Lesson Notes By Weeks and Term v4 - SHS 3
ALTERNATING CURRENT
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ALTERNATING CURRENT
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Subject: Physics
Class: SHS 3
Term: 2nd Term
Week: 6
Grade code: 3.3.1.LI.2
Strand code: 3
Sub-strand code: 2
Content standard code: 3.3.2.CS.1
Indicator code: 3.3.1.LI.2
Theme: ELECTRIC FIELD, MAGNETIC FIELD AND ELECTRONICS
Subtheme: ALTERNATING CURRENT
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This lesson introduces the fundamental concepts of Alternating Current (AC), the type of electricity that powers our homes, schools, and industries across Ghana, supplied by the Electricity Company of Ghana (ECG). Unlike the Direct Current (DC) from batteries that we use in our remote controls and torchlights, AC has unique properties that make it suitable for long-distance power transmission from sources like the Akosombo Dam. Understanding these properties, such as peak and RMS values, is essential for safely and effectively using electrical appliances and for understanding the principles behind our national power grid.
A. Alternating Current (AC) vs. Direct Current (DC)
The fundamental difference between AC and DC lies in the direction of current flow. Direct Current (DC): The electric current flows in one constant direction. Think of water flowing through a pipe from a high point to a low point without changing direction. Source: Batteries, solar cells, phone chargers (after conversion). Graph: A horizontal straight line on a Voltage/Time or Current/Time graph. Alternating Current (AC): The electric current periodically reverses its direction. It flows back and forth. Think of the blade of a saw moving forwards and backwards. Source: Power plants (hydroelectric like Akosombo, thermal), generators. This is the electricity supplied to our homes by ECG. Graph: A wave-like pattern that oscillates above and below the zero axis. B. Types of AC Waveforms
While the most common AC waveform is the sine wave, others exist and are important in electronics. Sinusoidal Wave: Description: This is a smooth, repetitive wave that follows the mathematical sine function. It represents a very efficient way to generate and transmit electricity. Source: Produced by rotating generators in power stations. It is the standard waveform for mains electricity worldwide, including the 230V, 50Hz supply in Ghana. Appearance: Square Wave: Description: The voltage or current instantaneously switches between two fixed levels (e.g., +5V and -5V) and stays at each level for a fixed duration. Use: Common in digital electronics, computer clocks, and signal processing. Appearance: Triangular Wave: Description: The voltage or current increases linearly to a maximum value and then decreases linearly back to a minimum value. Use: Used in sound synthesisers and for testing electronic amplifiers. Appearance: C. Important AC Values: Peak and RMS
Because AC voltage and current are constantly changing, we need special ways to describe their magnitude. Peak Voltage (V₀) and Peak Current (I₀) Definition: The peak value is the absolute maximum voltage or current that the AC signal reaches during one cycle. It is the amplitude of the waveform. On a graph, it is the distance from the horizontal axis (zero) to the highest point (crest) or lowest point (trough) of the wave. Root Mean Square (RMS) Voltage (V_rms) and Current (I_rms)