# Class 12 Physics MCQ – Representation of AC Current and Voltage by Rotating Vectors – Phasors

This set of Class 12 Physics Chapter 7 Multiple Choice Questions & Answers (MCQs) focuses on “Representation of AC Current and Voltage by Rotating Vectors – Phasors”.

1. Identify the function of a phasor from the following.
a) Phasor is a vector quantity used to represent a sinusoidal signal
b) Phasor is a scalar quantity used to represent a sinusoidal signal
c) Phasor is a vector quantity used to represent a cosine signal
d) Phasor is a scalar quantity used to represent a cosine signal

Explanation: Phasor is a vector quantity used to represent a sinusoidal signal. The vertical component of phasors represents the quantities that are sinusoidally varying for a given equation. The magnitude of the phasors represents the peak value of the voltage and the current.

2. What is the general expression of a sinusoidal signal?
a) A(t) = Am sin⁡(ωt – Φ)
b) A(t) = sin⁡(ωt + Φ)
c) A(t) = Am sin⁡(ωt + Φ)
d) A(t) = 2Am sin⁡(ωt + Φ)

Explanation: A sinusoidal signal is a mathematical curve that describes a smooth periodic oscillation. The general expression of a sinusoidal signal is given as follows:
A(t) = Am sin⁡(ωt + Φ)
Where ➔ Am = Peak Amplitude, ω = Angular Frequency, Φ = Phase Shift.

3. What does a phasor represent?
a) Current and resistance
b) Current and voltage
c) Voltage and resistance
d) Voltage and power

Explanation: Phasors represent alternating current and voltage of same frequency as vectors in a phasor diagram with the phase angle between them. Phasors can also be applied to impedance as well as other related complex quantities that are not dependent on time.

4. Phasors rotate in the clockwise direction.
a) True
b) False

Explanation: No, this statement is false. Phasors rotate in the anti-clockwise direction. Even though phasors are vectors, they represent scalar quantities such as voltage and current. It rotates about the origin with an angular speed ω. With the help of a phasor, we can easily detect whether one of two quantities are in the same phase or not.

5. Find the true statement.
a) Length of the phasor must be greater than the peak value of alternating voltage or alternating current
b) When the current reaches its maximum value after emf becomes maximum, then the current is considered to be leading ahead of emf
c) When the emf reaches its maximum value after current becomes maximum, then the emf is considered to be behind the current
d) Phasors for voltage and current are in the same direction when the phase angle between voltage and current is zero

Explanation: The phasors for voltage and current are in the same direction at all times when the phase angle between voltage and current is zero. All the other statements are not valid. Length of the phasor must be equal to the peak value of alternating voltage or alternating current. Also, when the current reaches its maximum value after emf becomes maximum, then the current is considered to be lagging behind the emf and when the emf reaches its maximum value after current becomes maximum, then the emf is considered to lead the current.

Sanfoundry Global Education & Learning Series – Physics – Class 12.

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