Alternating Current · SHS3 Physics

Lesson notes

Learning Objectives

Indicator: 3.3.2.LI.2 - Describe and distinguish between reactance and impedance.

By the end of the lesson, learners can:

Define reactance and impedance in their own words and state the SI unit (ohm, symbol Ω) for both quantities.
Derive and write the equations for capacitive reactance (Xc = 1/(2πfC)) and inductive reactance (XL = 2πfL) from the defining relationships for capacitors and inductors in AC circuits.
Use phasor diagrams to derive the impedance formula for a resistor-inductor series circuit (Z = √(R² + XL²)) and a resistor-capacitor series circuit (Z = √(R² + Xc²)).
Distinguish between reactance and impedance by explaining that reactance applies to a single reactive component (capacitor or inductor) while impedance applies to a combination of components (including at least a resistor) in a circuit.
Calculate the reactance of a capacitor or inductor, and the impedance of an R-L or R-C series circuit, given frequency, resistance, capacitance or inductance values.

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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.2 - Explain the a.c series connected circuit 3.3.2.LO.2 Illustrate and describe the behaviour of alternating current
Indicator: 3.3.2.LI.2 - Describe and distinguish between reactance and impedance.
Suggested placement: Semester 2, Week 5 (Week 25 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. 179

Exemplars (from the NaCCA curriculum)

Experiential learning:
- Guide learners to establish equations for resistive reactance, capacitive reactance and inductive reactance from exemplar 1.
- Let learners derive the formula for impedance using phasor diagrams for resistor-inductor in series, resistor-capacitor in series and resistor-capacitor-inductor in series.
Encourage the appreciation and valuing of the skills and abilities of individuals from diverse backgrounds, including gender, race, ethnicity, socioeconomic status, and disabilities.
Assessment (3.3.2.AS.2). 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