Alternating Current (AC) and Electronics

Grade 12 · Physics

Semester 2 | Period 4 | Week 23

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Subject: Physics

Semester: 2

Period: 4

Week: 23

School Name:

Teacher’s Name:

Subject: Physics

Grade Level: Grade 12

Week & Period: Week 23, Period IV

Date:

Main Topic: Alternating Current (AC) and Electronics
Sub-Topic: Semiconductor Devices – Transistors, Thermistors, and Their Applications

Learning Objectives:

By the end of the lesson, learners should be able to:

  1. Define and explain the function of transistors and thermistors.
  2. Distinguish between the types of transistors and their configuration.
  3. Describe how transistors work as amplifiers and switches.
  4. State the function of thermistors and their application in circuits.
  5. Build simple circuits using transistors and thermistors.

 

Instructional Materials:

  • NPN and PNP transistors (e.g., BC547, BC557)
  • Thermistors (NTC and PTC types)
  • Breadboards, batteries (9V)
  • Resistors (1kΩ, 10kΩ)
  • LEDs, switches
  • Multimeter
  • Circuit diagrams (printed or drawn)
  • Soldering kit (optional)

 

Anticipation (Warm-Up):

Ask learners:

“How does your TV remote respond instantly when you press a button? How do phone screens react to your touch?”

Transition into explaining how semiconductor devices like transistors and thermistors detect signals and control current.

 

Building Knowledge (Main Lesson):

  1. Transistors:

Definition:
A transistor is a semiconductor device used to amplify or switch electronic signals.

Types:

  • NPN Transistor: Current flows from collector to emitter when base is activated.
  • PNP Transistor: Current flows from emitter to collector when base is grounded.

Terminals:

  • Emitter (E)
  • Base (B)
  • Collector (C)

Functionality:

  • Amplifier: Small base current controls large collector current.
  • Switch: ON/OFF control of circuit by triggering base.
  1. Thermistors:

Definition:
A thermistor is a temperature-sensitive resistor.

Types:

  • NTC (Negative Temperature Coefficient): Resistance decreases as temperature increases.
  • PTC (Positive Temperature Coefficient): Resistance increases as temperature increases.

Applications:

  • Fire alarms
  • Temperature sensors
  • Battery chargers

 

Circuit Diagram Examples:

  1. Transistor as a switch to light an LED when button is pressed.
  2. Thermistor-based temperature alert: LED lights up when temperature increases (using NTC).

 

Experiment 1: Transistor as a Switch

Materials:

  • 9V battery, breadboard
  • NPN transistor (BC547)
  • 1kΩ resistor, LED
  • Push-button switch

Procedure:

  1. Connect collector to LED and then to power.
  2. Connect emitter to ground.
  3. Base connected to 1kΩ resistor and switch.
  4. Press switch → LED turns on.

Observation:
The LED glows only when switch is pressed — transistor acts as switch.

 

Experiment 2: Thermistor as a Temperature Detector

Materials:

  • NTC thermistor
  • 9V battery, LED, 10kΩ resistor
  • Heat source (e.g., hairdryer), multimeter

Procedure:

  1. Connect thermistor in series with LED and resistor.
  2. Measure voltage drop across thermistor.
  3. Heat it gently and observe brightness or voltage change.

Observation:
As temperature rises, resistance drops → more current flows → LED gets brighter.

 

Activities and Practice:

  • Learners label transistor terminals and draw diagrams of common-emitter configuration.
  • Build a working transistor-based amplifier circuit on breadboard.
  • Group project: Build a fire alarm circuit using a thermistor.

Assessment:

Classwork:

  1. Define transistor and thermistor.
  2. Differentiate between NPN and PNP transistors.
  3. Describe how a thermistor works.

Homework:

  1. Draw and label a transistor circuit used as a switch.
  2. Write out two real-life applications each of:
    • Transistors
    • Thermistors

 

Expanded Notes:

  • Transistors revolutionized electronics — smaller, faster than vacuum tubes.
  • Used in every computer, phone, calculator, and amplifier.
  • Thermistors are widely used in climate control, cars, fridges, and computers.
  • In logic gates, transistors control signals that represent 1s and 0s.

 

Differentiation:

  • Use analogies: transistor as a faucet (base = handle; collector = supply; emitter = flow).
  • Practical tasks with low-voltage batteries for kinesthetic learners.
  • Diagrams and simulation apps for visual learners.

 

Teacher’s Reflection:

  • Were learners able to build and explain transistor/thermistor circuits?
  • Did they understand the link between temperature and resistance?
  • How confident were they in distinguishing NPN and PNP types?