RRB JE Electrical question paper with solution 2013 | RRB JE Electrical

Ques.31. Commutator segments in a DC shunt machine equal to the number of

  1. Coil sides
  2. Turns
  3. Coils
  4. Slots

Answer.1. Coil sides

Explanation:-

  • The commutator is a mechanical rectifier, so the commutator collects induced EMF or current developed in the armature.
  • The commutator converts the alternating current generated in armature into the unidirectional current.
  • The commutator is made up of Copper segments, and each segment is insulated from each other by the thin layer of mica.
  • The number of commutator segments must be equal to the number of coils in the armature. That is, each commutator segment is in contact with two coil sides at all times. One coil side feeds the current to the segment, and the other side draws current from it.
  • Hence, the number of commutator segments is equal to twice the number of coil sides, which is equal to the number of coils.
  • Each commutator segment is connected to the armature conductor by using copper lugs.

construction of DC generator

 

Ques.32. What quantity of charge must be delivered by a battery with a potential difference of 220 V to do 880 J of work?

  1. 2 C
  2. 4 C
  3. 6 C
  4. 8 C

Answer.2. 4 C

Explanation:-

Given

Charge=?
voltage = 220 V
work = 880 joule

Work done by the battery

W = Q.V

Q = W/V = 880/220 = 4 C

 

Ques.33. The polarity of the supply voltage is reversed in which type of braking?

  1. Regenerative braking
  2. Dynamic braking
  3. Plugging
  4. None of these

Answer.3. Plugging

Explanation:-

Plugging:-  It is also called reverse current braking. The connections of the motor are reversed, reversing the direction of torque, thereby bring it to a quick stop. For this, either the direction of field current or the direction of armature current is reversed. The field circuit usually has a large time constant due to the high value of inductance, so the time taken to bring the field current to zero is large. Hence, it is usual to reverse the armature current. On reversing the armature current the supply voltage to the armature and back emf developed in it are additive. So, this condition is worse than the starting condition as the applied voltage to the armature is approximately double (V + E = 2V) to the supply voltage. Hence, an external resistance is inserted in the armature circuit simultaneously with the reversal of the armature current. Braking torque can be regulated by varying the magnitude of this resistance.

The disadvantages of this method are

  1. The kinetic energy of the motor is dissipated in the external resistance in the form of heat. So, this method of braking is inefficient.
  2. The braking in this method fails in case of failure of the supply.

 

Ques.34. Energy in the sun is produced as a result of

  1. Fusion
  2. Combustion
  3. Explosion
  4. Thermonuclear fission

Answer.1. Fusion

Explanation:-

The energy in the sun is produced by light nuclei combining together, leading to the production of helium from hydrogen. This process is called nuclear fusion. The hydrogen bomb developed a few years after World WarII has based on nuclear fusion the same process by which energy is generated in the sun.

 

Ques.35. The colors of thin film result due to

  1. Desperation of light
  2. Scattering of light
  3. Polarization of light
  4. Interference of light

Answer.4. Interference of light

Explanation:-

INTERFERENCE OF LIGHT

When two light waves of exactly equal frequency having a phase difference which is constant with respect to time travel in the same direction and overlap each other then the intensity is not uniform in space.

The phenomenon of non-uniform distribution of energy in the medium due to the superposition of two light waves is called `interference of light’.

At some points in the medium, the intensity of light is maximum (constructive interference), while at some other points the intensity is minimum (destructive interference). Usually, in the interference of the light waves. we obtain alternate bright and dark bands of light, called interference fringes.

It may be clearly understood that in interference of light, energy is neither created nor destroyed. The loss of energy at the points of destructive interference appears as the increase of energy at the points of constructive interference and vice-versa. Thus, interference of light, merely redistribution of energy takes place, the law of conservation of energy is followed.

Interference in thin film

Interference effects are commonly observed in thin films, such as the thin surface of a soap bubble or thin layers of oil on water. The varied colors observed when incoherent white light is incident on such films results from the interference of waves reflected from the two surfaces of the film.

Thin-film interference is the phenomenon that occurs when incident light waves reflected by the upper and lower boundaries of a thin film interfere with one another to form a new wave.

  • Light shining on the upper surface of a thin film with thickness is partly reflected at the upper surface.
  • Light transmitted through the upper surface is partly reflected at the lower surface.
  • The two reflected waves come together on the retina of the eye.
  • Depending on the phase relationship, they may interfere constructively or destructively. Different colors have different wavelengths, so the interference may be constructive for some colors and destructive for others.
  • That’s why we see colored rings or fringes. (which shows a thin film of oil floating on water) and in the photograph that opens this chapter (which shows thin films of soap solution that make up the bubble walls). The complex shapes of the colored rings in each photograph result from variations in the thickness of the film.

 

Ques.36. Polarization is a characteristic of

  1. Lightwave
  2. Sound Wave
  3. Water wave
  4. Heatwave

Answer.1. Lightwave

Explanation:-

Polarization is a property of waves that can oscillate with more than one orientation. Electromagnetic waves, such as light, and gravitational waves exhibit polarization; sound waves in a gas or liquid do not have polarization because the medium vibrates only along the direction in which the waves are traveling.

By convention, the polarization of light is described by specifying the orientation of the wave’s electric field at a point in space over one period of the oscillation.

 

Ques.37. Two copper spheres are same radii, one hollow and other solid are charged to the same potential which will hold more charge?

  1. Solid Sphere
  2. Hollow Sphere
  3. Both 1 and 2
  4. None of these

Answer.1. Both 1 and 2

Explanation:-

The capacitance of the charged sphere is given as

C = 4πε0 × R

From the above equation, it is clear that the capacitance is directly related to the radius and it is independent of the mass of the object hence both copper sphere has the same charge.

or

We know that the whole charge of a conductor remains on its outer surface. Since, both the spheres are of the same radii and are charged to the same potential, therefore both the spheres will hold the equal charge.

 

Ques.38. A combination circuit that selects one from many inputs

  1. Encoder
  2. Decoder
  3. Demultiplexer
  4. Multiplexer

Answer.4. Multiplexer

Explanation:-

Multiplexing means transmitting a large number of information units over a smaller number of channels or lines. A digital multiplexer is a combinational circuit that selects binary information from one of many input lines and directs it to a single output line. The selection of a particular input line is controlled by a set of selection lines.

 

Ques.39. A bar magnet is dropped vertically downward through a wire loop held horizontally. The acceleration of the magnet will be

  1. g
  2. Greater than g
  3. Less than g
  4. Zero

Answer.3. Less than g

Explanation:-

As the magnet falls, the magnetic flux linked with the ring changes. This sets up an induced emf and hence an induced current in the ring. The direction of the induced current is so as to oppose the downward motion of the magnet. Hence, the acceleration of the falling magnet is less than that due to gravity.

If the ring is cut, an induced emf exists but the induced current is zero. Hence, there will be no opposing force to the falling magnet, and it falls with the acceleration due to gravity.

 

Ques.40. Decibel is:-

  1. A measurement of sound level
  2. Wavelenght of noise
  3. A musical instrument
  4. The frequency of sound

Answer.1. A measure of sound level

Explanation:-

The unit used to measure the magnitude of the sound level is the decibel (dB). A decibel is a dimensionless unit used to express the logarithm of the ratio of a measured quantity to a reference quantity.

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