Lesson Notes By Weeks and Term v4 - SHS 3
DIRECT CURRENT
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DIRECT CURRENT
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Subject: Physics
Class: SHS 3
Term: 2nd Term
Week: 4
Grade code: 3.3.1.LI.3
Strand code: 3
Sub-strand code: 1
Content standard code: 3.3.1.CS.3
Indicator code: 3.3.1.LI.3
Theme: ELECTRIC FIELD, MAGNETIC FIELD AND ELECTRONICS
Subtheme: DIRECT CURRENT
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Welcome, learners. In our daily lives, from charging our phones to the work of an ECG technician checking our home meters, measuring electricity is crucial. But how do the devices that measure current (ammeters) and voltage (voltmeters) actually work? Today, we will discover that at the heart of these common instruments is a very sensitive device called a galvanometer. A galvanometer on its own is too delicate to measure the large currents and voltages in everyday circuits. We will learn the clever techniques used to modify, or 'adapt', a galvanometer to safely and accurately measure a wide range of currents and voltages.
Part 1: The Galvanometer - The Sensitive Heart
A moving-coil galvanometer is an instrument used to detect very small electric currents. Think of it like a very sensitive scale that can only measure the weight of a feather. If you try to weigh a bag of cement on it, you will break it.
Its operation is based on the principle that a coil carrying a current in a magnetic field experiences a torque, causing it to rotate. The amount of rotation (deflection of a pointer) is proportional to the current.
Every galvanometer has two critical properties we must know: Internal Resistance (Rg): The resistance of the wire used to make its coil. It's typically a small value, like 20 Ω or 50 Ω. Full-Scale Deflection Current (Ig): The maximum current the galvanometer can handle before its pointer reaches the end of the scale and the device gets damaged. This is a very small current, often in microamperes (µA) or milliamperes (mA).