Direct Current & Alternating Current

Direct Current

Direct Current (DC) is characterized by the unidirectional flow of electric charge carriers, such as electrons. The defining feature of direct current is that the flow of electric charge is constant and moves in the same direction around a circuit. This constancy means that the polarity of the electromotive force (e.m.f.)—the force that drives the electrons—remains unchanged over time.

Key Characteristics Of DC:

  • Constant Direction: In a direct current circuit, charge carriers move in one consistent direction, from the negative to the positive terminal, which distinguishes it from alternating current (AC) where the direction of flow changes periodically.
  • Source: Direct current is typically supplied by batteries, solar cells, or through rectification of AC power. The majority of electronic devices, such as smartphones and laptops, require direct current to operate because it provides a stable voltage level that can be effectively regulated.
  • Electromotive Force (e.m.f.): The e.m.f. in a direct current circuit does not change polarity, remaining constant over time, which ensures the unidirectional flow of current.

[A Level] Types Of Direct Current Waveforms:

Types of dc waveforms

Direct current can be represented in various waveform types, each depicting how the magnitude of the current might change over time, even though the direction of flow remains constant. These include pure direct current (a flat line indicating no change in magnitude), pulsed direct current (where the magnitude varies in pulses but always moves in one direction), and more complex modulated signals that still maintain a net direct flow.

[A Level] Alternating Current (AC)

500px Types of current.svg

In contrast, Alternating Current (AC) involves the periodic reversal of charge flow direction. Household electrical power is a prime example of alternating current, which typically operates at frequencies of 50 Hz or 60 Hz depending on the region. This means the direction of current flow (and thus the e.m.f. polarity) changes 50 or 60 times per second.

Key Characteristics Of Alternating Current:

  • Periodic Direction Change: The fundamental difference from DC is that AC’s flow of charge carriers reverses direction periodically, which can be advantageous for transmitting power over long distances.
  • Conversion Needs: Since many electronic devices are designed to operate on DC, AC supplied from the power grid must be converted to DC for such applications. This is achieved using rectifiers and power adapters.
  • Waveform Types: AC can be represented in different waveform shapes, including sinusoidal (the most common), square, triangular, and sawtooth waveforms. Each type has unique characteristics and applications in electronics and signal processing.

Features Of An Alternating Current Waveform:

Sinusoidal alternating current
  • Sinusoidal Representation: A typical AC waveform is sinusoidal, described mathematically as ($I = I_0 \sin(\omega t)$), where ($I_0$) is the peak current, ($\omega$) is the angular frequency, and ($t$) is time.
  • Cycle: A cycle in a sinusoidal AC waveform represents one complete oscillation from zero to positive peak, back through zero to negative peak, and return to zero.
  • Period (T): The period is the time it takes to complete one full cycle of the waveform. The inverse of the period gives the frequency ($f$) of the waveform.
  • Frequency (f): Frequency is measured in hertz (Hz) and represents the number of cycles per second.
  • Instantaneous Current: This refers to the current value at any given instant in time, which varies periodically in an AC circuit.
  • Peak and Peak-to-Peak Values: The peak current value ($I_0$) is the maximum amplitude of the waveform. The peak-to-peak value is double the peak value, representing the total magnitude difference between the positive and negative peaks.

[A Level] Comparison Of Direct Current & Alternating Current

FeatureDirect Current (DC)Alternating Current (AC)
DefinitionFlow of electric charge in a constant direction.Flow of electric charge that periodically reverses direction.
Direction of FlowUnidirectionalBi-directional, changes periodically
SourceBatteries, solar cells, rectifiersGenerators, power plants
PolarityConstantAlternates periodically
Current WaveformConstant or varying in magnitude but always in one directionSinusoidal, square, triangular, or sawtooth waveforms
ApplicationsElectronic devices, battery-powered equipment, DC motorsPower transmission, AC motors, household power supply
Transmission EfficiencyLess efficient over long distances without step-up capabilitiesMore efficient for long-distance transmission with transformers
ConversionAC to DC using rectifiersDC to AC using inverters
FrequencyZero (as the current does not oscillate)Varies (50 Hz, 60 Hz, or higher frequencies in some applications)
Key CharacteristicsStable and constant direction, ideal for electronic devicesCan be easily transformed to different voltages, ideal for power transmission

Worked Examples

Example 1: Battery-Powered Devices

A smartphone uses a lithium-ion battery as its power source. Would this device be powered by DC or AC, and why?

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The smartphone would be powered by Direct Current (DC). This is because lithium-ion batteries supply DC, providing a constant voltage and current flow in one direction, which is ideal for the sensitive electronic components in a smartphone.

Example 2: AC to DC Conversion

Your laptop charger is plugged into the wall outlet and your laptop is charging. What conversion process is happening inside the charger?

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The charger is converting Alternating Current (AC) from the wall outlet into Direct Current (DC) for the laptop. This process is known as rectification. Laptops require DC for their operation, and the rectifier circuit within the charger facilitates this conversion.

Example 3: Household Wiring

Why is Alternating Current (AC) used for household wiring instead of Direct Current (DC)?

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AC is used for household wiring primarily because it can be transmitted over longer distances more efficiently and economically than DC. AC can easily be transformed to higher or lower voltages with transformers, which reduces the energy losses that occur during transmission over long distances.

Example 4: Frequency of AC

If an AC power supply operates at 60 Hz, how many times does the current change direction in one minute?

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The current changes direction 120 times in one minute. Since the frequency is 60 Hz, it completes 60 cycles per second. Each cycle includes two changes in direction (from positive to negative and back to positive), resulting in 120 changes per minute.

Example 5: AC Waveform Application

Why might an engineer choose a square wave AC signal for a specific electronic application?

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An engineer might choose a square wave AC signal for applications requiring specific timing or digital signal processing where a clear and precise transition between high and low states is essential. Square waves are commonly used in digital electronics and signal processing because of their simplicity and ease of interpretation, which is ideal for clock signals, timing circuits, and switching applications.

Example 6: DC Motor Speed Control

How can the speed of a DC motor be controlled?

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The speed of a DC motor can be controlled by varying the voltage supplied to the motor. A lower voltage results in a slower rotation speed, while a higher voltage increases the speed. This is because the motor’s speed is directly proportional to the voltage applied across its terminals.

Example 7: Transformer Usage

Can a transformer be used to change the voltage of a DC supply? Explain your answer.

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No, a transformer cannot be used to change the voltage of a DC supply. Transformers work on the principle of electromagnetic induction, which requires a changing magnetic field to induce voltage. Since DC provides a constant current without any changes in direction or magnitude, it does not produce the varying magnetic field necessary for a transformer to operate.

Back To D.C. Circuits (O Level Physics)

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