Alternating Current

Introduction

Alternating Current (AC) is an electric current that periodically reverses its direction, unlike Direct Current (DC), which flows in only one direction. AC is the form of electricity commonly used for power transmission and household applications due to its efficiency in long-distance transmission. Understanding the principles of AC is crucial for NEET aspirants, as it plays a vital role in electrical circuits, transformers, and power systems.

Difference Between AC and DC

• Alternating Current (AC): Changes direction periodically, commonly used in power supply systems.

• Direct Current (DC): Flows in one direction, used in batteries and electronic devices.

• AC is preferred for power distribution as it can be easily transformed to different voltage levels using transformers.

AC Voltage and Current

• In an AC circuit, the voltage and current vary sinusoidally with time.

• The alternating voltage can be represented as a sine wave, characterized by:

  • Amplitude (Peak Value): Maximum value of voltage or current.

  • Frequency (f): Number of cycles per second, measured in Hertz (Hz).

  • Time Period (T): The time taken for one complete cycle (T = 1/f).

  • Phase Difference: Determines the relationship between voltage and current waveforms.

Root Mean Square (RMS) Value of AC

• Since AC fluctuates with time, an effective or RMS value is used to describe its magnitude.

• The RMS value of AC is the equivalent DC value that produces the same heating effect in a resistor.

• The household voltage supply is typically given in RMS value (e.g., 230V in India).

Phasor Representation of AC

• AC voltages and currents can be represented using phasor diagrams, which simplify the analysis of circuits with multiple AC sources.

• A phasor is a rotating vector that represents the amplitude and phase of a sinusoidal function.

Reactance and Impedance in AC Circuits

1. Resistor in AC Circuit:

   • In a purely resistive AC circuit, voltage and current are in phase (zero phase difference).

   • The circuit follows Ohm’s Law (V = IR).

2. Inductor in AC Circuit:

   • An inductor opposes changes in current, creating an opposition called inductive reactance (XL).

   • Current lags voltage by 90 degrees in an inductive circuit.

3. Capacitor in AC Circuit:

   • A capacitor stores energy in the form of an electric field, causing opposition known as capacitive reactance (XC).

   • Current leads voltage by 90 degrees in a capacitive circuit.

4. Impedance in AC Circuits:

   • In AC circuits, total opposition to current flow is called impedance (Z).

   • Impedance is a combination of resistance, inductive reactance, and capacitive reactance.

AC Circuits with LCR Components

• LCR Circuits (Inductor, Capacitor, and Resistor): Combination circuits with resistance, inductance, and capacitance.

• The total impedance is given by the vector sum of resistance and reactances.

• The circuit behavior depends on the resonance frequency, which determines how energy is stored and transferred between components.

Resonance in AC Circuits

• Resonance occurs when the inductive reactance and capacitive reactance cancel each other out, leading to maximum current flow.

• The frequency at which resonance occurs is called the resonant frequency (f₀).

• Resonance is used in radio tuning, oscillators, and frequency filtering.

Power in AC Circuits

• The power in AC circuits depends on voltage, current, and the phase difference (φ) between them.

• Types of Power in AC Circuits:

  • Instantaneous Power: Power at any given moment.

  • Average Power: Useful power consumed over time.

  • Reactive Power: Power stored and released in inductors and capacitors.

  • Apparent Power: Combination of real and reactive power.

• Power Factor:

  • Defined as the cosine of the phase angle (cos φ) between voltage and current.

  • A power factor closer to 1 means more efficient power usage.

Transformers and AC Power Transmission

• Transformers are devices used to step up (increase) or step down (decrease) AC voltage.

• Step-up Transformers: Increase voltage for long-distance transmission, reducing energy loss.

• Step-down Transformers: Reduce voltage for safe household and industrial use.

• Transformers work on the principle of mutual induction and only function with AC.

Applications of Alternating Current

• Electric Power Distribution: AC is used globally for power transmission.

• Transformers: Used in power stations and electronic devices.

• Induction Motors: Operate on AC principles for various appliances.

• Wireless Energy Transfer: Used in wireless charging and radio communication.

Conclusion

Alternating Current is the foundation of modern electrical power systems. The study of AC circuits, reactance, impedance, resonance, power, and transformers is essential for understanding how electricity is transmitted and utilized efficiently. Mastering these concepts will help NEET aspirants in physics and real-world applications.