SSC JE 2015 Electrical question paper Fully Solved

 

Ques 1. How much energy is stored in a 100 mH inductance when a current of 1A is flowing through it?

  1. 5.0 J
  2. 0.05
  3. 0.5 J
  4. 0.005 J 

The energy stored in the magnetic field of an inductor can be expressed as

E = 1/2 L I2

where

E = energy stored (joules, J)

L = inductance (henrys, H)

I = current (amps, A)

1/2(100 x 10-3 x 1)

50 x 10-3 J

0.05 J

 

Ques 2. For the circuit shown below, find the resistance between points P & Q

  1. 1 Ω  
  2. 2 Ω
  3. 3 Ω
  4. 4 Ω

 

Ques 3. The rate of change of current in a 4 H inductor is 2 Amps/sec. Find the voltage across inductor

  1. 8 V 
  2. 16 V
  3. 2 V
  4. 0.8 V
Here L = 4H and di/dt = 2 A

Hence voltage Across inductor

= V = L(di/dt)

= 4 x 2 = 8 V

 

Ques 4. Find the node voltage VA

  1. 6 V 
  2. 5.66 V
  3. 6.66 V
  4. 5 V

 

Ques 5. In a pure inductive circuit if the supply frequency is reduced to half the current will?

  1. Be four times as high
  2. Be doubled
  3. Be reduced to half
  4. Be reduced to one fourth
The amplitude of the current in a pure inductive circuit is given by

Im = Vm/ωl

And ω = 2πf

Im = Vm/2πfl

Im ∝ 1/f

Hence, when the frequency is halved, the amplitude of the current is doubled in a pure inductive circuit.

 

Ques 6. A 10 pole 25 Hz alternator is directly coupled to and is driven by 60 Hz synchronous motor then the number of poles in a synchronous motor is?

  1. 24 poles 
  2. 48 poles
  3. 12 Poles
  4. None of the above

Number of poles of alternator Pa = 10

F = 25 Hz (alternator)

F = 60 Hz (motor)

Then the number of poles of motor Pm =?

Since synchronous motor is directly coupled hence

Synchronous speed of an alternator = Synchronous speed of the motor

(120 x 25)/10 = (120 x 60)/ Pm

Pm = 24

 

Ques 7. When a source is delivering maximum power to the load the efficiency will be?

  1. Below 50 %
  2. Above 50%
  3. 50 %
  4. Maximum

Maximum Power Transfer theorem

The maximum power is transferred when the load resistance RL is equal to internal resistance Rth or the equivalent resistance Rth.

RL = Rth

Under this condition, the same amount of power is dissipated in the internal resistance Rth and hence the efficiency is 50%.

 

Ques 8. In the Maxwell Bridge as shown in the figure below the value of resistance Rx and inductance Lx of a coil is to be calculated after balancing the bridge. The component value is shown in the figure at balance the value of Rx and Lx will respectively be

  1. 37.5 ohms, 75 mH
  2. 75 ohm, 75 mH
  3. 375 ohm, 75 mH 
  4. 75 ohm, 150 mH

A Maxwell bridge is a modification to a Wheatstone bridge used to measure an unknown inductance (usually of low Q value) in terms of calibrated resistance and inductance or resistance and capacitance.

The balanced condition for Maxwell bridge is given as

Z1Z4 = Z2Z3

Here Z1 = Rx + jωLx

Z2 = 200Ω

Z3 = 750Ω

Z4 = R4 ||(1/ωC4)

 

Ques 9. The internal resistance of a voltage source is 10 ohm and has 10 volts and its terminals. Find the maximum power that can be transferred to the load

  1. 25 W
  2. 5 W
  3. 0.25 W
  4. 2.5 W 

For maximum power to be transferred from a source to a load resistance, the value of load resistance should be equal to the internal resistance of the source.

RL = R = 10Ω

current through the circuit = 10/(10+10) = 0.5 A

power transferred = I2R = 0.25 x 10 = 2.5 W

 

Ques 10. As the load is increased the speed of the DC shunt motor is

  1. Remain constant 
  2. Increase Proportionately
  3. Reduces Slightly
  4. Increase Slightly

For a dc shunt motor. field current is practically constant and thus the torque is directly proportional to the armature current. Hence the torque-armature current characteristic is a straight line passing through the origin. However, due to the armature reaction under the loaded conditions of the machine. the flux decreases slightly with the load.
Since flux is practically constant for DC shunt motor.The speed of DC shunt motor is

N = K (V – IaRa)

Armature current increases as the load on the motor is increased. Thus, the speed of the dc shunt motor will fall slightly with the armature current. As the speed characteristic is only slightly drooping, the dc shunt motor is normally regarded as a constant speed motor.