SHS3 Physics · Semester 2 Week 4: Alternating Current

Lesson notes

Learning Objectives

Indicator: 3.3.2.LI.3 - Using mathematical relationships define Alternating current and alternating voltage.

By the end of the lesson, learners can:

Define peak current (I₀) and peak voltage (V₀) using the equations i = I₀ sin ωt and v = V₀ sin ωt for a sinusoidal alternating current and voltage.
Define root mean square (rms) value of alternating current (Iᵣₘₛ) and alternating voltage (Vᵣₘₛ) and state the relationships Iᵣₘₛ = I₀/√2 and Vᵣₘₛ = V₀/√2.
Apply the mathematical relationships to calculate I₀, V₀, Iᵣₘₛ, and Vᵣₘₛ from given data in simple AC circuit problems.
Interpret the meaning and significance of peak and rms values in practical AC systems, explaining why rms values are used for power calculations.
Convert between peak and rms values in both directions, justifying each step of the calculation.

This lesson builds on the characteristics of AC (Week 3, LI.1) and the types of AC waveforms (Week 3, LI.2). It focuses purely on the mathematical definitions and relationships. The next lesson will move on to AC circuit connections with resistors, inductors and capacitors, so this lesson must firmly establish the meaning of peak, rms, and how they relate to each other.

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Curriculum details

Strand: Electric Field, Magnetic Field and Electronics (Strand 3)
Sub-strand: Alternating Current (3.2)
Content standard: 3.3.2.CS.1 - Demonstrate knowledge and understanding of Alternating Current Theory. 3.3.2.LO.1 Describe alternating current and utilise the associated mathematical relationships
Indicator: 3.3.2.LI.3 - Using mathematical relationships define Alternating current and alternating voltage.
Suggested placement: Semester 2, Week 4 (Week 24 of the year)
Our suggestion, laid out in curriculum order across three terms of twelve weeks. NaCCA does not fix the week, so follow your school's scheme of learning.
Curriculum reference: NaCCA curriculum document, p. 178

Exemplars (from the NaCCA curriculum)

Collaborative learning: Using the videos and simulation as well as internet/reference books let learners research to find the meaning of the following (i) Peak current Io, (ii) Peak voltage Vo, (iii) Root mean square value of alternating current Irms (iv) root mean square value of alternating voltage Vrms terms.
Explore contributions of vulnerable groups in the class.
Assessment (3.3.2.AS.3). The document marks these depth-of-knowledge levels for this indicator: Level 3 Strategic reasoning.
Teaching and Learning Resources:
- Audio-visuals
- Internet
- Projectors
- YouTube
- Interactive
- Reference virtual laboratory books