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.2
Strand code: 3
Sub-strand code: 1
Content standard code: 3.3.1.CS.2
Indicator code: 3.3.1.LI.2
Theme: ELECTRIC FIELD, MAGNETIC FIELD AND ELECTRONICS
Subtheme: DIRECT CURRENT
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In our study of electricity, we have often dealt with resistors that have a single, fixed value, like the ones inside your phone charger or a radio. However, in many real-life situations, we need to control the amount of current flowing or the voltage supplied to a part of a circuit. Think about changing the speed of a ceiling fan or adjusting the volume of a sound system. This control is achieved using a special component called a variable resistor. This lesson will introduce the two main types of variable resistors: the rheostat and the potentiometer. We will explore their construction, how they work, and how they are used to give us control over electrical circuits.
2.1. What is a Resistor? (A Quick Recap) Recall that a resistor is an electrical component that opposes the flow of electric current. Its resistance (R) is measured in Ohms (Ω). For a given voltage (V), a higher resistance allows less current (I) to flow, as described by Ohm's Law: V = IR. Most resistors we have used so far are fixed resistors, meaning their resistance value is constant. 2.2. Introduction to Variable Resistors A variable resistor is a resistor whose resistance value can be changed or adjusted. This is typically done by moving a sliding contact along a resistive element.
The fundamental principle is that the resistance of a conductor is directly proportional to its length (R ∝ L). By changing the length of the resistive material that the current must pass through, we can change the total resistance.
There are two main types of variable resistors we will study: the Rheostat and the Potentiometer. While they often look physically similar, their application and connection in a circuit are different. 2.3. The Rheostat: A Variable Current Controller
A rheostat is a variable resistor used to control the current in a circuit. The name comes from Greek words: *rheos* (stream or current) and *statis* (to stand or control). Construction: It consists of a coil of resistance wire (e.g., manganin or constantan) wound around an insulating cylinder. There is a sliding contact, often called a jockey or slider, which can be moved along the length of the coil. It has two terminals for connection in a circuit. Circuit Symbol: IEC Symbol: A rectangle with an arrow passing through it at an angle. `---[ ]--` ` / ` ` / ` ` / ` ` / ` US Symbol: A zigzag line with an arrow pointing to it. `---/\/\/\/\---` ` ^ ` ` | ` Working Principle: A rheostat is connected in series with the load (e.g., a light bulb) it is controlling. One terminal is connected to one end of the resistive wire, and the other terminal is connected to the sliding contact. When the slider is moved, it changes the length of the resistance wire included in the circuit. Longer length = Higher resistance = Less current (I = V/R). Shorter length = Lower resistance = More current (I = V/R). Circuit Diagram: Imagine we want to control the brightness of a bulb.