Lesson Notes By Weeks and Term v4 - SHS 3
ALTERNATING CURRENT
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ALTERNATING CURRENT
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Subject: Physics
Class: SHS 3
Term: 2nd Term
Week: 5
Grade code: 3.3.1.LI.3
Strand code: 3
Sub-strand code: 2
Content standard code: 3.3.1.CS.3
Indicator code: 3.3.1.LI.3
Theme: ELECTRIC FIELD, MAGNETIC FIELD AND ELECTRONICS
Subtheme: ALTERNATING CURRENT
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This lesson explores the practical application of a fundamental electrical instrument: the galvanometer. While extremely sensitive to small currents, a galvanometer on its own is not very useful for measuring the larger currents and voltages we find in everyday circuits, like those in our homes or used by an ECG (Electricity Company of Ghana) technician. We will learn the clever but simple techniques used to modify, or 'adapt', this sensitive device into the robust and essential tools of every physicist and electrician: the ammeter and the voltmeter. Understanding this conversion is key to appreciating how the multimeters used in our science labs and by professionals actually work.
A. The Moving-Coil Galvanometer
A galvanometer is a device used to detect the presence of small electric currents. A common type is the moving-coil galvanometer. Principle: It works on the motor principle. When a current flows through a coil placed in a magnetic field, the coil experiences a torque (turning force) and deflects. The angle of deflection is directly proportional to the current passing through it. Key Properties: Internal Resistance ($R_g$): The resistance of the wire used to make the coil. It is usually small (e.g., 20 Ω to 100 Ω). Full-Scale Deflection Current ($I_g$): The maximum current the galvanometer can handle before its pointer reaches the end of the scale (or the coil gets damaged). This current is very small, typically in the microampere (μA) or milliampere (mA) range. Limitations: It can only measure very small currents. A larger current (e.g., 1 Ampere) would burn the coil. Its low internal resistance means if you connect it in parallel to measure voltage, it will draw a significant current, changing the very circuit it is trying to measure. B. Adapting a Galvanometer into an Ammeter
An ammeter measures current and must be connected in series in a circuit. An ideal ammeter should have a very low (ideally zero) resistance so it does not affect the total current it is measuring.
The Challenge: How can we use a galvanometer that only allows a tiny current ($I_g$) to measure a much larger total current ($I$)?