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: 11
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 fundamental principles of electromagnetism: the turning effect (torque) experienced by a coil carrying an electric current when placed in a magnetic field. This principle is not just an abstract concept; it is the very foundation of countless devices we use daily in Ghana. From the ceiling fan that cools us down to the blender in the kitchen, and the starter motor in a car or *tro-tro*, this principle is at work. Understanding it allows us to appreciate the technology that powers our modern world.
A. Prerequisite Knowledge: The Motor Effect
Before we discuss the coil, let's remember the force on a single straight wire. The Motor Effect: When a wire carrying an electric current (I) is placed in a magnetic field (B), it experiences a force (F). Magnitude of the Force: The size of this force is given by the formula: ``` F = BILsinθ ``` Where: `F` is the force on the wire (in Newtons, N) `B` is the magnetic field strength (in Tesla, T) `I` is the current in the wire (in Amperes, A) `L` is the length of the wire inside the magnetic field (in metres, m) `θ` is the angle between the wire and the magnetic field lines. Direction of the Force: The direction of the force is found using Fleming's Left-Hand Rule. 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 resulting Thrust or Force. B. From a Single Wire to a Rectangular Coil
Now, imagine we bend this wire into a rectangle and place it in a uniform magnetic field, as shown below. Let the sides be PQ, QR, RS, and SP. The length of sides PQ and RS is *L*, and the width of sides QR and SP is *w*.
Let's analyse the forces on each side of the coil when current *I* flows as shown. Force on side PQ: Current flows into the page. The magnetic field goes from North (left) to South (right). Using Fleming's Left-Hand Rule: Forefinger points right (Field). Centre finger points into the page (Current). Thumb points downwards. So, side PQ experiences a downward force, `F_PQ`. Force on side RS: Current flows out of the page. The magnetic field is still from left to right. Using Fleming's Left-Hand Rule: Forefinger points right (Field). Centre finger points out of the page (Current). Thumb points upwards. So, side RS experiences an upward force, `F_RS`. Forces on sides QR and SP: On these sides, the current flows parallel or anti-parallel to the magnetic field lines. The angle `θ` between the wire and the field is either 0° or 180°. Since sin(0°) = 0 and sin(180°) = 0, the force on these sides is zero (`F = BILsin(0) = 0`). They do not contribute to the turning. C. The Concept of Torque