 ## SSC JE short note | Network Theory

Q. 1. Why do we use network theorems and techniques to solve electrical circuits?

Ans. Network theorems and techniques are often used to solve electrical circuits for the following reasons :

(i) Through their use, a complicated network can be reduced to a simple circuit.

(ii) Through their use, the analytical solution becomes very easy.

Q. 2. What is the proper use of the terms network? and circuit?

Ans. The term network is generally used in reference to an arbitrary arrangement of passive components while a circuit usually implies the presence of active sources and the current flow. However, there is no hard and fast rule for making these distinctions and the terms “network” and “circuit” are often used interchangeably.

Q.3. What is the difference between a mesh and a loop?

Ans. A loop is any closed path of a network. However, a mesh is the most elementary form of a loop and cannot be further divided into other loops.

Q. 4. What do you mean by a linear circuit element?

Ans. Circuit elements (e.g. resistance etc.) are linear when they follow Ohm’s law that doubling the voltage across them doubles the current through them.

Q. 5. What do you mean by a bilateral circuit element?

Ans. Circuit elements are bilateral if current can pass through them in either direction with the equal case.

Q. 6. What do you mean by an ideal voltage source and an ideal current source?

Ans. An ideal voltage source is one that maintains a constant terminal voltage no matter how much current is drawn from it. For example, an ideal 12 V voltage source theoretically maintains 12 V across its terminals whether you connect 1 M ohm or 0.01-ohm resistance across its terminals.
An ideal current source is one that will supply the same current to any resistance connected to its terminals. For example, a 2 A ideal current source will always supply 2 A to any value of resistance connected across its terminals.

Q. 7. What is nodal analysis?

Ans. The nodal analysis provides a general procedure for analyzing circuits using node voltages as the circuit variables. It is a convenient method and reduces the number of equations to be solved simultaneously.

Q. 8. Can the superposition theorem be used for power calculations?

Ans. No, because the superposition principle is based on linearity. However, the poster absorbed by a resistor depends on the square of voltage or current.

Q. 9. What are the applications of current sources?

Ans. Current sources have several applications (i) They are used to represent certain devices that exhibit a constant-current behavior. Transistors are examples of such devices. (ii) Current sources can also be used to represent equipment such as current-regulated power supplies. These supplies provide a constant current over some limited range of load resistance. (iii) They can be used in certain network analysis procedures.

Q. l0. Can you measure mesh currents?

Ans. Branch currents are the real currents because they actually flow in the branches and can be measured. However, mesh currents are fictitious and cannot be measured except in those instances where they happen to be identical to branch currents.

Q.11. What is the basis of nodal analysis?

Ans. Mesh analysis is based on Kirchhoff’s voltage law. However, nodal analysis is based on Kirchhoff’s current law.

Q.12. What are the advantages of nodal analysis over Kirchhoff’s laws and mesh analysis?

Ans. The advantages of nodal analysis over Kirchhoff’s laws and mesh analysis are :

1. generally, less number of equations are required to solve a circuit by nodal analysis.
2. Nodal analysis is convenient in circuits where various voltages, referred to as common ground, are to be determined.
3. Nodal analysis is a convenient approach to circuits that have current sources and conductances rather than voltage sources and resistances.

Q. 13 How is the reference node chosen in nodal analysis?

Ans. The choice of reference node in the nodal analysis is arbitrary but it is usually convenient to select the reference node as the one having the most components connected to it.

Q. 14. What are the advantages of Thevenin’s theorem?

Ans. Thevenin’s theorem offers the following advantages :

1. It reduces a complex circuit to a simple circuit viz a single source of e.m.f. ETh in series with a single resistance RTh.
2. It greatly simplifies the portion of the circuit of lesser importance and enables us to view the action of the output part directly.
3. The theorem is particularly useful to find current in a particular branch of a network . as the resistance of that branch is varied while all other resistances and e.m.f. sources remain constant.

Q. 15. What is the difference between Thevenin’s theorem and Norton’s theorem?

Ans. Although Thevenin’s theorem and Norton’s theorem can be used to solve a given network, the circuit approach differs in the following respects :

• Norton’s theorem is converse of Thevenin’s theorem in the respect that Norton’s equivalent circuit uses a current generator instead of a voltage generator and the resistance RN (which is the same as RTh) in parallel with the generator instead of being in series with it.
• Thevenin’s theorem is a voltage form of an equivalent circuit whereas Norton’s theorem is a current form of an equivalent circuit.
• The mathematical relation between the two circuit forms is RN = RTh ; IN = EN/ RTh

Q.20. Why is the power system never operated under maximum power conditions?

Ans. Under the conditions of maximum power transfer, the efficiency is low (50%) and there is a greater voltage drop in the lines. In a power system, the goal is higher efficiency rather than maximum power. For these reasons, maximum power transfer is not desired in a power system.

Q.21. Why are electronic circuits generally operated under maximum power transfer conditions?

Ans. In electronic circuits, it is often desirable to transfer maximum power e.g.
(i) The signal power available at the receiving antenna is very small. It is very important to recover the maximum possible amount of signal power from the receiving antenna.
(ii) In the public address system, it is desired that maximum power is transferred from the amplifier to the speaker (i.e. load) in order to operate the speaker. To meet such situations in electronic circuits, we adjust the circuit for maximum power transfer. The technique is to make the load resistance (e.g. speaker) equal to the source (e.g. amplifier) resistance. The circuit is then said to be matched.

Q. 22. Where do we use star/delta and delta/star transformation?

Ans. There are some networks in which resistances are neither in series nor in parallel. A familiar example is a three terminals network e.g. delta network or stat network. In such,.situations, it is not possible to simplify the network by series and parallel circuit rules. However, converting the delta network into a star and vice-versa often simplifies the network and makes it possible to apply series-parallel circuit techniques.

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