Network Theorems MCQ Quiz - Objective Question with Answer for Network Theorems - Download Free PDF

Norton Equivalent Current Source and Resistance:

In electrical circuit analysis, the Norton equivalent of a network is a simplified representation that consists of a single current source (Norton current source, I_N) in parallel with a single resistance (Norton resistance, R_N). This simplification is especially useful for analyzing complex circuits and determining current through or voltage across specific components.

To calculate the Norton equivalent for a given circuit, the following steps are performed:

1. Determine the Norton current (I_N): This is the current through the terminals of the circuit when they are short-circuited.
2. Determine the Norton resistance (R_N): This is the equivalent resistance of the circuit seen from the terminals with all independent sources turned off (voltage sources shorted and current sources opened).

Let us now analyze the problem step-by-step and calculate the Norton equivalent current source and resistance for the given circuit.

Step 1: Norton Current (I_N)

To find the Norton current, we need to short-circuit the output terminals of the given circuit and calculate the current through the short circuit. For the given problem, after performing the necessary circuit analysis (such as applying Ohm’s Law, Kirchhoff’s Voltage Law, or Kirchhoff’s Current Law), the value of the Norton current is found to be:

I_N = 0.4 A

Step 2: Norton Resistance (R_N)

The Norton resistance is determined by calculating the equivalent resistance of the circuit as seen from the output terminals with all independent sources turned off:

• Voltage sources: Replaced by short circuits.
• Current sources: Replaced by open circuits.

After simplifying the given circuit appropriately, the equivalent resistance is calculated to be:

R_N = 10 Ω

Final Norton Equivalent:

The Norton equivalent circuit for the given circuit is:

• Norton current source (I_N): 0.4 A
• Norton resistance (R_N): 10 Ω

Thus, the correct answer is:

Option 1: 0.4 A, 10 Ω

Explanation:

Analysis of Thevenin and Norton Equivalent Circuits

Definition: Thevenin and Norton equivalent circuits are techniques used in electrical engineering to simplify complex networks into more manageable forms for analysis. Thevenin's theorem represents a network as an ideal voltage source (V_TH) in series with a resistance (R_TH), while Norton’s theorem represents the same network as an ideal current source (I_N) in parallel with a resistance (R_N).

Key Relationship:

The resistances R_TH (Thevenin resistance) and R_N (Norton resistance) are always equal in value:

R_TH = R_N

This equivalence arises because both Thevenin and Norton transformations describe the same electrical network, but in different forms. The resistance remains unchanged during the transformation, as it represents the inherent impedance of the circuit.

Correct Option Analysis:

The correct option is:

Option 4: R_TH = R_N

This option is correct because the Thevenin equivalent resistance (R_TH) and Norton equivalent resistance (R_N) of a given passive linear network are always equal. This fundamental relationship ensures consistency in circuit transformations and analysis.

Explanation:

To achieve maximum power transfer, the load impedance must match the complex conjugate of the source impedance. In this problem, the source impedance is given as 10 Ω in series with 10 mH inductance. The complex impedance of the source can be expressed as:

Z_source = R + jωL

Here:

• R = 10 Ω (resistance)
• L = 10 mH = 0.01 H (inductance)
• ω = 10 rad/s (angular frequency)

Substituting the values:

Z_source = 10 + j(10 × 0.01) = 10 + j0.1 Ω

The complex conjugate of the source impedance is:

Z_load = R - jωL = 10 - j0.1 Ω 

To match this impedance, the load should consist of a 10 Ω resistor in series with a capacitive reactance that cancels out the inductive reactance of the source. The capacitive reactance is given by:

X_C = -X_L = -ωL

X_C = -0.1 Ω

The reactance of a capacitor is related to its capacitance by the formula:

X_C = 1 / (ωC)

Substituting the values:

-0.1 = 1 / (10 × C)

C = 1 / (10 × 0.1) = 1 F

Thus, the load impedance that matches the complex conjugate of the source impedance is a 10 Ω resistor in series with a 1 F capacitor. Therefore, the correct answer is Option 3.

Calculation:

Using Kirchhoff rule 

V = (I + I_o ) R 

⇒ I = V/R - I₀

at V =0 ⇒ I = - I_o

at I = 0 ⇒ V = I₀ R

Thus only correct graph is 

Kirchhoff's Voltage Law (KVL)

According to KVL, the algebraic sum of the voltages in a closed loop is always zero.

ΣV = 0

$-V_1+V_2+V_3+V_4=0$

Calculation

Applying KVL as per the direction ABCD:

+ 50 - 30 - V_S - 10 + 20 = 0

V_S = 30 V

Concept

The power transfer efficiency is:

​$\eta =\frac{I^2{R}_L}{VI}\times 100$

$\eta =\frac{I{R}_L}{V}\times 100$

The current across any resistor is given by:

$I=\frac{V}{R}$

where, I = Current

V = Voltage

R = Resistance

Calculation

Let the voltage and internal resistance of the voltage source be V and R respectively.

Case 1: When the current of 2 A flows through 5 Ω resistance.

$2=\frac{V}{5+R}$ .... (i)

Case 2: When the current of 1.6 A flows through 10 Ω resistance.

$1.6=\frac{V}{10+R}$ .....(ii)

Solving equations (i) and (ii), we get:

2(5+R)=1.6(10+R)

10 + 2R = 16 + 1.6R

0.4R = 6

R = 15Ω

Putting the value of R = 15Ω in equation (i):

V = 40 volts

Case 3: Current when the load is 15Ω

$I=\frac{V}{R+R_L}$

$I=\frac{40}{15+15}=\frac{4}{3}A$

$\eta =\frac{\frac{4}{3}\times 15}{40}\times 100$

η = 50%

Additional Information Condition for Maximum Power Transfer Theorem:

When the value of internal resistance is equal to load resistance, then the power transferred is maximum.

Under such conditions, the efficiency is equal to 50%.

Concept:

Thevenin's Theorem:

Any two terminal bilateral linear DC circuits can be replaced by an equivalent circuit consisting of a voltage source and a series resistor.

To find V_oc: Calculate the open-circuit voltage across load terminals. This open-circuit voltage is called Thevenin’s voltage (V_th).

To find I_sc: Short the load terminals and then calculate the current flowing through it. This current is called Norton current (or) short circuit current (i_sc).

To find R_th: Since there are Independent sources in the circuit, we can’t find Rth directly. We will calculate Rth using Voc and Isc and it is given by

${\mathrm{R}}_{\mathrm{t}\mathrm{h}}=\frac{{\mathrm{V}}_{\mathrm{o}\mathrm{c}}}{{\mathrm{i}}_{\mathrm{s}\mathrm{c}}}$  

Application:

Given: Load line equation = V + i = 100

To obtain open-circuit voltage (Vth) put i = 0 in load line equation 

⇒ Vth = 100 V

To obtain short-circuit current (isc) put V = 0 in load line equation

⇒ isc = 100 A

So, $R_{th}=\frac{V_{th}}{i_{sc}}=\frac{100}{100}=1\mathrm{\Omega }$

Equivalent circuit is

Current (i) = 100/2 = 50 A

Applying loop-law in the given circuit.

- V + i × R = 0

- V + I × 1 = 0

⇒ V = i

Given Load line equation is V + i = 100

Putting V = i 

then i + i = 100 

⇒ i = 50 A

Distributed Network:

• If the network element such as resistance, capacitance, and inductance are not physically separated, then it is called a Distributed network.
• Distributed systems assume that the electrical properties R, L, C, etc. are distributed across the entire circuit.
• These systems are applicable for high (microwave) frequency applications.

Lumped Network:

• If the network element can be separated physically from each other, then they are called a lumped network.
• Lumped means a case similar to combining all the parameters and considering it as a single unit.
• Lumped systems are those systems in which electrical properties like R, L, C, etc. are assumed to be located on a small space of the circuit.
• These systems are applicable to low-frequency applications.

Kirchoff's Laws:

• Kirchhoff’s laws are used for voltage and current calculations in electrical circuits.
• These laws can be understood from the results of the Maxwell equations in the low-frequency limit.
• They are applicable for DC and AC circuits at low frequencies where the electromagnetic radiation wavelengths are very large when we compare with other circuits. So they are only applicable for lumped parameter networks.

Kirchhoff's current law (KCL) is applicable to networks that are:

• Unilateral or bilateral 
• Active or passive 
• Linear or non-linear
• Lumped network

KCL (Kirchoff Current Law): According to Kirchhoff’s current law (KCL), the algebraic sum of the electric currents meeting at a common point is zero.

Mathematically we can express this as:

$\sum _{n=1}^M{i}_n=0$

Where in represents the nth current

M is the total number of currents meeting at a common node.

KCL is based on the law of conservation of charge.

Kirchhoff’s Voltage Law (KVL):

It states that the sum of the voltages or electrical potential differences in a closed network is zero. 

• According to Tellegen’s theorem, the summation of instantaneous powers for the n number of branches in an electrical network is zero.
• Let n number of branches in an electrical network have I1, I2, I3, ….. In respective instantaneous currents through them.
• These branches have instantaneous voltages across them are V1, V2, V3, ….. Vn respectively.
• According to Tellegen’s theorem, $\sum _{k=1}^n{V}_k.I_k=0$
• It is based on the conservation of energy.
• It is applicable to both linear and non-linear circuits.

Concept:

Maximum power transfer theorem:

Maximum power transfer theorem states that " In a linear bilateral network if the entire network is represented by its Thevenin's equivalent circuit then the maximum power transferred from source to the load when the load impedance is equal to the complex conjugate of  Thevenin's impedance.

Let's consider variable resistive load and Thevenin's equivalent network as shown below,

$P_m=\frac{V_{th}^2}{4R_{th}}$

Where, 

Pm is the maximum power 

Vth is the source voltage or Thevenin's voltage

Rth is the Thevenin's resistance (Rth = RL = RS)

The efficiency of the maximum power transfer theorem will be 50 %

The voltage across the load resistance/impedance is VL = VS / 2

Calculation:

Given the circuit diagram

Source voltage VS = 200 V

Va = 60 V

As V is the voltage across the load.

V = VS / 2 = 200 / 2 = 100 V

Load current i = V / RL (When maximum power is transferred RL = RS = Rth = 10 Ω) 

i = 100 / 10 = 10 A

By applying nodal analysis at node V

$-i+\frac{V}{20}+\frac{V-V_a}{R}=0$

$-10+\frac{100}{20}+\frac{100-60}{R}=0$

R = 8 Ω

Therefore, the value of R is 8 Ω when V_a is 60 V and maximum power is transferred from N₁ to N₂

Concept:

Superposition theorem is used to solve a circuit that contains multiple current and/or voltage sources acting together.

Theorem:

• The superposition theorem states that "in a linear circuit with several sources, the current and voltage for any element in the circuit is the sum of the currents and voltages produced by each source acting independently."
• The superposition theorem applies only when all the components of the circuit are linear, which is the case for resistors, capacitors, and inductors it is not applicable to networks containing nonlinear elements.

​Calculation:

case 1:

When 8 V source are there

I = 8 / 16 = 0.5 A

case 2:

When only 2 A current source present

Apply KVL in loop

6 I + 2 ( I - 2 )+ 8 I = 0

I = 0.25 A

case 3:

When 6 V source are there

I = - 6 / 16 = - 0.375 A

Now using superposition theorem

Total current I = 0.5 + 0.25 + ( - 0.375 ) A

= 0.375 A

= 375 mA

Important Points

Various Theorem and the circuits where they are applicable is shown below in the table:

Concept:

Maximum power transfer theorem:

• Maximum power transfer theorem states that " In a linear bilateral network if the entire network is represented by its Thevenin's equivalent circuit then the maximum power transferred from source to the load when the load resistance is equal to the Thevenin's resistance."
• P=VS2.RL(RS+RL)2" role="presentation" style="display: inline; position: relative;" tabindex="0">$\mathrm{P}=\frac{V_S^2.R_L}{{\left(R_S+R_L\right)}^2}$For maximum power transfer, RL = Rth 
• Then the maximum power transferred is given by ${\mathrm{P}}_{max}=\frac{V_S^2}{4R_L}$

Explanation:

Circuit Diagram

Given,

R_s = 5 to 25 Ω (variable)

R_L = 10 Ω (fixed)

Here Maximum Power Transfer theorem is not applicable as the load resistor is not variable.

Current, $I=\frac{V}{R_s+R_L}$

Power transferred to load R_L,

$P=I^2{R}_L=[\frac{V}{R_S+R_L}{]}^2\times R_L$

It is clear that for P to be maximum, R_S should be minimum.

∴ R_S = 5 Ω 

Additional Information 

Properties of maximum power transfer theorem: 

• This theorem is applicable only for linear networks i.e networks with R, L, C, transformer, and linear controlled sources as elements.
• The presence of dependent sources makes the network active and hence, MPPT is used for both active as well as passive networks.
• This theorem is applicable when the load is variable.

Maximum power transfers at RL = Rs

The current at this condition is,

$I_L=\frac{V_S}{2R_L}=\frac{V_S}{2R_S}$

The maximum value of current occurs at RL­ = 0 and is given by
$I_L=\frac{V}{R_S}$

Therefore, the current at maximum power is equal to 50% of the maximum current

Key Points

•  If source impedance is complex then load impedance has to be a complex conjugate of source impedance for maximum power transfer to occur.
•  Maximum efficiency is not related to maximum power transfer.

Reciprocity theorem:

Reciprocity theorem states that in any branch of a network, the current (I) due to a single source of voltage (V) elsewhere in the network is equal to the current through the branch in which the source was originally placed when the source is placed in the branch in which the current (I) was originally obtained.

In the circuit (a), the value I_a is obtained for a voltage source V. According to reciprocity theorem, this current is equivalent to I_b in the circuit B.

Limitations of reciprocity theorem:

• The network should be bilateral linear and time-invariant.
• It can apply only to the single-source network and not for multi-source.
• It is also applicable for passive networks consisting L,C.
• Not applicable for circuits containing dependent sources even if it is linear.

Concept:

Maximum power transfer for DC circuit:

According to the MPT the maximum power transfer to the load when the load resistance is equal to the source resistance or Thevenin resistance.

RL = Rth 

RL = load resistance

Rth = Thevenin or source resistance

The power at maximum power transfer (P_max) = V_th² / 4R_th

The maximum power transfer theorem is used in electrical circuits.

Calculation:

R_th = R_L

= ( 30 × 150 )  / 180

= 25 Ω 

V_th = V_ab 

= ( 150 × 180 ) / (150 + 30 )

= 150 V

From above concept,

$P_{max}=\frac{V_{th}^2}{4R_{th}}=\frac{{150}^2}{4\times 25}=225W$

Pmax = 225 W

Reciprocity theorem: It states that the current I in any branch of a network, due to single voltage source (E) anywhere in the network is equal to the current of the branch in which source was placed originally and when the source is again put in the branch in which current is obtained originally.

Limitations of reciprocity theorem:

• The network should be linear and time-invariant
• It can apply only to the single-source network