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: 8
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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This lesson explores two crucial concepts in Alternating Current (AC) circuits: Power and Resonance. The power supplied to our homes and schools by the Electricity Company of Ghana (ECG) is AC. However, not all the power delivered is converted into useful work (like light or motion). Understanding the 'power factor' helps us see how efficiently electrical power is being used. We will also investigate 'resonance,' the principle that allows you to tune your radio to receive your favourite station, like Peace FM or Joy FM, while ignoring all others. This knowledge is fundamental to electronics and power systems engineering.
This lesson is divided into two main parts: Power in AC Circuits and Resonance in AC Circuits. Part A: Power in AC Circuits
In DC circuits, power is simply calculated as P = IV. In AC circuits, it is more complex because the voltage and current may not be in phase with each other due to the presence of inductors and capacitors.
i. The Three Types of Power True Power (P): Also known as Real or Active Power. This is the power that actually does useful work, such as running a motor, lighting a bulb, or heating an element. It is measured in Watts (W) or Kilowatts (kW). Formula: `P = IV cos(φ)` Where `I` and `V` are the RMS values of current and voltage, and `φ` (phi) is the phase angle between them. Reactive Power (Q): This is the power that is stored and returned to the source by reactive components (inductors and capacitors). It does not perform any useful work. It is required to create the magnetic (in inductors) and electric (in capacitors) fields necessary for the device to operate. It is measured in Volt-Amperes Reactive (VAR) or kVAR. Formula: `Q = IV sin(φ)` Apparent Power (S): This is the "total" power that the source (like an ECG transformer) must supply. It is the vector sum of True Power and Reactive Power. It is the product of the total RMS voltage and RMS current in the circuit. It is measured in Volt-Amperes (VA) or kVA. Formula: `S = IV`
ii. The Power Triangle