# Basic Electrical Engineering Questions and Answers – Thevenin’s Theorem

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

**1. Thevenin’s theorem is true for __________**

**a) Linear networks**

b) Non-Linear networks

c) Both linear networks and nonlinear networks

d) Neither linear networks nor non-linear networks

**Explanation:** Only linear circuit elements, such as BJTs and semiconductors, are affected by Thevenin’s theorem.

**2. In Thevenin’s theorem Vth is __________**

a) Sum of two voltage sources

**b) A single voltage source**

c) Infinite voltage sources

d) 0

**Explanation:** A combination of voltage sources, current sources, and resistors is equivalent to a single voltage source V and a single series resistor R, according to Thevenin’s theorem.

**3. Vth is found across the ____________ terminals of the network.**

a) Input

**b) Output**

c) Neither input nor output

d) Either input or output

**Explanation:** Vth is found across the output terminals of a network, not the input terminals, according to Thevenin’s theorem.

**4. Which of the following is also known as the dual of Thevenin’s theorem?**

**a) Norton’s theorem**

b) Superposition theorem

c) Maximum power transfer theorem

d) Millman’s theorem

**Explanation:** The dual of Thevenin’s theorem is Norton’s theorem, because it contains short circuit current, which is the dual of power output, which is found in Thevenin’s theorem.

**5. Can we use Thevenin’s theorem on a circuit containing a BJT?**

a) Yes

**b) No**

c) Depends on the BJT

d) Insufficient data provided

**Explanation:** Only linear networks can benefit from Thevenin’s theorem. Because BJT is a non-linear network, we can’t use Thevenin’s theorem to solve it.

**6. Calculate the Thevenin resistance across the terminal AB for the following circuit.**

**basic-electrical-engineering-questions-answers-thevenins-theorem-q1**

a) 4.34 ohm

**b) 3.67 ohm**

c) 3.43 ohm

d) 2.32 ohm

**Explanation:** Thevenin resistance is found by opening the circuit between the specified terminal and shorting all voltage sources.

When the 10V source is shorted, we get:

Rth=(1||2)+3=3.67 ohm.

**7. Calculate Vth for the given circuit.**

**basic-electrical-engineering-questions-answers-thevenins-theorem-q1**

a) 5.54V

b) 3.33V

**c) 6.67V**

d) 3.67V

**Explanation:** 4 ohm is removed and then v across 2 ohm is calculated by voltage divider 2*10/(2+1) = 6.67V. Voltage between A and B i.e. Vth is equal to voltage across 4 ohm resistance since no current flow through 3 ohm resistance. So, Vth = 6.67V.

**8. Calculate the current across the 4 ohm resistor.**

**basic-electrical-engineering-questions-answers-thevenins-theorem-q1**

**a) 0.86A**

b) 1.23A

c) 2.22A

d) 0.67A

**Explanation:** Thevenin resistance is found by opening the circuit between the specified terminal and shorting all voltage sources.

When the 10V source is shorted, we get:

Rth=(1||2)+3=3.67 ohm.

Vth is calculated by opening the specified terminal.

Using voltage divider, Vth= 2*10/(2+1)=6.67V.

On drawing the Thevenin equivalent circuit, we get Rth, 4 ohm and Vth in series.

Applying Ohm’s law, I=Vth/(4+Rth) = 0.86A.

**9. The Thevenin voltage is the__________**

**a) Open circuit voltage**

b) Short circuit voltage

c) Open circuit and short circuit voltage

d) Neither open circuit nor short circuit voltage

**Explanation:** Thevenin voltage is obtained by connecting the specified terminals in an open circuit configuration. It is not the short circuit voltage because the voltage is zero when the specified terminals are shorted.

**10. Thevenin resistance is found by ________**

a) Shorting all voltage sources

b) Opening all current sources

**c) Shorting all voltage sources and opening all current sources**

d) Opening all voltage sources and shorting all current sources

**Explanation:** Ideal current sources have infinite internal resistance, so they act like an open circuit, whereas ideal voltage sources have zero internal resistance, so they act like a short circuit.

According to Thevenin’s Theorem, any linear circuit, no matter how complex, may be simplified to an equivalent circuit using only a single voltage source and series resistance coupled to a load. Ideal current sources have infinite internal resistance, so they act like an open circuit, whereas ideal voltage sources have zero internal resistance, so they act like a short circuit. When the load resistor is open, the Venin voltage VTH is defined as the voltage across the load terminals. When all sources are lowered to zero and the load resistor is open (RTH = ROC), thevenin resistance is defined as the resistance measured by an ohmmeter across the load terminals in the figure above.