Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define electrical resistance and its unit (Ohms).
• Explain the relationship between voltage, current, and resistance (Ohm's Law).
• Solve simple circuit problems using Ohm's Law.
• Explain how factors like wire length and thickness affect resistance.
• Identify the function of resistors in a circuit.

Lesson summary

This week, we delve deeper into electric circuits, specifically focusing on the relationship between resistance and current. Understanding this relationship is crucial because it explains how electrical appliances work and how we can control the flow of electricity in our homes and communities. Many load shedding issues and the lifespan of electrical devices relate directly to this topic. For instance, overloading circuits (too much current) can cause fires, and using energy-efficient appliances (with lower resistance) helps reduce our electricity consumption and costs. We'll learn how resistance affects current and how we can calculate these values in simple circuits.

Lesson notes

What is Electrical Resistance? Electrical resistance is the opposition that a material offers to the flow of electric current. Think of it like friction for electricity. Just as friction slows down a moving object, resistance slows down the flow of electrons in a circuit. The higher the resistance, the less current will flow for a given voltage. The unit of resistance is the Ohm, represented by the Greek letter Omega (Ω).

Ohm's Law: The Relationship Between Voltage, Current, and Resistance Ohm's Law is a fundamental principle that describes the relationship between voltage (V), current (I), and resistance (R).

It states: Voltage (V) = Current (I) x Resistance (R) or V = IR This means: Current (I) = Voltage (V) / Resistance (R) or I = V/R Resistance (R) = Voltage (V) / Current (I) or R = V/I Understanding Voltage, Current, and Resistance Voltage (V): The "electrical pressure" that pushes the current through the circuit. It's measured in Volts (V). Think of it as the force driving the water through a pipe.

Current (I): The flow of electric charge (electrons) through the circuit. It's measured in Amperes (Amps or A). Think of it as the amount of water flowing through a pipe per second.

Resistance (R): The opposition to the flow of current. It's measured in Ohms (Ω). Think of it as the width of the pipe – a narrow pipe offers more resistance to the flow of water. Factors Affecting Resistance The resistance of a wire depends on several factors: Length: Longer wires have higher resistance. Imagine a longer pipe offering more friction to the water flowing through it.

Cross-sectional Area (Thickness): Thicker wires have lower resistance. A wider pipe allows more water to flow through it with less resistance.

Material: Different materials have different resistances. Copper and silver are good conductors with low resistance, while materials like rubber and glass are insulators with high resistance.

Temperature: For most materials, resistance increases with temperature.

Worked example

Example 1:

A light bulb is connected to a 220V power source. The current flowing through the bulb is 0.5A. What is the resistance of the light bulb?

Solution:

We use Ohm's Law: R = V/I

V = 220V

I = 0.5A

R = 220V / 0.5A = 440 Ω