20.5 Alternating Current versus Direct Current
3 min read•june 18, 2024
(AC) and (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 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 , measured in Hertz (Hz), which is the number of cycles per second
- (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 )
- 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
- () values
- Equivalent DC value that produces the same heating effect as the AC waveform over one complete cycle
- For a sinusoidal waveform:
- voltage RMS is divided by square root of 2
- current RMS is 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 (approximately 1.414) to obtain the RMS value
- household outlet voltage has 120V peak, 84.85V RMS
- 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 , if voltage is high, current can be low
- Power loss in transmission lines is proportional to the square of the current 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 , which allows for efficient power generation and transformation
- Voltage transformation
AC Circuit Analysis
- : The total opposition to current flow in an AC circuit, combining resistance and reactance
- : The ratio of real power to in an AC circuit, indicating the efficiency of power transfer
- : The time difference between voltage and current waveforms in an AC circuit, affecting and circuit behavior
Key Terms to Review (32)
Acoustic impedance: Acoustic impedance is a measure of how much resistance an acoustic wave encounters as it travels through a medium. It is defined as the product of the density of the medium and the speed of sound in that medium.
Alternating Current: Alternating current (AC) is an electric current that periodically reverses direction, in contrast to direct current (DC) which flows consistently in one direction. AC is the standard form of electricity supplied to homes and businesses, powering a wide range of electrical devices and equipment.
Ampere: An ampere (A) is the unit of electric current in the International System of Units (SI). It represents the flow of one coulomb of charge per second.
Ampere: The ampere (symbol: A) is the base unit of electric current in the International System of Units (SI). It is defined as the constant flow of one coulomb of electrical charge per second, and it is a fundamental quantity used to describe the movement of electric charge.
Apparent Power: Apparent power is a concept in electrical engineering that describes the total power flow in an alternating current (AC) circuit, including both the real power that is converted into useful work and the reactive power that is temporarily stored and returned to the source. It is a measure of the total capacity of the electrical system to perform work.
Beat frequency: Beat frequency is the frequency at which two waves of slightly different frequencies interfere with each other, resulting in a modulation pattern perceived as a periodic variation in amplitude. It is calculated as the absolute difference between the frequencies of the two interfering waves.
Direct current: Direct current (DC) is a type of electrical current where the flow of electric charge is unidirectional. It is commonly used in batteries, electronics, and other applications requiring a stable and constant voltage.
Direct Current: Direct current (DC) is an electric current that flows in a single direction, with a constant polarity. It is in contrast to alternating current (AC), which reverses direction periodically. Direct current is the type of electricity commonly used in batteries and powers many electronic devices.
Electromagnetic Induction: Electromagnetic induction is the process by which a changing magnetic field induces an electromotive force (EMF) in a conductor, causing an electric current to flow. This phenomenon is the fundamental principle behind the operation of many electrical devices and systems, including transformers, generators, and motors.
Frequency: Frequency is a fundamental concept in physics that describes the number of occurrences of a repeating event per unit of time. It is a crucial parameter in various areas of study, including radiation, oscillations, waves, sound, and electromagnetic phenomena.
Impedance: Impedance is a measure of the opposition to the flow of alternating current (AC) in an electrical circuit. It combines the effects of resistance, inductance, and capacitance, and determines the overall opposition to the current flow in an AC circuit.
Nikola Tesla: Nikola Tesla was a Serbian-American inventor, electrical engineer, and futurist best known for his contributions to the design of the modern alternating current (AC) electricity supply system. He played a pivotal role in the transition from direct current (DC) to AC power, which enabled the widespread distribution of electricity over long distances and the development of many electrical devices and applications.
Ohm's Law: Ohm's law is a fundamental principle in electrical engineering that describes the relationship between the voltage, current, and resistance in an electrical circuit. It states that the current flowing through a conductor is directly proportional to the voltage applied across it, and inversely proportional to the resistance of the conductor.
Peak Current: Peak current refers to the maximum value of the electric current in an alternating current (AC) circuit. It represents the highest point or amplitude of the current waveform, which oscillates between positive and negative values over time, unlike the constant flow of direct current (DC).
Peak Voltage: Peak voltage refers to the maximum value or amplitude of an alternating current (AC) waveform, representing the highest point of the voltage cycle. It is a critical parameter in the analysis and understanding of AC circuits and systems.
Phase angle: Phase angle is the measure of the difference in phase between two alternating current (AC) waveforms, typically measured in degrees or radians. It represents how far one waveform leads or lags behind another.
Phase Angle: The phase angle is a measure of the time delay or shift between two periodic signals, such as voltage and current, in an alternating current (AC) circuit. It represents the angular difference between the peaks of the two waveforms and is expressed in degrees or radians.
Power factor: Power factor is the ratio of real power used by a circuit to the apparent power flowing into the circuit. It indicates how effectively electrical power is being converted into useful work output.
Power Factor: Power factor is a measure of the efficiency of power transmission in an alternating current (AC) electrical system. It represents the ratio of the real power (the power that does useful work) to the apparent power (the total power being supplied) in an AC circuit.
Reactive Power: Reactive power is the portion of electrical power in an AC circuit that does not produce net positive work but is instead stored and returned to the source, oscillating between the source and load. It is essential in the operation of many electrical devices and systems that rely on alternating current.
Rectifier: A rectifier is an electrical device that converts alternating current (AC) to direct current (DC) by allowing current to flow in only one direction. It is a crucial component in power supply circuits, enabling the conversion of the widely available AC power to the DC power required by many electronic devices.
RLC Circuit: An RLC circuit is an electrical circuit that contains a resistor (R), an inductor (L), and a capacitor (C) connected in series or parallel. These components interact to create complex behaviors that are important in the study of alternating current (AC) circuits.
RMS: RMS, or Root Mean Square, is a statistical measure that represents the square root of the mean of the squares of a set of values. It is commonly used in the context of alternating current (AC) and direct current (DC) to compare the effective values of these two types of electrical currents.
Rms current: RMS current (Root Mean Square current) is the effective value of an alternating current (AC). It represents the equivalent direct current (DC) that would produce the same power dissipation in a resistive load.
Rms voltage: RMS Voltage (Root Mean Square Voltage) is the effective value of an alternating current (AC) voltage, which represents the equivalent direct current (DC) voltage that would deliver the same power to a load. It is calculated as $V_{rms} = V_{peak} / \sqrt{2}$ for sinusoidal waveforms.
Root Mean Square: The root mean square (RMS) is a statistical measure that quantifies the magnitude of a varying quantity. It is particularly useful in the context of alternating current (AC) and direct current (DC) as it provides a way to compare the effective values of these two types of electrical currents.
Sine Wave: A sine wave is a mathematical function that describes a smooth, periodic oscillation. It is characterized by a repeating pattern of sinusoidal curves, often used to represent alternating current (AC) and other wave-like phenomena in physics and engineering.
Sinusoidal: Sinusoidal refers to a waveform or function that varies in a smooth, repeating pattern resembling a sine wave. This term is particularly relevant in the context of uniform circular motion, simple harmonic motion, and alternating current versus direct current.
Step-down transformer: A step-down transformer is a device that decreases the voltage from the primary coil to the secondary coil while increasing the current. It is commonly used to convert high voltage electricity from power lines to a lower, safer voltage for use in homes and businesses.
Thomas Edison: Thomas Edison was an American inventor and businessman who is considered one of the most prolific inventors of all time. He is best known for his contributions to the development of the electric power distribution system and for inventing devices that greatly influenced modern life, such as the phonograph and the long-lasting, practical electric light bulb.
Transformer: A transformer is a device that transfers electrical energy from one alternating current (AC) circuit to another through the process of electromagnetic induction, without the need for direct electrical connection. It is a crucial component in the transmission and distribution of electrical power, as well as in various electronic devices.
Volt: A volt is the unit of electric potential difference, which measures the amount of electric potential energy per unit charge between two points in an electric circuit. It helps us understand how much energy is available to move electric charges through a conductor, connecting it to concepts like power, energy storage, and circuit behavior.