Department of Electrical and Electronics Engineering.
Electric Circuits
UNIT 1 (Basic Circuit concepts)
 What do you mean by an electric network and an electric circuit
Interconnection
of two or more simple circuit elements (voltage sources, resistors, inductors
and capacitors) is called an electric network. If a network contains at least
one closed path, it is called an electric circuit.
 Classify the network elements
i)
Active or passive
ii)
Unilateral or bilateral
iii)
Linear or non linear
iv)
Lumped or distributed
 Differentiate active and passive elements
Energy
sources (voltage or current sources) are active elements, capable of delivering
power to some external device. The passive elements are those which are capable
only of receiving power.
 Differentiate bilateral and unilateral elements
In
the bilateral element, the voltage  current relation is the same for current
flowing in either direction. In contrast, a unilateral element has different
relations between voltage and current for the two possible directions of
current.
 Differentiate linear and nonlinear elements
An
element is said to be linear, if it satisfies the linear voltage current
relationship. That means the current through the element is multiplied by some
constant ‘α’, and results in the multiplication of voltage across the element
by the same constant.
An
element which does not satisfy this relation is called a nonlinear element.
 Define lumped and distributed elements in electric circuits
The
elements which are separated physically are known as lumped elements, like
resistors, capacitors and inductors. Distributed elements, on the other hand,
are those which are not separable for analytical purposes.
 Write the equations of energy stored by an inductor and capacitor
Energy stored by
an inductor is,
W = LI^{2} / 2
I à
Current flow through the inductor
Energy stored by
a capacitor
W = CV^{2} / 2
V à
Voltage across capacitor
 State Kirchhoff’s voltage and current law
Kirchhoff’s
voltage law:
It
states that the algebraic sum of voltages around any closed path in a circuit
is always zero.
Kirchhoff’s current law:
Kirchhoff’s current law:
It
states that the sum of currents entering into any point is equal to the sum of
the currents leaving that point.
 Write the equation for voltage for the given circuit using voltage division technique
 Write the equation for current for the given circuit using current division technique
 How can a practical voltage source, having an ideal voltage V_{S }and internal series resistance R_{V} be replaced by a current source?
The
practical voltage source, having an ideal voltage V_{s }and internal
series resistance R_{V} be replaced by a current source I = V_{S}
/ R_{V }in parallel with an internal resistance R_{I} = R_{V}
UNIT II (Sinusoidal steady
state analysis)
 Write the equation for Instantaneous active power, Average power, Apparent power
Instantaneous active
power,
P(t)
= V_{m}I_{m}[Cos(2ωt+θ)+Cos θ]
/ 2
Average power,
P_{av}
= V_{m}I_{m }Cos θ / 2
Apparent power,
P_{app}
= V_{eff }I_{eff}
 Define Power factor
It
is defined as the ratio of average power to the apparent power, whereas the
apparent power is the product of the effective values of the current and the
voltage.
Power factor = P_{av} / (V_{eff} I_{eff)}
It
is also defined as the factor with which the volt amperes are to be multiplied
to get true power in the circuit
 Draw the power triangle
 Give any one application of resonance
In
the area of communications, the ability of a radio receiver to select a certain
frequency, transmitted by a station and to eliminate frequencies from other
stations is based on the principle of resonance.
 Define resonance frequency for a series RLC circuit
The frequency at
which resonance occurs is called the resonant frequency.
f_{r} = 1 / (2π√LC)
L
à
Inductance of the circuit
C
à
Capacitance of the circuit
 Define Bandwidth of a RLC circuit
It
is the range of frequencies for which the current or output voltage is equal to
70.7 % of its value at the resonant frequency.
 Define the Q factor of a RLC series circuit
The
Quality factor, Q, is the ratio of the reactive power in the inductor or
capacitor to the true power in the resistance in series with the coil or
capacitor.
Q
= 2π * (maximum energy stored / energy dissipated per cycle)
 Define Magnification in resonance
The
ratio of voltage across either L or C to the voltage applied at resonance can
be defined as magnification.
Magnification
= (V_{L} / V) or (V_{C} / V)
 What is tank circuit?
The
parallel resonant circuit is generally called a tank circuit, because of the
fact that, the circuit stores energy in the magnetic field of the coil and in
the electric field of the capacitor. The stored energy is transferred back and
forth between the capacitor and coil, and viceversa.
 Write the types of balanced and unbalanced loads in a three phase system?
Types of
balanced loads:
i)
Balanced delta connected load
ii)
Balanced star connected load
Types of
unbalanced loads:
i)
Unbalanced delta connected load
ii)
Unbalanced three wire star connected load
iii)
Unbalanced four wire star connected load
 The neutral point of an unbalanced three wire star connected load is called as a floating neutral point. Why?
The
potential of the load star point is different from that of the supply star
point. The result is that the load phase voltages are not equal to the supply
phase voltage and they are not only unequal in magnitude, but also subtend
angles other than 120^{o} with one another. The magnitude of each phase
voltages depends upon the individual phase loads. The potential of the load
neutral point changes according to changes in the impedances of the phases,
that is why sometimes the load neutral is also called floating neutral point.
 When the wattmeter will give down scale reading? How to measure the power under such condition?
If
the phase angle between the line voltage and current is greater than 90^{o},
the corresponding wattmeter would indicate downscale reading. To obtain upscale
reading, the connections of either the current coil or the pressure coil have
to be interchanged. Reading
obtained after reversal of coil connection should be taken as negative. Then,
the algebraic sum of the two wattmeter readings gives the total power.
Unit – III (Circuits
and theorems)
1.
What do
you mean by a planar and a non planar circuit?
A circuit is said to be planar,
if it can be drawn on a plane surface without crossovers. A non planar circuit
cannot be drawn on a plane surface without a crossover.
2.
State
Superposition theorem
It states that in any linear
network containing two or more sources, the response in any element is equal to
the algebraic sum of the responses caused by individual sources acting alone,
while the other sources are non operative; that is, while considering the
effect of individual sources, other ideal voltage sources and ideal current
sources in the network are replaced by short circuit and open circuit across
their terminals. This theorem is valid only for linear systems.
The Superposition theorem is not
valid for power responses. It is applicable only for computing voltage and
current responses.
3.
State
Thevenin’s theorem
It states that any circuit having
a number of voltage sources, resistances (impedances) and open output terminals
can be replaced by a simple equivalent circuit consisting of a single voltage
source in series with a resistance (impedances), where the value of the voltage
source is equal to the open circuit voltage across the output terminals, and resistance
(impedance) is equal to the resistance seen in to the network across the output
terminals.
4.
State
Norton’s theorem
It states that any circuit with
voltage sources, resistances (impedances) and open output terminals can be
replaced by a single current source in parallel with single resistance
(impedance), where the value of current source is equal to the current passing
through the short circuit output terminals and the value of the resistance
(impedance) is equal to the resistance seen into the output terminals.
5.
State
Reciprocity theorem
According to this theorem, if we
apply some input( from either a current source, or a voltage source) to a
circuit which consists of resistors, inductors, capacitors and transformers,
including coupled circuits, the ratio of response(output) in any element to the
input is constant even when the position of input and output terminals are
interchanged. This is called the Reciprocity theorem.
6.
State
Compensation(Substitution) theorem
It states that any element in the
network, linear or non linear, may be replaced by a voltage source of magnitude
equal to line current, passing through the element multiplied by the value of
the element, provided the currents and voltages in other parts of the circuit
remain unaltered.
This theorem is useful to find
the changes in current or voltage when the value of resistance is changed in
the circuit.
7.
State
Reciprocity theorem
It states that maximum power is
delivered from a source to a load when the load resistance is equal to the
source resistance.
8.
State
Millman’s theorem
Unit – IV (Response of Electric Circuits)
 What is transient state?
If a network contains energy
storage elements, with change in excitation, the current and voltages change
from one state to other state. The behaviour of the voltage (or) current when
it is changed from one state to another state is called transient state.
 What is natural response?
If a circuit containing storage
elements which are independent of sources, the response depends upon the nature
of the circuit, it is called natural response.
 What is meant by natural frequency?
If the damping is made zero then
the response oscillates with natural frequency without any opposition, such a
frequency is called natural frequency of oscillation, denoted as ω_{n}.
 Define Damping ratio.
It is the ratio of actual
resistance R in the circuit to the critical resistance R_{C}. It is
denoted by greek letter zeta ζ.
.
 Write down few applications of RL, RC and RLC circuits.
The few applications of RL,RC and
RLC circuits are given below.
Coupling circuits,
Phaseshift circuits,
Filters,
Resonant circuits,
AC bridge circuits, and
Transformers.
 What is the timeconstant for RL and RC circuits?
Time constant for RL circuit, =L/R
Time constant for RC circuit, =1/RC
 How does an inductor act at t=0^{+} and t=∞?
An inductor will act as a open
circuit at t=0^{+ }and will act as a short circuit at t=∞.
 Obtain the natural frequency and time constant of an RLC series circuit with R=1K, L=100mH and C=0.1F.
 What is meant by over damped and under damped system?
 When do we get transient response?
Whenever a circuit is switched from
one condition to another either by a change in the applied source (or) change
in the circuit elements there is a transitional period during which the branch
currents and voltage change from their values to new ones, this period is
called transient.
 Distinguish between steady state and transient response of an electric circuit.
SL.No

STEADY STATE RESPONSE

TRANSIENT RESPONSE

1

Amplitude will not change.

Amplitude may change.

2

Frequency will not change.

Frequency may change.

3

Constant voltage and current with time.

Change from one steady state to another.

4

Algebraic equations are used.

Integrodifferential equations are used

 What are the causes of transient behavior occurring in a circuit?
The causes are
a) It
may be due to the sudden change of applied voltage.
b) When
the voltage source is shorted.
c) When
a circuit is connected or disconnected and
d) Due
to storage elements in the circuit.
 Define Decrement factor or logarithmic decrement.
 What is the Laplace transform ?
 What is the cause of free response in electric circuits?
The presence of storage element
such as inductors or capacitors or both in a network in the absence of external
energy source is the cause of free response.
 Find the poles and zeros for the function , and also obtain the polezero plot.
Unit – V (Two port network and Filters)
 Define oneport network.
A pair of terminals at which a
signal may enter (or) leave a terminal is called a port. A network having only
one such pair of terminals is called oneport network.
 Define transmission parameters.
 Write ABCD parameters in terms of Zparameters.
ABCD parameters in terms of
Zparameters is given below.
 What is steady state?
A circuit consisting of constant
sources is said to be in steady state if the voltages and currents do not
change with time.
 What is meant by free response?
Free response is due to the
internal energy stored in the network. It depends upon the type of elements,
their size etc. This response is independent of the source. This response dies gradually, i.e., it approaches
zero as time becomes infinity. Free response is also known as natural (or)
transient response.
 Distinguish between free and forced response.
When a circuit contains storage
elements which are independent of the sources, the response depends upon the
nature of circuit. This response is called natural (or) free response.
The storage elements deliver the
energy to the resistances. So the response changes with time, gets saturated
after some time. It is referred to as the transient response. When we consider
sources acting on a circuit, the response depends on the nature of such
sources. This response is called forced response.
10. What is a Low
pass filter?
11. Draw the characteristics of ideal low
pass filter.


12. What is a high pass filter?
A highpass
filter is a filter that passes high frequencies
well, but attenuates
(reduces the
amplitude of) frequencies lower than the cutoff
frequency.
13. Draw a high
pass and low pass filter using RC.
14. Give the applications of filter.
Highpass and lowpass filters are also
used in digital image processing to perform
transformations in the spatial frequency
domain.
Most highpass filters have zero gain
(inf dB) at DC. Such a highpass filter with
very low cutoff frequency can be used to
block DC from a signal that is undesired
in that signal (and pass nearly
everything else). These are sometimes called DC
blocking filters.
 A constant k high pass filter is required for a cutoff frequency of 2500Hz. The
resistance of load circuit is
600Ω. Determine the value of the components
required.
SOLUTION:
 Each of the series elements of a Ttype Low pass filter consists of an inductance of 30mH having negligible resistance and the shunt element has a capacitance of 0.2µF. Calculate the cutoff frequency and determine the iterative impedance at a frequency of 1000Hz.
GIVEN:
L/2=30mHL=60mH, C=0.2µF
SOLUTION:
CONFIGURATION OF TSECTION
CONFIGURATION OF TSECTION
Iterative impedance (or)
Characteristic impedance,