Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define current, voltage, and resistance
• Draw and interpret circuit diagrams
• Construct and describe series and parallel circuits
• Predict how changes affect circuits
• Measure current and voltage

Lesson summary

Electricity powers so much of our lives in South Africa, from the lights in our homes and schools to the cellphones we use to communicate. Understanding how electricity works and how circuits are designed allows us to use energy safely and efficiently. Unfortunately, electricity is not always readily available to everyone in South Africa, and it's important to understand how we can use it responsibly when we do have access to it. This week, we'll be focusing on building circuits, understanding how current flows, and learning how to measure electricity.

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 amount of water flowing per second is similar to the amount of charge flowing per second in a circuit. This flow of charge is due to the movement of electrons.

Definition: The rate of flow of electric charge.

Symbol: I Unit: Ampere (A) – often abbreviated as "amps." How to measure: An ammeter is used to measure current. It must be connected in series with the circuit element you want to measure the current through. 2.2 Voltage (Potential Difference): Voltage, also known as potential difference, is the "push" or "electrical pressure" that drives the electric current through a circuit. It's the difference in electric potential between two points in a circuit.

Definition: The electrical potential difference between two points in a circuit that drives the flow of current.

Symbol: V Unit: Volt (V)

How to measure: A voltmeter is used to measure voltage. It must be connected in parallel across the circuit element you want to measure the voltage across. 2.3 Resistance: Resistance is the opposition to the flow of electric current. Think of it like a narrow section in a pipe, making it harder for water to flow through. A resistor is a component that is designed to have a specific resistance. Light bulbs also have resistance.

Definition: The opposition to the flow of electric current.

Symbol: R Unit: Ohm (Ω) – the Greek letter omega.

Factors Affecting Resistance: The resistance of a wire depends on its material, length, and thickness. Longer wires have higher resistance, and thinner wires have higher resistance. 2.4 Circuit Diagrams and Symbols: It's important to use standard symbols when drawing circuit diagrams so that everyone can understand them.

Here are some common symbols: Cell: | - (short line is negative, long line is positive)

Battery: | - | - (multiple cells connected)

Bulb: (A circle with an "X" inside)

Switch (Open): ---/ /--- (switch is open, circuit is broken)

Switch (Closed): ------- (switch is closed, circuit is complete)

Resistor: ----/\/\/\---- Ammeter: (Circle with an "A" inside)

Voltmeter: (Circle with a "V" inside)

Connecting Wire: Straight Line 2.5 Series and Parallel Circuits: Series Circuit: A circuit where all components are connected in a single loop. The current is the same at all points in a series circuit. If one component fails (e.g., a bulb burns out), the entire circuit is broken, and everything stops working. Think of old Christmas tree lights where if one bulb blew, they all went out.

Parallel Circuit: A circuit where components are connected along multiple paths. The voltage is the same across all branches in a parallel circuit. If one component fails, the other branches continue to function. Most household wiring is done in parallel. 2.6 Conductors and Insulators: Conductors: Materials that allow electric current to flow easily. Examples include copper, aluminum (used in power lines), and steel. Most metals are good conductors.

Insulators: Materials that resist the flow of electric current. Examples include rubber, plastic (used to coat wires), and glass. These are important for safety, preventing electric shocks. 2.7 Worked

Examples: Example 1: Simple Series Circuit Consider a series circuit with a 6V battery and two identical light bulbs. What happens to the brightness of the bulbs if one bulb burns out?

Solution: In a series circuit, the current flows through both bulbs. If one bulb burns out, it creates an open circuit. This means the path for the current is broken, and no current can flow.

Therefore, both bulbs will go out.

Example 2: Simple Parallel Circuit Consider a parallel circuit with a 6V battery and two identical light bulbs. What happens to the brightness of the remaining bulb if one bulb burns out?

Solution: In a parallel circuit, each bulb has its own independent path to the battery. If one bulb burns out, it only affects its own branch of the circuit. The other branch remains complete, and the remaining bulb will continue to shine with the same brightness.

Example 3: Measuring Current in a Series Circuit A series circuit contains a 9V battery, a switch, a resistor, and an ammeter. The ammeter reads 0.5

A. Draw a circuit diagram and state what the current is at all points in the circuit.

Solution: Circuit Diagram: (Draw a simple series circuit with the components described and label the battery 9V and the ammeter reading 0.5A)

Explanation: In a series circuit, the current is the same at all points.

Therefore, the current through the resistor is also 0.5A. The current leaving the battery is 0.5A and the current returning to the battery is also 0.5

A. Guided Practice (With Solutions)

Question 1: Draw a circuit diagram showing a battery connected to two resistors in series, an ammeter measuring the current flowing from the battery, and a voltmeter measuring the voltage across one of the resistors.