22324 Electrical Circuits Viva Questions with Answers
Q.1. What do you
mean by electric network and electric circuit?
Answer: An electric network is any possible
interconnection of electric circuit elements (e.g. R, L, C) or branches. An
electric circuit is a closed energized network. A network is not necessarily a
circuit example T network.
Q.2. Define
current, voltage and power.
Answer: The time rate of flow of electric charge across a cross-sectional boundary is termed as current. Voltage is defined as work done in moving a unit positive charge once around the closed path. The instantaneous power delivered to circuit elements is the product of the instantaneous value of voltage and current of the element. In terms of energy, power is defined as “The time rate of change of energy is called power”.
Q.3. What is
self-inductance (L)?
Answer: The property of coil due to which it opposes any increase or decrease of current or flux through it is called self-inductance.
Q.4. What is
mutual-inductance (M)?
Answer: Mutual-inductance is the property of two coil
because of which each opposes any change in the current flowing through the
other by developing an induced emf.
Q.5. What are
lumped network?
Answer: A network in which physically separate resistors, capacitors and inductors can be represented.
Q.6. What are
distributed network?
Answer: A network in which resistors, capacitors and
inductors cannot be electrically separable and isolated as individual elements.
A transmission line has distributed resistor, inductor and capacitor which are
not isolated from the network.
Q.7. What is the
difference between the unilateral and bilateral network?
Answer: A unilateral network is one whose elements
follow different laws relating to voltage and current for different direction
of voltage and current polarities. For example, vacuum tubes, crystal and metal
rectifiers are unilateral elements. A bilateral network is defined as those
whose elements can transmit well in either direction. For example, elements
made of high conductivity material like iron core conductor is bilateral
elements.
Q.8. What are
linear networks?
Answer: A linear network is one for which the principle of superposition holds. A circuit element is linear if the relation between voltage and current involves a constant coefficient. Therefore, it is concluded that a linear network must be bilateral but a bilateral network is not necessarily linear. For example, an iron core conductor is bilateral but it is non linear.
Q.9. What is a
current source?
Answer: A generator which maintains its output current independent of the voltage across its terminals. It is indicated by a circle enclosing an arrow for reference current direction. An ideal current source has infinite internal resistance.
Q.10. What is a
voltage source?
Answer: A generator which maintains its value of potential independent of the output current. An AC source is indicated by a circle enclosing a wavy line. An ideal voltage source has zero internal resistance.
Q.11. What should
be the internal resistance of an ideal voltmeter and ammeter?
Answer: Ideally the internal resistance of a
voltmeter should be infinite and ideally the internal resistance of an ammeter
should be zero.
Q.12. State
Kirchhoff’s current law (KCL).
Answer: It is based on conservation of charge. It states that, “The algebraic sum of currents at any node of a circuit is zero”.
Q.13. State
Kirchhoff’s voltage law (KVL).
Answer: It is based on conservation of energy. It
states that, “The algebraic sum of voltages in any closed path of network that
is traversed in any single direction is zero”.
Q.14. Explain the
switching action of an inductor?
Answer: The current through an inductor cannot change
instantaneously. If an energy source is suddenly connected to an inductor then
it will not cause current to flow initially. Thus inductor acts as open circuit
for the new applied source at the instant of switching. If a current of vale IO
flows in the inductor at the instant switching takes place, that current will
continue to flow. For the initial instant, the inductor may be considered as a
current source of current IO.
Q.15. Explain the
switching action of a capacitor?
Answer: The voltage across a capacitor cannot change instantaneously. Since, we have V=q/c, hence zero charge corresponds to zero voltage. Thus for a suddenly applied energy source, voltage across capacitor is initially zero i.e. equivalent to a short circuit. Hence on connecting an uncharged capacitor to an energy source, a current flows instantaneously. But with an initial charge in the system, the capacitor is equivalent to a voltage source of vale VO= qo/c, where qo the initial charge.
Q.16. State clearly
the Thevenin’s theorem.
Answer: This theorem states that a linear, bilateral
network consisting of active and passive elements can be replaced by a voltage
source connected in series with impedance. The value of voltage source is equal
to the open circuited voltage seen across the terminals and the impedance is
equal to the impedance seen across the open circuited terminals with all
sources replaced by their internal impedances.
Q.17. Explain the
usefulness of Thevenin’s theorem in linear network?
Answer: Thevenin’s theorem is very useful in linear network analysis because using Thevenin’s theorem a large part of network, often a complicated and uninteresting part, can be replaced by a very simple equivalent which enables us to make rapid calculations. It is very useful when it is desired to determine the current through or voltage across any one element in a network instead of going through the lengthy method of soling the network.
Q.18. State clearly
the superposition theorem.
Answer: This theorem states that the voltage across or current through any element due to multiple sources present in a linear network is equal to the algebraic sum of voltage across or current through that element due to individual source with all other sources replaced by their internal impedances.
Q.19. Explain the
usefulness of superposition theorem in linear network?
Answer: Superposition theorem is very useful in linear network analysis because a complex network can be solved easily by breaking it in simpler networks and using superposition theorem. This theorem is very useful to calculate the voltage and current when more than one active source is present in the network.
Q.20. State
Tellegen’s theorem and its application.
Answer: It states that for any given time, the sum and power delivered to each branch of any electric network is zero. In other words, the summation of instantaneous power or summation of complex power of sinusoidal sources in a electrical network is zero. Tellegen’s theorem is applicable for any lumped network having elements which are linear or non-linear, active or passive, time-varying or time-invariant.
Q.21. What is
Reciprocity theorem?
Answer: This theorem states that the current in
branch X due to voltage source in branch Y of a linear bilateral network is
same as the current in branch Y when the same voltage source is connected in
branch X of the network.
§
Only one
source should be present in the network.
§
Initial
condition must be zero.
§
Network must
be time invariant.
§ Dependent source are not permitted.
Q.22. State
Norton’s Theorem.
Answer: It states that, any two terminals of a
network containing linear passive and active elements may be replaced by an
equivalent current source IN in parallel with a resistance RN where IN is the
current flowing through a short circuit placed across the terminals and RN is
the equivalent resistance of the network as seen from the two terminals with
all the independent sources suppressed.
Q.23. State
Millman’s Theorem.
Answer: This theorem states that, any number of current sources in parallel may be replaced by a single current source, whose current is the algebraic sum of individual source currents and source resistance is the parallel combination of individual source resistance.
Q.24. What is the
condition for maximum power transfer to take place?
Answer: For maximum power transfer to take place the
resistance of the load should be equal to the source resistance and the
reactance of the load should be equal to that of source in magnitude but
opposite in sign. It means, if load is capacitive then source is inductive and
vice-versa.
Q.25. Where the
study of resonance is very useful?
Answer: The study of resonance is very useful in the area of communication particularly. For example, 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.