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: 10
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 the fundamental principle that makes electric motors work. We see electric motors everywhere in our daily lives in Ghana, from the fan that cools us down, to the blender in the kitchen, to the water pump that fills our Polytank, and even the starter motor in a car or "trotro". By understanding why a current-carrying coil spins in a magnetic field, we are unlocking the secret behind these essential modern machines. We will build on our previous knowledge of the force on a single wire to understand the turning effect, or torque, on a complete coil.
A. Recap: Force on a Single Current-Carrying Wire
Before we look at a whole coil, let's remember the basics. When a straight wire of length L carrying a current I is placed in a magnetic field of strength B, it experiences a force F. Formula: `F = BILsinθ` B: Magnetic flux density (in Tesla, T) I: Current (in Amperes, A) L: Length of the wire in the field (in metres, m) θ: The angle between the wire and the magnetic field lines. Direction of Force: We use Fleming's Left-Hand Rule to find the direction of this force. Thumb: Direction of the Thrust or Force. Forefinger: Direction of the magnetic Field (North to South). Centre finger: Direction of the Current (conventional current, + to -). 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, like the one between the North and South poles of a magnet. Let's label the corners A, B, C, and D. The coil has a length L and a width w. Current I flows around the coil.
Let's analyse the forces on each of the four sides: Side AB (Length L): Current flows into the page. The magnetic field goes from N to S (left to right). Using Fleming's Left-Hand Rule: Field (Forefinger): Points Right. Current (Centre finger): Points Into the page. Force (Thumb): Points DOWNWARDS. Magnitude of Force: `F_AB = BIL` (since the wire is perpendicular to the field, sin90° = 1). Side CD (Length L): Current flows out of the page. The magnetic field is still left to right. Using Fleming's Left-Hand Rule: Field (Forefinger): Points Right. Current (Centre finger): Points Out of the page. Force (Thumb): Points UPWARDS. Magnitude of Force: `F_CD = BIL` (also perpendicular, so sin90° = 1). Sides BC and DA (Width w): On these sides, the current is flowing parallel or anti-parallel to the magnetic field lines. For side BC, the angle θ between current and field is 180°. Since sin(180°) = 0, the force `F_BC = 0`. For side DA, the angle θ is 0°. Since sin(0°) = 0, the force `F_DA = 0`. Conclusion: The sides parallel to the magnetic field experience no force. C. The Couple and Torque