedance (ZL = Z0), the line is perfectly matched. Therefore, no wave is reflected, so the reflection coefficient Γ = 0. Also, the standing wave ratio (SWR) becomes 1, which is the minimum possible value. Hence the correct answer is 0 and 1.
Electromagnetic waves may be diffracted
A)
by reflection from the ground
B)
when encountered with a spherical wave front
C)
around the edge of a sharp obstacle
D)
while passing through a large slot
Diffraction is the bending/spreading of waves when they pass around an obstacle or through an aperture. Electromagnetic waves can therefore be diffracted around the edge of a sharp obstacle. Reflection from the ground is reflection, not diffraction. A spherical wave front is just a wave shape, not a cause of diffraction. Passing through a large slot may produce only negligible diffraction because diffraction is significant when the opening is comparable to the wavelength.
Creeping error in single-phase energy meter can be minimized by
A)
adjusting the braking magnet
B)
using short circuited loops on the outer limbs of the shunt magnet
C)
drilling two holes in the disc on the opposite side of the spindle
D)
adjusting the shading band
Creeping is the slow continuous rotation of the energy meter disc at no-load or very light load. It is commonly minimized by making two opposite holes/slots in the disc near the rim, so the disc tends to stop when one of these holes comes under the driving system. Hence option (c) is correct. Option (a) relates to braking torque adjustment, (b) is used in induction wattmeters/meters for phase compensation, and (d) is for shading band adjustment, not creeping prevention.
The electric potential due to an electric dipole of length L at point distance r away from it will be doubled if the
A)
L is doubled
B)
r is doubled
C)
r is halved
D)
L is halved
For an electric dipole, the potential at a point on the axial line is proportional to the dipole moment p = qL, and inversely proportional to r^2. Thus, V ∝ L/r^2 (for fixed charge q and distance r). To double the potential, L must be doubled. If r is doubled, potential becomes one-fourth; if r is halved, potential becomes four times; if L is halved, potential becomes half.
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.
The magnitude of statically induced e.m.f. depends on
A)
the magnitude of flux
B)
the rate of change of flux
C)
the resistance of the coil
D)
none of these
A statically induced e.m.f. is produced when the magnetic flux linking a stationary coil changes with time. By Faraday’s law, the induced e.m.f. is proportional to the rate of change of flux linkage, not to the flux magnitude itself or the coil resistance. Therefore, the correct answer is (b).
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.
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.
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.
Two parallel wires separated by a distance d are carrying a current I in the same direction. The magnetic field along a line running parallel to these wires and midway between them
A)
depends upon I
B)
is zero
C)
depends upon d
D)
depends upon the permeability of the medium between the wires
For a long straight current-carrying wire, the magnetic field at a point is proportional to I and inversely proportional to distance from the wire. Here the two parallel wires carry equal currents in the same direction, and the point on the line midway between them is at equal distance from both wires. By the right-hand rule, the magnetic fields produced by the two wires at the midpoint are equal in magnitude but opposite in direction, so they cancel out. Therefore, the net magnetic field along that midway line is zero.
The direction of the magnetic lines of force is
A)
from positive to negative charges
B)
from south to north poles
C)
from one end of the magnet to the other
D)
from north pole to the south pole
Magnetic field lines outside a magnet emerge from the north pole and enter the south pole. So the direction of magnetic lines of force is from north pole to the south pole. Option (a) refers to electric field lines, (b) is the opposite of the correct magnetic direction, and (c) is too vague and not the standard scientific description.
In a reed type frequency meter, all the reeds
A)
have the same natural frequency
B)
have different natural frequency
C)
have different natural frequency with a specific difference of the frequencies of adjacent reeds
D)
none of the above
A reed type frequency meter contains a set of reeds, each tuned to a slightly different natural frequency. The reeds are arranged so that adjacent reeds have a specific small frequency difference, allowing the reed whose natural frequency is closest to the supply frequency to vibrate the most. Hence, the correct choice is (c). Option (a) is wrong because all reeds cannot have the same natural frequency, and option (b) is incomplete because the reeds are not just different randomly; they are spaced by a specific frequency interval.