# Network Theory Questions and Answers – Thevenin’s Theorem

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This set of Network Theory Multiple Choice Questions & Answers (MCQs) focuses on “Thevenin’s Theorem”.

1. Thevenin’s voltage is equal to the _____________ voltage across the _______________ terminals.
a) short circuit, input
b) short circuit, output
c) open circuit, output
d) open circuit, input

Explanation: Thevenin’s voltage is equal to open circuit voltage across output terminals not the short circuit voltage across output terminals.

2. Consider the circuit shown below. The expression of Thevenin’s voltage (VTh) is? a) V(Z1/(Z1+Z2))
b) V(Z2/(Z1+Z2))
c) V(Z1)
d) V(Z2)

Explanation: Thevenin’s theorem gives us a method for simplifying a given circuit. The thevenin’s voltage is VTh = V(Z2/(Z1+Z2)).

3. The value of ZTh in the circuit shown below is? a) Z3+(Z1Z2/(Z1+Z2))
b) Z1+(Z3Z2/(Z3+Z2))
c) Z2+(Z1Z3/(Z1+Z3))
d) (Z1Z2/(Z1+Z2))

Explanation: The thevenin’s equivalent form of any complex impedance consists of an equivalent voltage source and an equivalent impedance. The thevenin’s impedance is ZTh = Z3+(Z1Z2/(Z1+Z2)).

4. In the circuit shown below, find the thevenin’s voltage across ‘ab’ terminals. a) 48.5∠40.35⁰
b) 48.5∠-40.35⁰
c) 49.5∠-40.35⁰
d) 49.5∠40.35⁰

Explanation: Though the thevenin’s equivalent circuit is not same as its original circuit, the output current and voltage are the same in both cases. The thevenin’s voltage is equal to the voltage across (4+j6)Ω impedance. VTh =50∠0o×(4+j6)/((4+j6)+(3-j4))=49.5∠40.35⁰V.

5. Find the thevenin’s impedance in the circuit shown below. a) 4.83∠-1.13⁰
b) 5.83∠1.13⁰
c) 4.83∠1.13⁰
d) 5.83∠-1.13⁰

Explanation: The impedance is equal to the impedance seen into the network across the output terminals. ZTh = (j5-j4)+(3-j4)(4+j6)/(3-j4+4+j6) = 4.83∠-1.13⁰Ω.

6. Determine the Thevenin’s voltage across ‘ab’ terminals in the circuit shown below. a) 41.86∠0⁰
b) 42.86∠0⁰
c) 43.86∠0⁰
d) 44.86∠0⁰

Explanation: The voltage across the points a and b is called thevenin’s equivalent voltage. Thevenin’s equivalent voltage Vab=100∠0o×j3/(j3+j4) = 42.86∠0o V.

7. Find the Thevenin’s impedance across ‘ab’ terminals in the circuit shown below. a) j4.71
b) j5.71
c) j6.71
d) j7.71

Explanation: The impedance is equal to the impedance seen into the network across the output terminals. Zab=j5 + (j4)(j3)/j7 = j6.71Ω.

8. Determine the load current across j5Ω in the circuit shown below. a) 3.66∠90⁰
b) 3.66∠-90⁰
c) 4.66∠90⁰
d) 4.66∠-90⁰

Explanation: The load current is the ratio of thevenin’s equivalent voltage and thevenin’s equivalent impedance. The load current IL = (42.86∠0o)/(j6.71+j5) = 3.66∠-90o A.

9. Determine the thevenin’s voltage in the circuit shown below. a) 18∠146.31⁰
b) 18∠-146.31⁰
c) 19∠146.31⁰
d) 19∠-146.31⁰

Explanation: The voltage across (-j4) Ω resistor is = (5∠90o)/((2+j2)) (-j4) = 7.07∠-45o V. The voltage across ‘ab’ = -10∠0o+5∠90o-7.07∠-45o=18∠146.31o V.

10. Find the Thevenin’s impedance in the following circuit. a) 11.3∠45⁰
b) 12.3∠45⁰
c) 11.3∠-45⁰
d) 12.3∠-45⁰

Explanation: The impedance is equal to the impedance seen into the network across the output terminals. Zab = 4 + (2+j6)(-j4)/(2+j2) = 11.3∠-45o Ω.

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