SSC JE electrical question paper explained 2016-2017

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).

Explanation:

Consider the given circuit

• 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°.

Explanation:

The total impedance of the series LCR circuit is given as

Z = R + j (X₁ – X_²)

where X₁ is inductive reactance

and X₂ is capacitive reactance.

At a particular frequency (resonant frequency), we find that X₁=X₂ because 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”.

Explanation:

Flux in a magnetic circuit also depends on the opposition that the circuit presents to it. Reluctance (R_M), 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,

R_M = 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 R_M, while nonmagnetic materials have low permeability and high R_M.

R_M = L/μA

Explanation:

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

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

Explanation:

For series RC circuit time constant is

τ = RC

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.

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.

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

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.