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 focuses on the practical application of electromagnetism by exploring the galvanometer, a sensitive instrument for detecting small electric currents. While the broader topic is Alternating Current, understanding how we measure current and voltage is fundamental. The instruments we use daily, like the multimeter used by an ECG technician checking your home's voltage or a car electrician diagnosing a battery issue, are all based on the principles we will learn today. We will learn how to modify a delicate galvanometer to measure much larger currents (as an ammeter) and voltages (as a voltmeter), which are essential skills for any future engineer, electrician, or scientist.
2.1 The Galvanometer
A galvanometer is a highly sensitive electrical instrument used to detect the presence of small electric currents. A common type is the moving-coil galvanometer. Principle: It works on the principle that a coil carrying a current in a magnetic field experiences a torque, which causes it to rotate. The amount of rotation (deflection of a pointer) is directly proportional to the current passing through the coil. Key Characteristics: Internal Resistance (R_g): The coil is made of a long, thin wire, so it has its own resistance. This is usually small, perhaps 10-100 Ω. Current for Full-Scale Deflection (I_g): This is the maximum current the galvanometer can handle before its pointer reaches the end of the scale and the coil is at risk of burning out. This current is very small, typically in the range of microamperes (µA) or milliamperes (mA).
A galvanometer on its own is not very useful for measuring the large currents and voltages found in everyday circuits (like the current drawn by a pressing iron or the voltage from a wall socket). We must adapt it. 2.2 Converting a Galvanometer into an Ammeter
An ammeter is an instrument used to measure current in a circuit. It must be connected in series in the circuit. For an ideal ammeter, its resistance should be zero so it doesn't affect the current it is trying to measure.