Lesson Notes By Weeks and Term v4 - SHS 2
ELECTRONIC COMPONENTS AND CIRCUITS
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ELECTRONIC COMPONENTS AND CIRCUITS
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Subject: Applied Technology
Class: SHS 2
Term: 2nd Term
Week: 20
Grade code: 2.5.2.LI.11
Strand code: 5
Sub-strand code: 2
Content standard code: 2.5.2.CS.1
Indicator code: 2.5.2.LI.11
Theme: ELECTRICAL AND ELECTRONIC TECHNOLOGY
Subtheme: ELECTRONIC COMPONENTS AND CIRCUITS
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This lesson introduces the Moving Iron (MI) instrument, a fundamental device used for measuring electrical current and voltage in both AC and DC circuits. In our daily lives in Ghana, from the electrician who checks the wiring in our homes to the technician at the Volta River Authority (VRA) monitoring the power grid, the ability to measure electricity is crucial. Understanding how these basic meters work and how they can be adapted to measure different ranges of current and voltage is a foundational skill for any student of technology.
2.1. The Moving Iron (MI) Instrument
A Moving Iron instrument is an electrical measuring device that uses the magnetic effect of electric current to produce a deflection. A key feature is that it can measure both Alternating Current (AC) and Direct Current (DC).
Principle of Operation: The operation is based on a simple principle: when a piece of soft iron is placed in a magnetic field, it gets magnetised. If the magnetic field is created by a current-carrying coil, the iron piece will be either attracted to the coil or repelled by another piece of magnetised iron. The force of this attraction or repulsion causes the iron piece to move, and this movement is used to deflect a pointer against a calibrated scale. Attraction Type: A fixed coil (electromagnet) attracts a single piece of soft iron when current flows through it. The stronger the current, the stronger the magnetic field, and the greater the attraction and deflection. Repulsion Type: Two soft iron pieces (one fixed, one movable) are placed inside the coil. When current flows, both become magnetised with the same polarity. They repel each other, causing the movable iron to move away, deflecting the pointer.
Construction: A typical MI instrument consists of: Fixed Coil: A coil of insulated copper wire that carries the current to be measured, creating the magnetic field. Moving Iron: A small piece or vane of soft iron that is attached to a spindle and is free to move. Pointer and Scale: A lightweight pointer attached to the spindle moves over a calibrated scale to give the reading. The scale is usually non-uniform (cramped at the beginning). Control System: A spring (usually a hairspring) provides the controlling torque. It opposes the deflecting force and brings the pointer back to zero when the current is off. Damping System: Air friction damping is commonly used. A light aluminium vane attached to the spindle moves in a closed air chamber to prevent the pointer from oscillating too much, allowing it to settle on a reading quickly.