Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define electric current and resistance.
• Explain the relationship between voltage, current, and resistance using Ohm's Law.
• Solve problems using Ohm's Law.
• Identify factors that affect the resistance of a conductor.
• Distinguish between conductors, insulators, and semiconductors.

Lesson summary

This week, we delve into the crucial concepts of resistance and current within electric circuits. Understanding these concepts is fundamental to understanding how electricity powers our homes, schools, and communities. From the lights in your room to the appliances that make life easier, a grasp of resistance and current allows you to appreciate and troubleshoot electrical issues safely and effectively. South Africa, with its diverse energy landscape, requires informed citizens who can understand energy consumption and contribute to sustainable energy solutions.

Lesson notes

2.1 Electric Current (I): The Flow of Charge Electric current is the rate of flow of electric charge through a conductor. Think of it like water flowing through a pipe. The amount of water flowing per second is analogous to the electric current. Specifically, current is the amount of charge (measured in Coulombs, C) passing a point in the circuit per unit time (measured in seconds, s).

Formula: `I = Q / t` Where: `I` is the electric current (measured in Amperes, A) `Q` is the electric charge (measured in Coulombs, C) `t` is the time (measured in seconds, s) Conventional Current vs.

Electron Flow: Historically, it was thought that current flowed from positive to negative. This is conventional current.

However, we now know that electrons (which carry a negative charge) actually flow from negative to positive. While the direction is technically opposite, we usually use conventional current direction for circuit analysis. Think of it as an agreed-upon convention. 2.2 Resistance (R): Opposition to Current Flow Resistance is the opposition to the flow of electric current in a circuit. Materials that allow current to flow easily have low resistance (conductors), while materials that resist the flow of current have high resistance (insulators).

Factors Affecting Resistance: Material: Different materials have different inherent resistances. Copper and silver are good conductors (low resistance), while rubber and plastic are good insulators (high resistance).

Length: The longer the conductor, the greater the resistance. Imagine pushing water through a long, thin pipe; it's harder than pushing it through a short pipe. Resistance is directly proportional to length.

Cross-sectional Area: The thicker the conductor, the lower the resistance. A thicker pipe allows more water to flow easily. Resistance is inversely proportional to the cross-sectional area.

Temperature: For most materials, resistance increases with increasing temperature. The increased thermal energy makes it harder for electrons to flow smoothly.

Formula (Relationship to Resistivity): `R = ρL / A` Where: `R` is the resistance (measured in Ohms, Ω) `ρ` (rho) is the resistivity of the material (measured in Ohm-meters, Ω⋅m) – a material property `L` is the length of the conductor (measured in meters, m) `A` is the cross-sectional area of the conductor (measured in square meters, m²) 2.3 Ohm's Law: The Relationship Between Voltage, Current, and Resistance Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in a circuit. It states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them.

Formula: `V = I * R` Where: `V` is the voltage (potential difference) across the conductor (measured in Volts, V) `I` is the current flowing through the conductor (measured in Amperes, A) `R` is the resistance of the conductor (measured in Ohms, Ω) This formula can be rearranged to solve for I or R: `I = V / R` `R = V / I` 2.4 Conductors, Insulators, and Semiconductors Conductors: Materials that allow electric current to flow easily through them. They have low resistance.

Examples: copper, silver, aluminum. These are used in wiring in our homes.

Insulators: Materials that resist the flow of electric current. They have high resistance.

Examples: rubber, plastic, glass. Used to coat wires and prevent shocks.

Semiconductors: Materials with conductivity between that of conductors and insulators. Their conductivity can be controlled by adding impurities.

Examples: silicon, germanium. Used in electronics like computers and cell phones.

Worked example

Example 1:

A light bulb in a South African household is connected to a 220V power supply. If the current flowing through the bulb is 0.45A, what is the resistance of the bulb?

Solution:

We use Ohm's Law: `V = I * R`