20.5 Alternating Current versus Direct Current

Alternating Current (AC) and Direct Current (DC) are two fundamental types of electrical flow. AC periodically reverses direction, while DC maintains a constant flow. Understanding their characteristics is crucial for grasping how electricity works in various applications.

AC power is widely used in homes and for long-distance transmission due to its ability to be easily transformed. DC, on the other hand, is common in electronic devices and battery-powered systems. Knowing the differences helps in choosing the right power source for specific needs.

Alternating Current (AC) and Direct Current (DC)

AC vs DC characteristics

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• Alternating Current (AC)
  • Current periodically reverses direction flows back and forth in a sinusoidal pattern
  • Voltage polarity alternates between positive and negative values over time
  • Typically generated by mechanical means through the rotation of a generator or alternator
  • Can be transformed to different voltage levels using transformers enables efficient long-distance transmission
  • Used in household electrical systems (outlets, appliances) and long-distance power transmission on the grid
  • Characterized by its frequency, measured in Hertz (Hz), which is the number of cycles per second
• Direct Current (DC)
  • Current flows in a single direction maintains a constant flow from negative to positive terminal
  • Voltage polarity remains constant does not alternate, always has a fixed positive and negative terminal
  • Produced by sources such as batteries (AA, lithium-ion), solar cells (photovoltaic panels), and rectified AC (converted with a rectifier)
  • Cannot be easily transformed to different voltage levels without specialized electronic circuits
  • Used in electronic devices (smartphones, laptops), battery-powered systems (flashlights, remote controls), and some industrial applications (electroplating, welding)

RMS values in AC circuits

• Root Mean Square (RMS) values
  • Equivalent DC value that produces the same heating effect as the AC waveform over one complete cycle
  • For a sinusoidal waveform:
    • $V_{rms} = \frac{V_{peak}}{\sqrt{2}}$ voltage RMS is peak voltage divided by square root of 2
    • $I_{rms} = \frac{I_{peak}}{\sqrt{2}}$ current RMS is peak current divided by square root of 2
  • RMS values are used to compare AC and DC quantities in terms of power and energy allows for equivalent comparisons
• Calculating RMS values
  • Determine the peak value of the voltage or current waveform from the sinusoidal graph or given values
  • Divide the peak value by $\sqrt{2}$ (approximately 1.414) to obtain the RMS value
  • $V_{rms} = \frac{120V}{\sqrt{2}} = 84.85V$ household outlet voltage has 120V peak, 84.85V RMS
  • $I_{rms} = \frac{10A}{\sqrt{2}} = 7.07A$ appliance drawing 10A peak current has 7.07A RMS

Advantages of AC power transmission

• Advantages of AC for long-distance power transmission
  • Voltage transformation
    • AC voltage can be easily stepped up or down using transformers no moving parts, just coils of wire
    • High-voltage transmission (100kV+) minimizes power losses over long distances (hundreds of miles) by reducing current
    • Lower voltages (120V/240V) can be used for distribution and end-user consumption safer and easier to work with
  • Reduced transmission losses
    • Higher voltages result in lower currents for the same power since $P = VI$, if voltage is high, current can be low
    • Power loss in transmission lines is proportional to the square of the current $P_{loss} = I^2R$ so reducing current has a big impact
    • Lower currents lead to reduced power losses and more efficient transmission less wasted energy and lower costs
  • Simplified generation and distribution
    • AC generators (alternators) are simpler and more efficient than DC generators easier to design and maintain
    • AC power can be easily distributed using a three-phase system provides balanced power and cancels neutral current
    • Three-phase AC allows for more efficient power transmission and motor operation used in industrial settings
  • Based on the principle of electromagnetic induction, which allows for efficient power generation and transformation

AC Circuit Analysis

• Impedance: The total opposition to current flow in an AC circuit, combining resistance and reactance
• Power factor: The ratio of real power to apparent power in an AC circuit, indicating the efficiency of power transfer
• Phase angle: The time difference between voltage and current waveforms in an AC circuit, affecting power factor and circuit behavior