Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define electric current and resistance and their units.
• Explain the relationship between voltage, current, and resistance using Ohm's Law.
• Calculate resistance using voltage and current.
• Explain factors that affect the resistance of a conductor.
• Predict how changes in resistance affect current.

Lesson summary

Electricity is the lifeblood of modern South Africa. From the lights in our homes and schools to the cellphones we use to connect with the world, almost everything we rely on depends on electric circuits. Understanding how these circuits work, particularly the concepts of resistance and current, is crucial not only for understanding the technology around us but also for making informed decisions about energy usage and safety. Many South African homes and businesses face challenges related to load shedding and managing electricity consumption; a strong understanding of resistance and current allows for better energy management and troubleshooting of simple electrical issues.

Lesson notes

Electric Current: Electric current is the flow of electric charge through a circuit. Think of it like water flowing through a pipe. The electric charge is carried by tiny particles called electrons. In South Africa, like most of the world, the standard electrical system uses alternating current (AC), meaning the direction of the flow of electrons changes periodically.

However, for the purposes of our simple circuit analysis in Grade 9, we often use the convention of conventional current, which assumes that current flows from the positive terminal of a battery to the negative terminal. The unit of electric current is the ampere (A), often shortened to "amp". One ampere represents one coulomb of charge flowing per second (1 A = 1 C/s). We use the symbol I to represent current in equations.

Resistance: Resistance is the opposition to the flow of electric current. It's like a narrow section in a water pipe that restricts the flow of water. All materials resist the flow of electric current to some extent. Conductors, like copper wires, have low resistance, allowing current to flow easily. Insulators, like rubber and plastic, have high resistance, preventing current from flowing. Resistors are components specifically designed to provide a particular amount of resistance in a circuit. The unit of resistance is the ohm (Ω). We use the symbol R to represent resistance in equations.

Factors affecting resistance: Length: The longer the conductor, the greater the resistance. Imagine a long, narrow pipe – it offers more resistance to water flow than a short, narrow pipe.

Cross-sectional Area: The thicker the conductor, the lower the resistance. A thick water pipe allows more water to flow through easily.

Material: Different materials have different inherent resistances. Copper is a much better conductor than iron, meaning it has lower resistance.

Temperature: For most materials, resistance increases with temperature. Think of the atoms vibrating more vigorously at higher temperatures, hindering the flow of electrons.

Ohm's Law: Ohm's Law is a fundamental relationship between voltage (V), current (I), and resistance (R) in a circuit. It states that the voltage across a conductor is directly proportional to the current flowing through it, with the constant of proportionality being the resistance. Mathematically, it's expressed as: V = I * R Where: V is the voltage measured in volts (V) I is the current measured in amperes (A) R is the resistance measured in ohms (Ω) Ohm's Law can be rearranged to solve for current or resistance: I = V / R R = V / I

Worked example

Example 1: A light bulb in a shack uses a 12V battery and draws a current of 0.5A. What is the resistance of the light bulb?

Solution:

We are given: V = 12V, I = 0.5A

We want to find: R

Using Ohm's Law: R = V / I

R = 12V / 0.5A

R = 24 Ω

Therefore, the resistance of the light bulb is 24 ohms.