All questions of R-L & R-C Transients for Electrical Engineering (EE) Exam

Three inductors having inductance values 3H, 4H and 5H are connected in series, calculate the equivalent inductance.
  • a)
    10H
  • b)
    12H
  • c)
    3H
  • d)
    5H
Correct answer is option 'B'. Can you explain this answer?

Zoya Sharma answered
When inductances are connected in series, the equivalent inductance is equal to the sum of all the individual inductance values.
Hence Leq= L1+L2+L3= 12H.

Initially, when the switch in a series RL circuit is closed, the inductor acts as?
  • a)
    Open circuit
  • b)
    Short circuit
  • c)
    Resistor
  • d)
    Capacitor
Correct answer is option 'A'. Can you explain this answer?

Alok Verma answered
Initially, when the switch in a series RL circuit is closed, the inductor acts as an open circuit. This is because the current gradually increases as the time increases. At t=0, the current in the inductor is zero, hence it acts as an open circuit.

What is the total applied voltage in an inductive circuit?
  • a)
    V=Ri+Ldi/dt
  • b)
    V=Ri+di/dt
  • c)
    V=i+Ldi/dt
  • d)
    V=R+Ldi/dt
Correct answer is option 'A'. Can you explain this answer?

Lavanya Menon answered
The total voltage in an inductive circuit is the sum of the voltage due to the resistor which is Ri and the voltage due to the inductor which is Ldi/dt. Hence V=Ri+Ldi/dt.

What is the expression for current in the given circuit?​
  • a)
     i=2(e-2t)A
  • b)
     i=2(1-e-2t)A
  • c)
     i=2(e2t)A
  • d)
    i=2(1+e-2t)A
Correct answer is option 'B'. Can you explain this answer?

At t = 0+ the current in the circuit is zero. Therefore at t = 0+, i = 0, 0 = c + 2, c = -2. Substituting the value of c in the current equation, we have i = 2(1-e-2t)A.

What happens to the inductance when the current in the coil becomes double its original value?
  • a)
    Becomes half
  • b)
    Becomes four times
  • c)
    Becomes infinity
  • d)
    Becomes double
Correct answer is option 'D'. Can you explain this answer?

Lavanya Menon answered
The formula for magnetic field strength in a coil is:
H=iN/l
The inductance is: directly proportional to magnetic field strength, hence as the current value doubles, the inductance also doubles.

An induced emf is said to be ________
  • a)
    Inductive
  • b)
    Capacitive
  • c)
    Resistive
  • d)
    Cannot be determined
Correct answer is option 'A'. Can you explain this answer?

Nayanika Kaur answered
Any circuit in which a change of current is accompanied by a change of flux, and therefore by an induced emf, is said to be inductive.

 Calculate the value of emf in an inductor if the value of inductance is 15H and an average current of 5A flows through it in 10s.
  • a)
    15V
  • b)
    7.5V
  • c)
    10V
  • d)
    5.5V
Correct answer is option 'B'. Can you explain this answer?

Arya Mukherjee answered
The expression for emf in an inductive circuit is:
emf= LI/2t, where I/2 is the average current.
Substituting the values from the given question, we get emf= 2.5V.

Among the following, which is the right formula for inductance?
  • a)
    L=emf*t/I
  • b)
    L=emf/t*I
  • c)
    L=emf*I/t
  • d)
    L=emf*t*I
Correct answer is option 'A'. Can you explain this answer?

Sanya Agarwal answered
The average emf induced is proportional to the current per unit time, the constant of proportionality being L. Hence emf=LI/t. Making L the subject of the formula, we get: L=emf*t/I.

 At steady state, inductor acts as ____________
  • a)
    Open circuit
  • b)
    Short circuit
  • c)
    Resistor
  • d)
    Capacitor
Correct answer is option 'B'. Can you explain this answer?

Yash Patel answered
At steady state, inductor acts as a short circuit because at steady state maximum current flows across the inductor. Hence it acts as a zero resistance source or an open circuit.

Which, among the following, is the correct formula to fing coupling coefficient?
  • a)
    k=M/sqrt(L1L2)
  • b)
    k=M/sqrt(L12)
  • c)
    k=M/sqrt(L22)
  • d)
    k=M/(L1L2)
Correct answer is option 'A'. Can you explain this answer?

Nilesh Joshi answered
The correct formula for coupling coefficient is k=M/sqrt(L1L2). Where, L1 and L2 are the inductance values of the first and second coil respectively and M is the mutual inductance.

When inductors are connected in series, the voltage across each inductor is _________
  • a)
    Equal
  • b)
    Different
  • c)
    Zero
  • d)
    Infinity
Correct answer is option 'B'. Can you explain this answer?

Ameya Gupta answered
In a series circuit, the current across all elements remain the same and the total voltage of the circuit is the sum of the voltages across all the elements.

Calculate the emf induced in an inductor if the inductance is 10H and the current is 2A in 4s.
  • a)
    2.5V
  • b)
    1.5V
  • c)
    3.5V
  • d)
    5V
Correct answer is option 'A'. Can you explain this answer?

Malavika Nair answered
The expression for emf in an inductive circuit is:
emf= LI/2t
Substituting the values from the given question, we get emf= 2.5V.

The charging time constant of a circuit consisting of an inductor is the time taken for the voltage in the inductor to become __________% of the initial voltage.
  • a)
    33
  • b)
    63
  • c)
    37
  • d)
    36
Correct answer is option 'B'. Can you explain this answer?

Kunal Sharma answered
We know that: V=V0(1-e-t /time constant).
When time constant=t, we have: V=V0(1-e-1)= 0.63*V0.
Hence the time constant is the time taken for the charge in an inductive circuit to become 0.63 times its initial charge.

The discharging time constant of a circuit consisting of an inductor is the time taken for the voltage in the inductor to become __________% of the initial voltage.
  • a)
    33
  • b)
    63
  • c)
    37
  • d)
    36
Correct answer is option 'D'. Can you explain this answer?

Pranab Basu answered
We know that: V=V0(e-t/time constant).
When time constant=t, we have: V=V0(e-1)= 0.36*V0.
Hence the time constant is the time taken for the charge in an inductive circuit to become 0.36 times its initial charge.

 At DC, inductor acts as _____________
  • a)
    Open circuit
  • b)
    Short circuit
  • c)
    Resistor
  • d)
    Inductor
Correct answer is option 'B'. Can you explain this answer?

Shivam Das answered
At DC, the inductor acts as short circuit because the inductive resistance is zero. The frequency of a DC circuit is 0. The inductive resistance=(2*pi*f*L). Therefore, if the frequency is 0, the inductive resistance is zero and it acts as an short circuit.

 Can the coupling coefficient practically ever be equal to 1?
  • a)
    Yes
  • b)
    No
  • c)
    Depends on current in coil 1
  • d)
    Depends on current in coil 2
Correct answer is option 'B'. Can you explain this answer?

Dipika Basak answered
Coupling coefficient can never be equal to 1 because all the flux of coil 1 can never link with coil 2. There are bound to be losses.

 What is the unit for inductive reactance?
  • a)
    Henry
  • b)
    Ohm
  • c)
    Farad
  • d)
    Volts
Correct answer is option 'B'. Can you explain this answer?

Sanya Agarwal answered
Inductive reactance is nothing but the impedance. Impedance is the AC equivalent of resistance, hence the unit for inductive reactance is ohm.

Among the following, which is the right formula for inductance?
  • a)
    L=emf*t/I
  • b)
    L=emf/t*I
  • c)
    L=emf*I/t
  • d)
    L=emf*t*I
Correct answer is option 'A'. Can you explain this answer?

Avik Iyer answered
The average emf induced is proportional to the current per unit time, the constant of proportionality being L. Hence emf=LI/t. Making L the subject of the formula, we get: L=emf*t/I.

Calculate the initial current in an inductor having inductance 0.004H, number of turns is 200 and the flux changes from 760 to 800 ∫Wb. Current changes to 7A.
  • a)
    10A
  • b)
    2A
  • c)
    5A
  • d)
    3A
Correct answer is option 'C'. Can you explain this answer?

From the formula of incremental inductance, we know that:
L=(Change in flux/Change in current)*Number of turns
Substituting the values from the given question, we get change in current= 2A.
Change in current= final current- initial current.
2=7-initial current.
Initial current= 5A.

The discharging time constant of a circuit consisting of an inductor is the time taken for the voltage in the inductor to become __________% of the initial voltage.
  • a)
    33
  • b)
    63
  • c)
    37
  • d)
    36
Correct answer is option 'D'. Can you explain this answer?

We know that: V=V0(e-t/time constant).
When time constant=t, we have: V=V0(e-1)= 0.36*V0.
Hence the time constant is the time taken for the charge in an inductive circuit to become 0.36 times its initial charge.

What is the time constant of an inductive circuit?
  • a)
    LR
  • b)
    R/L
  • c)
    1/LR
  • d)
    L/R
Correct answer is option 'D'. Can you explain this answer?

Nikhil Iyer answered
The time constant in an inductive circuit is the time taken for the voltage across the inductor to become 63 percent of its initial value. It is given by: Time constant= L/R.

The flux linkage in coil 1 is x Wb-t and it has 500 turns and the current in coil 2 is 2A, calculate the value of x if the mutual inductance is 750H.
  • a)
    1Wb-t
  • b)
    2Wb-t
  • c)
    3Wb-t
  • d)
    4Wb-t
Correct answer is option 'C'. Can you explain this answer?

We know that mutual inductance is the product of the number of turns in one coil and the flux linkages of that coil, divided by the current in the other coil.
φ=750*2/500= 3Wb-t.

 If the switch is closes at t=0, what is the current in the circuit?
  • a)
    0A
  • b)
    10A
  • c)
    20A
  • d)
    30A
Correct answer is option 'A'. Can you explain this answer?

Due to the presence of inductor current won't rise immediately when the switch is closed. inductor current= integral V dt. over 0 seconds results in zero

In an inductive circuit, when the XL value increases, the circuit power factor?
  • a)
    Increases
  • b)
    Decreases
  • c)
    Remains the same
  • d)
    Becomes zero
Correct answer is option 'B'. Can you explain this answer?

Avik Iyer answered
 The power factor, that is, cos(phi) is equal to the resistance divided by the impedance in the circuit, hence as impedance increases, the power factor decreases.

If the current in one coil becomes steady, the magnetic field is?
  • a)
    Zero
  • b)
    Infinity
  • c)
    Doubles
  • d)
    Halves
Correct answer is option 'A'. Can you explain this answer?

Jaya Rane answered
A magnetic field is produced when there is a changing electric field. Hence the magnetic field is zero when the current is steady.

If the current in one coil is steady, what happens to the mutual inductance?
  • a)
    Zero
  • b)
    Infinity
  • c)
    Doubles
  • d)
    Halves
Correct answer is option 'A'. Can you explain this answer?

Lavanya Menon answered
A magnetic field is produced when there is a changing electric field. Hence the magnetic field is zero when the current is steady. When the magnetic field is zero there is no current induced in the other coil, thus electric field is zero.

What happens to the effective inductance when a ferromagnetic core is introduced?
  • a)
    Increases
  • b)
    Decreases
  • c)
    Remains the same
  • d)
    Becomes zero
Correct answer is option 'A'. Can you explain this answer?

The effective inductance increases when a ferromagnetic core is introduced within an inductor because the flux increases. Inductance varies directly with the flux hence it increases.

The flux linkage in coil 1 is 3Wb-t and it has x turns and the current in coil 2 is 2A, calculate the value of x if the mutual inductance is 750H.
  • a)
    300
  • b)
    400
  • c)
    500
  • d)
    700
Correct answer is option 'C'. Can you explain this answer?

Avik Saha answered
We know that mutual inductance is the product of the number of turns in one coil and the flux linkages of that coil, divided by the current in the other coil.
N=750*2/3= 300 turns.

What is Helmholtz equation?
  • a)
     i=I(1-eRt/L)
  • b)
     i=I(1-e-Rt/L)
  • c)
     i=I(1+e-Rt/L)
  • d)
     i=I(e-Rt/L)
Correct answer is option 'B'. Can you explain this answer?

Helmholtz equation is an equation which gives the formula for the growth in an inductive circuit. Hence the Helmholtz formula is: i=I(1-e-Rt/L).

Chapter doubts & questions for R-L & R-C Transients - 6 Months Preparation for GATE Electrical 2025 is part of Electrical Engineering (EE) exam preparation. The chapters have been prepared according to the Electrical Engineering (EE) exam syllabus. The Chapter doubts & questions, notes, tests & MCQs are made for Electrical Engineering (EE) 2025 Exam. Find important definitions, questions, notes, meanings, examples, exercises, MCQs and online tests here.

Chapter doubts & questions of R-L & R-C Transients - 6 Months Preparation for GATE Electrical in English & Hindi are available as part of Electrical Engineering (EE) exam. Download more important topics, notes, lectures and mock test series for Electrical Engineering (EE) Exam by signing up for free.

Top Courses Electrical Engineering (EE)