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: 5
Grade code: 3.3.1.LI.2
Strand code: 3
Sub-strand code: 2
Content standard code: 3.3.1.CS.3
Indicator code: 3.3.1.LI.2
Theme: ELECTRIC FIELD, MAGNETIC FIELD AND ELECTRONICS
Subtheme: ALTERNATING CURRENT
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Welcome, future engineers and scientists! In our homes, the electricity we get from the Electricity Company of Ghana (ECG) is Alternating Current (AC), not the Direct Current (DC) we find in batteries. This AC power is what runs our refrigerators, fans, televisions, and charges our phones. However, the components inside these appliances don't all use the full 230 Volts from the wall socket. The voltage and current must be divided and directed to different parts of the internal circuits. Today, we will learn the fundamental principles of how voltage and current are shared or 'divided' in AC circuits.
Part 1: From Resistance (DC) to Impedance (AC)
In our study of DC circuits, the only thing that opposed the flow of current was Resistance (R). In AC circuits, the situation is more complex. We have three types of components that affect current flow: Resistors (R): Their opposition to current is still called Resistance. It is independent of the frequency of the AC supply. Inductors (L): These are coils of wire. They oppose changes in current. Their opposition is called Inductive Reactance (X L ). It increases with frequency. Formula: `X_L = 2πfL` Where: `f` = frequency (in Hertz, Hz), `L` = inductance (in Henry, H). Capacitors (C): These components store charge. They oppose changes in voltage. Their opposition is called Capacitive Reactance (X C ). It *decreases* with frequency. Formula: `X_C = 1 / (2πfC)` Where: `f` = frequency (in Hz), `C` = capacitance (in Farads, F).
Key Definition: Impedance (Z) Impedance (Z) is the *total opposition* to current flow in an AC circuit. It is the combined effect of resistance and reactance. Like resistance and reactance, it is measured in Ohms (Ω).
For a simple series circuit with a resistor and an inductor: `Z = √(R² + X_L²)`