# 31 Lecture

## Alternating Current

Alternating current (AC) is a type of electrical current that periodically reverses direction, unlike direct current (DC), which flows in only one direction. AC is widely used in power generation, transmission, and distribution systems, as well

## Important Mcq's Midterm & Finalterm Prepration Past papers included

What is the phase difference between voltage and current in an ideal inductor in an AC circuit?

A. 0 degrees

B. 90 degrees

C. 180 degrees

D. 270 degrees

What is the unit of frequency in an AC circuit?

A. Hertz

B. Volt

C. Ampere

D. Ohm

What is the term for the opposition of an AC circuit to the flow of current?

A. Resistance

B. Reactance

C. Impedance

D. Conductance

Which of the following components is commonly used to reduce the reactance of an AC circuit?

A. Resistor

B. Capacitor

C. Inductor

D. Transformer

What is the effect of increasing the frequency of an AC circuit?

A. Increases the reactance of inductors

B. Decreases the reactance of inductors

C. Increases the reactance of capacitors

D. Decreases the reactance of capacitors

Answer: B. Decreases the reactance of inductors

What is the term for the measure of the amount of AC power being used in a circuit?

A. Amplitude

B. Frequency

C. Power factor

D. RMS voltage

What is the relationship between the voltage and current in a purely resistive AC circuit?

A. They are in phase

B. Voltage leads current by 90 degrees

C. Voltage lags current by 90 degrees

D. Voltage and current are 180 degrees out of phase

Answer: A. They are in phase

Which of the following is true for a parallel AC circuit?

A. Voltage is the same across all components

B. Current is the same across all components

C. Resistance increases as more components are added

D. Capacitance decreases as more components are added

Answer: A. Voltage is the same across all components

What is the unit of power in an AC circuit?

A. Watt

B. Volt-ampere

C. Ohm

D. Joule

What is the term for the time delay between the peak voltage and peak current in an AC circuit?

A. Phase angle

B. Power factor

C. Impedance

D. Reactance

## Subjective Short Notes Midterm & Finalterm Prepration Past papers included

What is meant by alternating current?

Answer: Alternating current is an electric current that periodically reverses direction, constantly changing its magnitude and direction.

What is the frequency of AC current in India?

Answer: The frequency of AC current in India is 50 Hz.

Define RMS value of AC current.

Answer: RMS (Root Mean Square) value of AC current is the value of the current which when passed through a resistor for a given time produces the same amount of heat as produced by the corresponding DC current when passed for the same time through the same resistor.

What is the phase difference between the voltage and current in a purely resistive circuit?

Answer: In a purely resistive circuit, the voltage and current are in phase with each other, i.e., there is no phase difference between them.

What is an inductor in an AC circuit?

Answer: An inductor is a passive electrical component that stores energy in a magnetic field when electric current flows through it in an AC circuit.

What is the reactance of a capacitor in an AC circuit?

Answer: The reactance of a capacitor in an AC circuit is given by Xc = 1/(2?fC), where f is the frequency of the AC signal and C is the capacitance of the capacitor.

What is the power factor of a purely resistive circuit?

Answer: The power factor of a purely resistive circuit is unity or 1.

What is meant by the resonance frequency of an AC circuit?

Answer: The resonance frequency of an AC circuit is the frequency at which the circuit offers maximum impedance to the flow of current.

What is the phase difference between the voltage and current in an inductive circuit?

Answer: In an inductive circuit, the current lags behind the voltage by 90 degrees.

What is an LC circuit?

Answer: An LC circuit is a resonant circuit consisting of an inductor (L) and a capacitor (C) connected together. It can store electrical energy oscillating at its resonant frequency.

### Alternating Current

Alternating current (AC) is a type of electrical current that periodically reverses direction, unlike direct current (DC), which flows in only one direction. AC is widely used in power generation, transmission, and distribution systems, as well as in many household and industrial appliances. AC is generated by a device called an alternator, which produces a varying electromotive force (EMF) in a wire loop by rotating a magnet inside a coil. This EMF changes direction periodically, resulting in a sinusoidal voltage and current waveform. The frequency of AC is measured in hertz (Hz), which is the number of cycles per second. One of the key advantages of AC over DC is that it can be easily transformed from one voltage level to another using a transformer. This allows for efficient long-distance power transmission and the ability to adjust the voltage level for different applications. Another important characteristic of AC is its impedance, which is the opposition to the flow of current caused by the electrical properties of the circuit. Impedance depends on the frequency of the AC signal, as well as the resistance, capacitance, and inductance of the circuit components. The magnitude and phase of the impedance can be represented by a complex number called the impedance vector. In addition to its use in power systems, AC is also used in many electrical devices, such as motors, transformers, and generators. AC motors are widely used in industrial applications due to their efficiency and ease of control. Transformers are used to step up or step down AC voltage levels, while generators are used to convert mechanical energy into electrical energy. The behavior of AC circuits is characterized by several key parameters, including voltage, current, power, and power factor. The voltage and current in an AC circuit are sinusoidal waveforms that are out of phase with each other. The power in an AC circuit is the product of the voltage and current, but it is not simply the sum of the instantaneous power values due to the phase shift between the two waveforms. The power factor of an AC circuit is a measure of the efficiency of power transfer and is defined as the ratio of the real power (which is the power that is actually consumed by the load) to the apparent power (which is the product of the voltage and current magnitudes). A power factor of 1 indicates that all of the power is being consumed by the load, while a power factor of less than 1 indicates that some of the power is being lost due to the reactive properties of the circuit. In conclusion, AC is an essential part of modern electrical systems, providing a convenient and efficient way to transmit and distribute power over long distances. Its sinusoidal waveform and impedance characteristics require special considerations for circuit analysis and design, but also enable the use of a wide range of electrical devices and applications.