Circuit and Machines · SHS1 Engineering

Lesson notes

Learning Objectives

Indicator: 1.2.1.LI.5 - Compute power in dc and single-phase ac circuits.

By the end of the lesson, learners can:

Compute power in a dc circuit using P = VI, P = I²R, and P = V²/R, given any two of the three quantities (voltage, current, resistance).
Distinguish between true power (watts), reactive power (VAR), and apparent power (VA) in a single-phase ac circuit.
Calculate true power, apparent power, and power factor in a single-phase ac circuit using the equations P = VI cos θ and S = VI.
Solve word problems involving power in both dc and ac circuits that require multi-step strategic reasoning, such as finding an unknown quantity after first computing another.
Justify their answers in small groups and explain their reasoning to the whole class using correct engineering terminology.

This lesson builds directly on the circuit element identification from Week 14, the passive and active element classification from Week 15, and the Kirchhoff’s laws work from Weeks 16 and 17. The simulation of these circuits using software is reserved for Week 19.

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Curriculum details

Strand: Energy Systems (Strand 2)
Sub-strand: Circuit and Machines (2.1)
Content standard: 1.2.1.CS.1 - Demonstrate knowledge of simple electric circuits based on first principles and through simulation tools. 1.2.1.LO.1 Identify and explain the functions of the elements of dc and ac circuits. 1.2.1.LO.2 Analyse simple dc and single-phase ac circuits.
Indicator: 1.2.1.LI.5 - Compute power in dc and single-phase ac circuits.
Suggested placement: Semester 1, Week 18 (Week 18 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. 37

Exemplars (from the NaCCA curriculum)

Initiating Talk for Learning: The facilitator draws and explains the power training, including the power factor. The facilitator further provides all relevant equations.
Problem-based Learning: Present various dc and ac circuit problems for the class to collectively discuss and solve. Learners sit in small mixed-ability groups to solve circuit problems and share their solutions with the whole class,

Assessment (1.2.1.AS.5). The document marks these depth-of-knowledge levels for this indicator: Level 3 Strategic reasoning.

Teaching and Learning Resources:
- Projector
- Laptop
- Videos on dc and ac circuit elements
- Assorted dc and ac electric circuit elements (e.g., Inductors, capacitors, resistors, switches, etc.)
- Whiteboard and marker
- Whiteboard and marker
- Simulation software (e.g., Proteus)