Lesson Notes By Weeks and Term v4 - SHS 2
ELECTROMAGNETISM
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ELECTROMAGNETISM
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Subject: Physics
Class: SHS 2
Term: 2nd Term
Week: 8
Grade code: 2.3.2.LI.3
Strand code: 3
Sub-strand code: 2
Content standard code: 2.3.2.CS.2
Indicator code: 2.3.2.LI.3
Theme: ELECTRIC FIELD, MAGNETIC FIELD AND ELECTRONICS
Subtheme: ELECTROMAGNETISM
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This lesson explores the fascinating application of the motor effect, a fundamental principle of electromagnetism. We will investigate how the force on a current-carrying wire in a magnetic field is harnessed to create continuous rotation in an electric motor and precise measurement in a moving coil galvanometer. These devices are not just abstract concepts; they are the heart of many machines we use daily in our homes and communities in Ghana, from the ceiling fan that cools us to the instruments an ECG technician uses to check our electrical wiring. Understanding these principles is key to appreciating the technology that powers our modern lives.
2.1 Foundational Principle: The Motor Effect
Before we can understand a motor or a galvanometer, we must understand the force that makes them work. Definition: The motor effect is the phenomenon where a conductor carrying an electric current experiences a force when placed in a magnetic field. Condition: The force is maximum when the conductor is perpendicular (at 90°) to the magnetic field and zero when it is parallel to the field. Fleming's Left-Hand Rule: This rule helps us predict the direction of the force (or motion). Imagine holding your left hand with your thumb, forefinger, and middle finger all at right angles (90°) to each other. Thumb represents the direction of the Thrust or Force (Motion). Forefinger represents the direction of the magnetic Field (from North pole to South pole). Centre (middle) finger represents the direction of the Current (conventional current, from + to -). 2.2 The Simple DC Electric Motor
An electric motor is a device that converts electrical energy into mechanical energy (rotational motion). Structure and Parts A simple DC motor consists of the following essential parts: Permanent Magnets (or Electromagnet): Two strong magnets placed with opposite poles facing each other (e.g., North and South). Their purpose is to create a strong, uniform magnetic field in the region where the coil will rotate. Armature (or Coil): A rectangular coil of insulated copper wire with many turns, wound on a soft iron core. The soft iron core concentrates the magnetic field lines, increasing the force on the coil. Split-Ring Commutator: This is the most crucial part for continuous rotation. It is a metallic ring split into two halves (e.g., C1 and C2), insulated from each other. Each end of the armature coil is connected to one half of the commutator. Its function is to reverse the direction of the current in the coil every half rotation. Carbon Brushes: Two small blocks of carbon (a good conductor and a solid lubricant) that are held by springs to press gently against the rotating commutator. Their function is to conduct electricity from the power source to the commutator and then to the coil. Power Source (DC Supply): A battery or DC power pack that supplies the electric current.
*Diagram of a Simple DC Motor:* (Teacher should draw this on the board for clarity) Working Principle