All questions of Electrical Circuits for Electrical Engineering (EE) Exam

The number of mesh currents required in the circuit given below is:
  • a)
    3
  • b)
    4
  • c)
    5
  • d)
    6
Correct answer is option 'C'. Can you explain this answer?

Naroj Boda answered
In the given network
Number of branches (b) = 9
Number of nodes (n) = 5
Number of mesh currents = b – n + 1
= 9 – 5 + 1 = 5

Which of the following is not the same as watt?
  • a)
    Joule/sec
  • b)
    (Ampere)2 × ohm
  • c)
    (Volt)2/ohm
  • d)
    Volt/Ampere
Correct answer is option 'D'. Can you explain this answer?

Jaya Datta answered
Watt is the unit of power. We can represent the power as follows
Hence Volt/Ampere is not same as watt, it is equal to ohm.

If each branch of star connection is 3 Ω. Then its corresponding delta connection will contain ______.
  • a)
    3 Ω
  • b)
    1/3Ω
  • c)
  • d)
    1/9Ω
Correct answer is option 'C'. Can you explain this answer?

Rishika Sen answered
Understanding Star and Delta Connections
In electrical engineering, star (Y) and delta (Δ) connections are two fundamental configurations used to connect three-phase systems. The relationship between resistances in these configurations is critical for circuit analysis.
Star Configuration
- In a star connection, each branch has a resistance of 3 Ω.
- The total resistance between any two phases in a star connection can be calculated using the formula: R_Y = R_phase.
Delta Configuration
- In a delta connection, the resistance is calculated differently. The resistances in the delta configuration are related by the formula: R_Δ = 3 * R_Y.
Calculating Delta Resistance
- Given: R_Y = 3 Ω (each branch in star).
- To find R_Δ:
R_Δ = 3 * R_Y
R_Δ = 3 * 3 Ω
R_Δ = 9 Ω
This indicates that each branch in the delta connection will be 9 Ω.
Conclusion
Thus, when each branch of a star connection has a resistance of 3 Ω, the corresponding resistance in the delta connection will indeed be 9 Ω. The correct answer is option 'C', which is 9 Ω. This relationship is crucial for engineers when converting between these two types of connections in three-phase systems.

What is the difference between an atom and an ion?
  • a)
    Ions have always larger mass than the atoms of the same element
  • b)
    Ions are neutral particles while atoms always carry a positive charge
  • c)
    Ions are always charged particles while the atoms are neutral as a whole
  • d)
    Ions can only exist in liquid solutions
Correct answer is option 'C'. Can you explain this answer?

Pranab Basu answered
An ion is a particle or collection of particles with a net positive or negative charge.
An atom is the basic unit of an element. The identity of an element is determined by the number of positively charged protons in the atom’s nucleus. A stable atom contains the same number of electrons as protons and no net charge.

Which statement about electrical network and electrical circuit is CORRECT?
  • a)
    Electrical network and electrical circuit are same
  • b)
    Electrical network and electrical circuit are different
  • c)
    Every electrical circuit is a network, but all networks are not circuits
  • d)
    Every network is a circuit
Correct answer is option 'C'. Can you explain this answer?

Prasad Verma answered
Explanation:

Difference between Electrical Network and Electrical Circuit:
- Electrical network and electrical circuit are different:
- An electrical network is a collection of interconnected components (such as resistors, capacitors, and inductors) where each component is represented by a node and the connections between components are represented by branches.
- An electrical circuit, on the other hand, is a closed loop path that contains a power source and various components connected in a specific way to perform a specific function.
- Every electrical circuit is a network, but all networks are not circuits:
- Every electrical circuit is essentially an electrical network because it consists of interconnected components.
- However, not all networks can be considered circuits as they may not form a closed loop path or may not have a power source to perform a specific function.
- Examples:
- A simple series circuit with a battery, resistor, and connecting wires is both an electrical circuit and an electrical network.
- A complex network of interconnected components without a closed loop path or power source may be considered an electrical network but not a circuit.
In conclusion, while every electrical circuit is a network due to its interconnected components, not all networks can be classified as circuits as they may lack the necessary elements to form a complete functional loop.

In a four-branch parallel circuit, there are 10 mA of current in each branch. If one of the branches is open, the current in each of the other three branches is:
  • a)
    13.3 mA
  • b)
    10 mA
  • c)
    0 A
  • d)
    30 mA
Correct answer is option 'B'. Can you explain this answer?

All the four branches are connected in parallel, so the voltage across every branch is same. If one of the branch is open, it doesn’t affect the other branches current. Because the voltage and impedance across remaining branches will remain same.

A filament lamp of 40 W, 110 V is connected in series with resistance R. if the supply voltage is 230 V, the value of resistance R should be:
  • a)
    330 ohm
  • b)
    340 ohm
  • c)
    130 ohm
  • d)
    240 ohm
Correct answer is option 'A'. Can you explain this answer?

Sakshi Roy answered


Given data:
- Power of the filament lamp (P) = 40 W
- Voltage across the lamp (V) = 110 V
- Supply voltage (Vs) = 230 V

Calculating resistance of the filament lamp:
Using the formula P = V^2 / R, we can calculate the resistance of the filament lamp:
R = V^2 / P
R = (110)^2 / 40
R = 302.5 ohm

Calculating the resistance in series with the lamp:
Let the resistance in series with the lamp be R.
Total voltage across the circuit = Voltage across lamp + Voltage across R
Vs = V + V_R
230 = 110 + 110 + R
R = 230 - 220
R = 330 ohm

Therefore, the value of resistance R should be 330 ohm to be connected in series with the filament lamp.

If the length of a wire having uniform resistance R is stretched in m times with volume remaining same. What will be new resistance value?
  • a)
    R/m
  • b)
    Rm2
  • c)
    mR
  • d)
    R/m2
Correct answer is option 'B'. Can you explain this answer?

Given that, the length of a wire having uniform resistance R is stretched in m times with volume remaining same.
Let the length of the wire is L1
and the area of the wire is A1
For volume remains to be same
L1A1 = L2A2
⇒ L11 = (mL1) (A2)
⇒ A1 = mA2

For parallel RLC circuit, which of the following statements is not correct?
  • a)
    The bandwidth of the circuit decreases, if R is increased
  • b)
    The bandwidth of the circuit remains the same, if L is increased
  • c)
    At resonance, input impedance is a real quantity
  • d)
    At resonance, the magnitude of input impedance attains its minimum value
Correct answer is option 'D'. Can you explain this answer?

Bijoy Mehta answered
Parallel RLC Circuit and Bandwidth

A parallel RLC circuit consists of a resistor (R), an inductor (L), and a capacitor (C) connected in parallel. This type of circuit is commonly used in various electrical systems and applications. One of the important characteristics of a parallel RLC circuit is its bandwidth, which refers to the range of frequencies over which the circuit operates effectively.

Bandwidth and Resistance (Option A)
When the resistance (R) in a parallel RLC circuit is increased, it affects the circuit's bandwidth. The bandwidth is defined as the difference between the upper and lower cutoff frequencies, where the circuit's response decreases by a specified amount (usually -3 dB). As the resistance increases, the circuit becomes more damped, resulting in a decrease in the bandwidth. Therefore, option A is correct.

Bandwidth and Inductance (Option B)
Increasing the inductance (L) in a parallel RLC circuit does not affect the circuit's bandwidth. The bandwidth is primarily determined by the resistance and capacitance values in the circuit. The inductance affects the resonant frequency but does not directly impact the bandwidth. Therefore, option B is incorrect.

Input Impedance and Resonance (Option C)
At resonance, the input impedance of a parallel RLC circuit becomes a real quantity. Resonance occurs when the inductive reactance (XL) is equal to the capacitive reactance (XC). At this frequency, the imaginary components cancel each other out, resulting in a purely resistive impedance. Therefore, option C is correct.

Input Impedance at Resonance (Option D)
Option D states that at resonance, the magnitude of the input impedance attains its minimum value. This statement is incorrect. At resonance, the magnitude of the input impedance in a parallel RLC circuit is at its maximum value, not minimum. The impedance is purely resistive at resonance, and its magnitude is determined by the resistance value. Therefore, option D is incorrect.

Conclusion
In summary, option D is the incorrect statement regarding a parallel RLC circuit. At resonance, the magnitude of the input impedance is at its maximum value, not minimum. The correct statements are:
- The bandwidth of the circuit decreases if the resistance is increased.
- The bandwidth of the circuit remains the same if the inductance is increased.
- At resonance, the input impedance is a real quantity.

The sum of currents entering a junction is 9 A. If the current leaves the junction form 3 different paths having the same resistance, the current leaving from any one of the path will be:
  • a)
    1 A
  • b)
    9 A
  • c)
    3 A
  • d)
    It cannot be determined
Correct answer is option 'C'. Can you explain this answer?

Sahana Sarkar answered
By KCL,
The sum of current entering a junction is equal to the sum of current leaving at that junction.
⇒ Sum of currents leaving = 9 A
This 9 A current will flow through three resistances having same value.
The current will be some through each resistor

If the energy is supplied form a source, whose resistance is 1 Ohm, to a load of 100 Ohms the source will be
  • a)
    A voltage source
  • b)
    A current source
  • c)
    Both a above
  • d)
    None of the above
Correct answer is option 'A'. Can you explain this answer?

The internal resistance of an ideal voltage source should be zero and practically it is very low.
Given that, source resistance is 1 ohm
Hence the source will be a voltage source

An ac voltage has frequency of 50 Hz with peak amplitude of 100 V. In how many seconds after the zero value, voltage will attain the value of 86.6 V?
  • a)
    1/300 second
  • b)
    1/100 second
  • c)
    1/150 second
  • d)
    1/600 second
Correct answer is option 'A'. Can you explain this answer?

Given data:
Frequency of ac voltage = 50 Hz
Peak amplitude = 100 V
Required voltage = 86.6 V

Calculation:
The ac voltage can be represented by the equation:
V(t) = V_m * sin(ωt)

Where:
V(t) = instantaneous voltage at time t
V_m = peak amplitude of the voltage
ω = angular frequency = 2πf
t = time

In this case, V_m = 100 V and f = 50 Hz, so ω = 2π * 50 = 100π rad/s.

We need to find the time at which the voltage attains the value of 86.6 V.

So, we can set up the equation:
86.6 = 100 * sin(100πt)

To solve this equation, we can rearrange it as:
sin(100πt) = 0.866

Using the inverse sine function, we can find the time:
sin^(-1)(sin(100πt)) = sin^(-1)(0.866)

100πt = sin^(-1)(0.866)

t = (sin^(-1)(0.866)) / (100π)

Now, we can calculate the value of t using a calculator:
t ≈ 0.01 seconds

Therefore, the voltage will attain the value of 86.6 V after approximately 0.01 seconds.

Answer:
The correct option is (A) 1/300 second.

The voltage source in the given circuit supplies 24W of power. The current source supplies 6.0 W. Determine the value of the resistance R and R2:
  • a)
    R= 24Ω and R2 = 2.5Ω
  • b)
    R= 18Ω and R2 = 3.0Ω
  • c)
    R= 24Ω and R2 = 3.0Ω
  • d)
    R= 18Ω and R2 = 2.5Ω
Correct answer is option 'B'. Can you explain this answer?

Given that, voltage source supplies 24 W
By applying KVL,
- 24 + I1 R1 + I2 R2 = 0
⇒ I2 R2 = 24 – R1 → (1)
Given that, current source supplies 6 W
By KVL in second loop
I2 R2 = 6 → (2)                
From (1) and (2)
⇒ R1 = 18 Ω
By KCL at point ‘O’
⇒ I2 = 1 + I1 = 2 A
We know that, I2 R2 = 6
⇒ R2 = 3 Ω

The temperature coefficient of resistance of material is given as:
 
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'A'. Can you explain this answer?

We know that, R = R0[1+α(T1−T2)]
Where α is a temperature coefficient of resistance
If the temperature coefficient of resistance is positive, it indicates that the resistance increases with the increase in temperature
If the temperature coefficient of resistance is negative, it indicates that the resistance decreases with the increase in temperature

The Norton current at terminals a and b of the circuit shown at Figure:
  • a)
    1 A
  • b)
    -2 A
  • c)
    2 A
  • d)
    3 A
Correct answer is option 'A'. Can you explain this answer?

Megha Datta answered
To find Norton’s current, we need to find the short circuit current across the terminals a and b.

By source transformation,
By KVL,
-180 + 20 ISC + 120 +40 ISC = 0
⇒ ISC = 1 A

Consider the following circuit and find the current through the 6Ω resistor.
  • a)
    1.23 A
  • b)
    2.03 A
  • c)
    1.11 A 
  • d)
    0.31 A
Correct answer is option 'B'. Can you explain this answer?

Kalyan Patel answered
Let the current passing through 6 Ω resistor is I.
By source transformation,


By source transformation,
By current division, 

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