Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define electric current and resistance and state their units.
• Describe the relationship between voltage, current, and resistance using Ohm's Law.
• Apply Ohm's Law to solve problems in simple circuits.
• Explain factors affecting the resistance of a conductor.
• Identify the function of resistors in circuits.

Lesson summary

Electric circuits are all around us, powering our homes, schools, and communities. Understanding resistance and current is crucial for comprehending how these circuits work, how to use electricity safely, and even how to design and maintain electronic devices. From charging your cellphone in Soweto to powering the lights in your classroom in Durban, the principles of resistance and current are always at play. This week, we'll delve into these fundamental concepts, focusing on how resistance affects the flow of current in a circuit. Understanding this relationship allows us to control and use electrical energy safely and efficiently.

Lesson notes

2.1 Electric Current: Electric current is the flow of electric charge through a conductor. Think of it like water flowing through a pipe. The "water" in this case is made up of tiny particles called electrons, which carry a negative electric charge. These electrons move through the conductor, typically a wire made of metal like copper or aluminum, when a voltage is applied.

Definition: The rate of flow of electric charge.

Symbol: I Unit: Ampere (A). One Ampere is defined as one Coulomb of charge flowing per second (1 A = 1 C/s).

Explanation: A higher current means more charge is flowing per unit time. In South Africa, many household circuits are rated for 15A or 20

A. Exceeding this can cause the circuit breaker to trip. 2.2 Resistance: Resistance is the opposition to the flow of electric current in a circuit. It's like a narrowing in the pipe that restricts the water flow. Some materials allow electrons to flow through them easily (conductors), while others hinder the flow (insulators). Resistors are components specifically designed to provide a certain amount of resistance in a circuit.

Definition: A measure of how difficult it is for electric current to flow through a material.

Symbol: R Unit: Ohm (Ω)

Explanation: Higher resistance means it's harder for the current to flow. Think about trying to push water through a thin, crumpled straw – that's high resistance. A thick, smooth pipe offers low resistance.

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

Length: Longer wires have higher resistance. Imagine a long pipe – the water has to travel further, so there's more friction. The longer the wire, the more opportunities for electrons to collide with atoms in the material, thus increasing resistance.

Cross-sectional Area (Thickness): Thicker wires have lower resistance. A thicker pipe allows more water to flow through easily. A thicker wire allows more electrons to flow, decreasing resistance.

Temperature: For most conductors, resistance increases with temperature. As the temperature rises, the atoms in the material vibrate more, making it harder for electrons to move through. 2.3 Ohm's Law: Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in a circuit. It's a fundamental law that governs how circuits behave.

Statement: The voltage across a conductor is directly proportional to the current flowing through it, provided the temperature remains constant.

Formula: V = I R Where: V = Voltage (in Volts) I = Current (in Amperes) R = Resistance (in Ohms)

Understanding Ohm's Law: If you increase the voltage (V) while keeping the resistance (R) constant, the current (I) will increase proportionally. If you increase the resistance (R) while keeping the voltage (V) constant, the current (I) will decrease. Ohm’s law can be rearranged to solve for current: I = V/R and Resistance: R = V/I

Worked example

Example 1:

A cellphone charger plugged into a South African 220V outlet has an internal resistance of 10 Ohms. Calculate the current flowing through it.

Given: V = 220 V, R = 10 Ω

Required: I = ?

Formula: I = V / R

Solution: I = 220 V / 10 Ω = 22 A

Answer: The current flowing through the cellphone charger is 22 Amperes. This is quite high, so the charger has internal circuitry to reduce this. Real chargers draw much less current from the mains.

Example 2:

A light bulb connected to a 6V battery has a current of 0.5A flowing through it. Calculate the resistance of the light bulb.

Given: V = 6 V, I = 0.5 A

Required: R = ?

Formula: R = V / I

Solution: R = 6 V / 0.5 A = 12 Ω

Answer: The resistance of the light bulb is 12 Ohms.