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: 9
Grade code: 3.3.2.LI.3
Strand code: 3
Sub-strand code: 2
Content standard code: 3.3.2.CS.2
Indicator code: 3.3.2.LI.3
Theme: ELECTRIC FIELD, MAGNETIC FIELD AND ELECTRONICS
Subtheme: ALTERNATING CURRENT
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In our daily lives in Ghana, from charging our phones to watching television or using a fan, we are constantly using alternating current (AC) supplied by the Electricity Company of Ghana (ECG). Unlike the direct current (DC) from batteries, AC has unique characteristics, especially when it flows through devices with coils (like motors in fans and fridges) and capacitors. This lesson explores how power is consumed in AC circuits, focusing on efficiency through the concept of the power factor. We will also discover the fascinating phenomenon of resonance, the principle that allows you to tune your radio to your favourite station, like Joy FM or Peace FM, from a sea of other signals.
This lesson is divided into two main parts: Power in AC Circuits and Resonance in AC Circuits. Part A: Power in AC Circuits, The Power Triangle, and Power Factor
In DC circuits, power is simple: P = IV. In AC circuits containing inductors and capacitors, the voltage and current are often out of phase, which complicates things. This leads to three different types of power. True Power (P) Definition: This is the actual power used or dissipated by the resistive component of a circuit to do useful work (e.g., create heat, light, or motion). Unit: Watts (W) or Kilowatts (kW). Formula: `P = I²R` or `P = IV cos(φ)` Where `I` is the RMS current, `V` is the RMS voltage, `R` is the resistance, and `φ` is the phase angle between voltage and current. Reactive Power (Q) Definition: This is the power that is stored and then returned to the source by the reactive components of the circuit (inductors and capacitors). It does not perform any real work. Think of it as "sloshing" back and forth in the circuit. Unit: Volt-Ampere Reactive (VAR). Formula: `Q = I²X` or `Q = IV sin(φ)` Where `X` is the net reactance (X = XL - XC). Apparent Power (S) Definition: This is the "total" power that appears to be supplied to the circuit, calculated simply as the product of the total RMS voltage and current. It is the vector sum of True Power and Reactive Power. Unit: Volt-Ampere (VA) or Kilovolt-Ampere (kVA). Formula: `S = IV` or `S = I²Z` Where `Z` is the total impedance of the circuit.
The Power Triangle
These three types of power can be represented by a right-angled triangle, which is a powerful visual tool. The adjacent side represents True Power (P). The opposite side represents Reactive Power (Q). The hypotenuse represents Apparent Power (S). The angle between the adjacent and the hypotenuse is the phase angle (φ).