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: 9
Grade code: 2.3.2.LI.2
Strand code: 3
Sub-strand code: 2
Content standard code: 2.3.2.CS.2
Indicator code: 2.3.2.LI.2
Theme: ELECTRIC FIELD, MAGNETIC FIELD AND ELECTRONICS
Subtheme: ELECTROMAGNETISM
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This lesson explores one of the most important applications of electromagnetism: the turning effect (torque) experienced by a current-carrying coil in a magnetic field. This principle is not just an abstract concept; it is the fundamental engine of our modern world. In Ghana, from the electric fans that cool our homes and classrooms, to the blenders used in making our favourite dishes, and even the starter motors in trotros and taxis, this principle is at work. Understanding it helps us appreciate the technology that powers our daily lives, from the electricity generated by VRA at Akosombo to its final use as mechanical motion in our homes.
2.1. Recap: The Motor Effect and Fleming's Left-Hand Rule
Before we discuss the turning coil, let's remember the basic principle: the motor effect. Definition: When a conductor (like a wire) carrying an electric current is placed in a magnetic field, it experiences a force. Conditions: For this force to exist, the current must flow at an angle to the magnetic field lines. The force is maximum when the current and field are perpendicular (90°) to each other. The force is zero when they are parallel (0°). Fleming's Left-Hand Rule: This rule helps us predict the direction of the force. Hold your left hand with the Thumb, Forefinger, and Centre finger mutually perpendicular (at 90°) to each other. Forefinger points in the direction of the Magnetic Field (North to South). Centre finger points in the direction of the Current. The Thumb points in the direction of the Thrust or Force. 2.2. Torque on a Rectangular Current-Carrying Coil
Now, let's move from a single wire to a rectangular coil. Imagine a coil of wire, labelled `ABCD`, placed in a uniform magnetic field between the North and South poles of a magnet. A current `I` flows through the coil.
Let's analyse the forces on each side of the coil: Side AB: The current flows from A to B. The magnetic field is from North to South (left to right). Using Fleming's Left-Hand Rule: Forefinger (Field) points right. Centre finger (Current) points into the page. Thumb (Force) points DOWNWARDS. Side CD: The current flows from C to D. The magnetic field is still from left to right. Using Fleming's Left-Hand Rule: Forefinger (Field) points right. Centre finger (Current) points out of the page. Thumb (Force) points UPWARDS. Sides BC and DA: On side BC, the current is flowing away from the viewer, parallel to the field at some points. On side DA, the current is flowing towards the viewer, parallel to the field at other points. In these positions, the force on these sides is either zero (when parallel to the field) or directed outwards along the axis of rotation, so they do not contribute to the turning effect. They simply try to stretch the coil.