UNIT - 111
ONE MARK OUESTIONS
1. Transient disturbance is produced in a circuit whenever
(a) it is suddenly connected or disconnected from the supply
(c) its applied voltage is changed suddenly
(b) it is shorted
(6)all of the above.
2. There are no transients in pure resistive circuits because they
(a) Offer high resistance
(b) obey Ohm's law (c) have no stored energy (4are linear circuits.
3. Transient currents in electrical circuit are associated with
(a) Inductors
(b) capacitors
(c) resisters
(4both (a) and (b).
4. The transients which are produced due to sudden but energetic changes fiom one steady state of a
circuit to another are called ....... transients.
(c) relaxation
(4subsidence
(a) Initiation
(b) transit ion
5. A capacitor does not allow sudden changes
(a) In currents
(b) in voltages
(c) in both currents and voltages
(d)none
6. In an R-L circuit connected to an alternating sinusoidal voltage, size of transient current primarily
depends on
(a) the instant in the voltage cycle at which circuit is closed (b) the peak value of steady-state current
(d) the voltage frequency.
(c) the circuit impedance
7. Transient current in a circuit results fiom
a) voltage applied to the circuit
b) impedance of the circuit
c) changes in the stored energy in inductors and capacitors d) resistance of the circuit
8. Double-energy transients are produced in circuits consisting of
(a) Two or more resistors
( b ) resistance and inductance
(c) Resistance and capacitance
(4) resistance, inductance and capacitance.
9. The transient current in a loss-free L-C circuit when excited fiom an ac source is a/ an ..... sine
wave.
( d )critically damped.
(a) over damped
(b) undamped
(c) under damped
10. Inductors does not allow sudden changes
(a) In currents
(b) in voltages
(c) in both currents and voltages
11. The time constant of a series RL circuit
(a) LR
(b) L I R
(c) R / L
(4 e-R/L
(d)none
12. The time constant of a series RC circuit
(a)lIRC
(b) R I C
(c) RC (6)emRC
13. A circuit has total resistance of 10 ohms and an inductance of 50 H. In how many seconds will
the current reach its steady-state value?
(a)25 sec
(b) 5 sec
(c) 2 sec
(d) 15 sec
14. The time taken for the circuit to change fiom one steady state to another steady state is
called ---------------(a)initial time
(b) transient time
(c) final time (d) delay time
15. Define time constant of RLC circuit.
16. What is an initial condition of the current in R-L series circuit
( 0
(b) V/R
(c) infinity
(d) e-WL
17. The final value theorem is used to find the
(a) Steady state value of the system output
(c) Transient behavior of the system output
(b)Initial value of the system output
(d) none
18. value of the system output What is the steady state value of the current in R-C series circuit
(a) 0
(b) V/R
(c) infinity
(d) e-WL
19. The convolution of .f(t)*g(t) = ----------20. Transient current in an R-L-C circuit is oscillatory when
(a) R = 0
( b ) R >2 L I C
( c ) R <2 L I C
(d)R= 2 L I C
TWO MARK QUESTIONS
1. Derive the time constant of a series RL circuit
2. What is natural response?
3. Explain the behavior of inductor under short and open circuit conditions.
4. What is Convolution integral.
5. Distinguish between steady state and transient response.
6 . Why transients occur in electric circuits?
7. Two coils having equal resistance but different inductances are connected in series. Determine the time
constant of the series combination.
8. Define time constant of RL circuit.
9. Derive the time constant of a series RC circuit
10. A RL series circuit has the following values: V = 75 V dc, R = 50 ohms, and L = 15 H.
(a) Find the time constant. (b) How long after the circuit is energized will the current reach a
steady value?
FIVE MARK QUESTIONS
1. Derive the current expression of a series RC circuit when it is excited by unit impulse .
2. In a series W C circuit, R=6 ohms, L=l H, C=l F. A DC voltage of 40 V is applied at t=O.
Obtain the expression for i(t)
3. A Series RLC circuits with R=300 ohm, L=lH and C=lOOxlO-6 F has a constant voltage of 50V
applied to it at t= 0. Find the maximum value of current ( Assume zero initial conditions)
4. Derive the current expression of a series RL circuit when it is excited by unit pulse .
5. A source voltage of 12 V is applied to .series RL circuit at t = 0 as shown in figure 1. UsingLaplace
Transform method, determine the circuit current i(t) for all t LO. Assume zero initial conditions
6. A 1.0 H choke has a resistance of 50 R . This choke is supplied with an a.c. voltage given by e = 141
sin 3 14 t. Find the expression for the transient component of the current flowing through the choke
after the voltage is suddenly switched on.
7. A coil of resistance 12 nand inductance of 18 H is suddenly connected to a dc supply of 30V.
calculate: (i) The initial rate of change of current (ii) ii) The time constant (iii) The current after 3s
8. A series RC circuit consists of 10Q and capacitor of 0.1 F as shown in figure. A constant
voltage of 20V is applied to the circuit at t = 0. Obtain the current equation. Determine the
voltage across the capacitor and resistor.
9. Derive the current expression of a series RL circuit when it is excited by sinusoidal input.
10. A series R-L circuit has R=20 ohms and L=8 H. The circuit is connected across a DC voltage source
of 120 V at t=O. Calculate the time at which the voltage drops across R and L are the same
TEN MARK QUESTIONS
1. A circuit of resistance 10 L-2 and inductance 0.1 H in series has a direct voltage of 200 V suddenly
applied to it. Find the voltage drop across the inductance at the instant of switching on and at 0.01
second. Find also the flux-lmkages at these instants.
2. A coil of 10 H inductance and 5S2 resistance is connected in parallel with a 20 S2 resistor across a
100-V d.c. supply which is suddenly disconnected. Find
(a) the initial rate of change of current after switching.
(b) the voltage across the 20 R resistor initially and after 0.3 s.
(c) the voltage across the switch contacts at the instant of separation and
(d) the rate at which the coil is losing stored energy 0.3 second after switching.
3. In the circuit shown in figure, switch S is in position 1 for a long time and brought to position 2 at
time t=O. Detennine the circuit current.
4. Derive an expression for current response of R~ and RC series circuit transients.
5. For the circuit shown in figure, find the voltage across the resistor 0.5 ohm when the switch, S is
opened at t=O. Assume that there is no charge on the capacitor and no current in the inductor before
switching.
6. Derive an expression for current response of FUC series circuit transient.
7. A 1.0 H choke has a resistance of 50 ll . This choke is supplied with an a.c. voltage given b y e = 141
sin 314 t. Find the expression for the transient component of the current flowing through the choke
after the voltage is suddenly switched on.
8. For a source free RLC series circuit, the initial voltage across C is 10V and the initial current
through L is zero. If L = 20rnH, C=0.5 microfarad and R=100 ohm. Evaluate i(t).
9. An alternating voltage v = 400 sin (3 14 t + Y ) is suddenly applied across a coil of resistance 0.2 ll
and inductance 6.36 mH.Determine the first peak value of the resultant current when the transient
current has maximum value.
10. A step voltage V(t) = 100 u(t) is applied to a series RLC circuit with L=IOH, R=2ohm and C= 5F.
The initial current in the circuit is zero but there is an initial voltage of 50V on the capacitor in a
direction which opposes the applied source. Find the expression for the current in the circuit