This set of Basic Electrical Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Resistance and Inductance in Series”.

1. A resistance of 7 ohm is connected in series with an inductance of 31.8mH. The circuit is connected to a 100V 50Hz sinusoidal supply. Calculate the current in the circuit.

a) 2.2A

b) 4.2A

c) 6.2A

d) 8.2A

View Answer

Explanation: X

_{L}=2*π*f*L = 10 ohm. Z

^{2}=(R

^{2}+X

_{L}

^{2})

Therefore the total impedance Z = 12.2ohm.

V=IZ, therefore I=V/Z=100/12.2 = 8.2A.

2. A resistance of 7 ohm is connected in series with an inductance of 31.8mH. The circuit is connected to a 100V 50Hz sinusoidal supply. Calculate the phase difference.

a) -55.1

b) 55.1

c) 66.1

d) -66.1

View Answer

Explanation: φ=tan

^{-1}(XL/R)=55.1

Since this is an inductive circuit, the current will lag, hence φ= -55.1.

3. A resistance of 7 ohm is connected in series with an inductance of 31.8mH. The circuit is connected to a 100V 50Hz sinusoidal supply. Calculate the voltage across the resistor.

a) 31.8V

b) 57.4V

c) 67.3V

d) 78.2V

View Answer

Explanation: X

_{L}=2*π*f*L = 10 ohm. Z

^{2}=(R

^{2}+X

_{L}

^{2})

Therefore, the total impedance Z = 12.2ohm.

V=IZ, therefore I=V/Z=100/12.2 = 8.2A. Voltage across resistor = 8.2*7 = 57.4V.

4. A resistance of 7 ohm is connected in series with an inductance of 31.8mH. The circuit is connected to a 100V 50Hz sinusoidal supply. Calculate the voltage across the inductor.

a) 52V

b) 82V

c) 65V

d) 76V

View Answer

Explanation: X

_{L}=2*π*f*L = 10 ohm. Z

^{2}=(R

^{2}+X

_{L}

^{2})

Therefore, the total impedance Z = 12.2ohm.

V=IZ, therefore I=V/Z=100/12.2 = 8.2A. Voltage across inductor = 8.2*10 = 82V.

5. A resistance of 7 ohm is connected in series with an inductance of 31.8mH. The circuit is connected to a x V 50Hz sinusoidal supply. The current in the circuit is 8.2A. Calculate the value of x.

a) 10V

b) 50V

c) 100V

d) 120V

View Answer

Explanation: X

_{L}=2*π*f*L= 10 ohm. Z

^{2}=(R

^{2}+X

_{L}

^{2})

Therefore, the total impedance Z = 12.2ohm.

V=IZ, therefore V = 12.2*8.2 = 100V.

6. Which, among the following, is the correct expression for φ.

a) φ=tan^{-1} (XL/R)

b) φ=tan^{-1} (R/XL)

c) φ=tan^{-1} (XL*R)

d) φ=cos^{-1} (XL/R)

View Answer

Explanation: From the impedance triangle, we get tanφ= XL/R.

Hence φ=tan

^{-1}(XL/R).

7. For an RL circuit, the phase angle is always ________

a) Positive

b) Negative

c) 0

d) 90

View Answer

Explanation: For a series resistance and inductance circuit the phase angle is always a negative value because the current will always lag the voltage.

8. What is φ in terms of voltage?

a) φ=cos^{-1}V/VR

b) φ=cos^{-1}V*VR

c) φ=cos^{-1}VR/V

d) φ=tan^{-1}V/VR

View Answer

Explanation: From the voltage triangle, we get cosφ= VR/V.

Hence φ=cos

^{-1}VR/V.

9. What is sinϕ from impedance triangle?

a) X_{L}/R

b) X_{L}/Z

c) R/Z

d) Z/R

View Answer

Explanation: In Impedance triangle, Base is R, Hypotenuse is Z, Height is X

_{L}.

So, sinϕ = X

_{L}/Z.

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