SSC JE electrical question paper explained 2016-2017

Ques 21. The relative permittivity of paramagnetic material is

  1. Greater than one
  2. Less than one
  3. 1
  4. Infinite

Answer.1. Greater than one

Explanation:

Paramagnetism is a form of magnetism that occurs only in the presence of an externally applied magnetic field. Paramagnetic materials are attracted to magnetic fields, and hence have a relative magnetic permeability greater than one (or, equivalently, a positive magnetic susceptibility).

 

Ques 22. The phase difference between voltage and current wave through a circuit element is given as 30. The essential condition is that

  1. Both waves must have the same frequency
  2. Both waves must have identical peak values
  3. Both waves must have zero value at the same time
  4. None of the above

Answer.1. Both waves must have same frequency

Explanation:

Consider the given circuit

circuit digram

  • The current would be 0 if the voltage of both the sources is the same and the  AC sources are in phase i.e at the same frequency.
  • However, if the same frequency sources are connected together but if they have phase differences even then current will flow.
  • If you connect voltage sources with the different phases, voltage, frequencies then the high magnitude of current will flow in the circuit. This can damage the sources. This can cause metallic wires to melt because of heating.

Hence if both waves have the same frequency then the phase difference between voltage and the current wave is 30°.

 

Ques 23. In a series resonant circuit the impedance of the circuit is

  1. Minimum
  2. Maximum
  3. Zero
  4. None of these

Answer.1. Minimum

Explanation:

The total impedance of the series LCR circuit is given as

Z = R + j (X1 – X²)

where X1 is inductive reactance

and X2 is capacitive reactance.

At a particular frequency (resonant frequency), we find that X1=Xbecause the resonance of a series RLC circuit occurs when the inductive and capacitive reactances are equal in magnitude but cancel each other because they are 180 degrees apart in phase.

Thus, at the resonant frequency, the net reactance is zero because X1=X2. The circuit impedance Z becomes minimum and is equal to the resistance R. “Since the impedance is minimum, the current will be maximum”.

 

Ques 24. Magnetic reluctance depends upon

  1. Cross-sectional area
  2. Length of material
  3. Material properties
  4. All of the above

Answer.3. All of the above

Explanation:

Flux in a magnetic circuit also depends on the opposition that the circuit presents to it. Reluctance (RM), is the opposition a magnetic circuit offers to the formation of magnetic flux.

The opposition depends on the dimensions of the core and the material of which it is made. As the resistance of a wire, reluctance is directly proportional to length (L) and inversely proportional to cross-sectional area (A). In equation form,

RM = L/A (Ampere-Weber)

The unit of Reluctance is Ampere-Weber or A/Wb

To change the ‘proportional’ sign into an ‘equals’ sign we must introduce a constant. This constant is called ‘magnetic reluctivity‘ and is directly equivalent to ‘resistivity’ in an electric circuit. However, it’s far more common to use it’s reciprocal instead, which we call ‘absolute permeability(μ)‘ Which is equivalent to conductivity.

Permeability is a measure of how easy it is to establish the flux in a material. Ferromagnetic materials have high permeability and hence low RM, while nonmagnetic materials have low permeability and high RM.

R= L/μA

 

Ques 25. In the R-L-C series resonant circuit magnitude of resonance frequency can be changed by changing the value of

  1. R only
  2. L only
  3. C only
  4. L or C

Answer.4. L or C

Explanation:

By changing the value of either capacitor or inductor in the series resonant circuit then the frequency of the circuit will change accordingly.

 

Ques 26. If the length of the material remains same while diameter of the material becomes half, what will be the new resistance

  1. Four times
  2. Doubles
  3. Half
  4. No change

Answer.1. Four Times

Explanation:

Resistance = R= pL/A

Where p is resistivity of material

L is length

A is cross-section area

A =πr2

Therefore if we reduce the diameter by 1/2, i.e { A = Π (D/2)2} then the resistance becomes 4 times

 

Ques 27. The time constant of the series RC circuit is given by

  1. R/C
  2. RC2
  3. RC
  4. R2C

Answer.3. RC

Explanation:

For series RC circuit time constant is

τ = RC

 

Ques 28. Why copper is preferred in the transmission lines?

  1. Has less resistance
  2. Has more reliable
  3. Has higher conductivity
  4. Has more cost
    1. 1,2 correct
    2. Only 1 correct
    3. Only 3 correct
    4. All are correct

Answer.D. All are correct

Explanation:

Advantages of copper in transmission lines are

  1. It has the best conductivity than other metals.
  2. It has a higher current density, hence for the given current rating lesser cross-sectional area of the conductor is required.
  3. The metal is homogeneous.
  4. It has low specific resistance.
  5. It is durable and has a higher scrap value.

Disadvantage of copper in transmission lines is

  1. it is expensive and heavy.

 

Ques 29. Which of the following is not a unit of inductance?

  1. Henry
  2. Columb/Volt ampere
  3. Volt second per ampere
  4. All of the above

Answer.3. Volt second per ampere

Explanation:

The unit of measurement for the inductance of an inductor is the henry, which is equal to 1 volt-second per ampere. The voltage across an inductor is proportional to the rate of change of current (measured in amps/second) through it. The larger the inductance, the larger the voltage across the inductor for the same rate of change of current. If the current is flowing left to right through an inductor and increasing (respectively decreasing) with time, then the potential on the left side of the inductor is higher (respectively lower) than the potential on the right side.

$1H = \frac{{1V \cdot 1s}}{{1A}}$

 

 

Ques 30. Why is the rotor skewed?

  1. To prevent starting current
  2. To provide stability
  3. To reduce magnetic turn
  4. To reduce rotor locking tendency

Answer.4. To reduce rotor locking tendency

Explanation:

Rotor conductors are skewed because of these two main reasons-

1. Primarily to prevent the cogging phenomenon. It is a phenomenon in which, if the rotor conductors are straight, there are chances of magnetic locking or strong coupling between the rotor & stator.

2. To avoid crawling. Crawling is a phenomenon where harmonic components introduce oscillations in torque.

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