Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Draw series, parallel, and combination circuit diagrams.
• Calculate total resistance, current, and voltage.
• Identify and explain different types of switches.
• Construct and test a combination circuit.
• Explain the pros and cons of series and parallel circuits.

Lesson summary

This week, we delve deeper into electrical systems, moving beyond simple circuits to explore more complex arrangements and different types of switches. Understanding these concepts is crucial because electrical systems are fundamental to modern life in South Africa, powering our homes, schools, industries, and transportation. From the lighting in your classroom to the appliances in your kitchen, these systems rely on the principles we will learn.

Furthermore, a basic understanding of circuits and switches allows for informed decision-making when troubleshooting electrical issues and encourages safer and more responsible energy consumption.

Lesson notes

2.1 Series Circuits: In a series circuit, components are connected one after the other, forming a single path for the current to flow. Think of it like a single lane road – all the cars (electrons) must follow the same route.

Current (I): The current is the SAME at all points in a series circuit. What flows in, flows out. Imagine the number of cars passing any point on the single-lane road remains constant.

Voltage (V): The voltage is DIVIDED across the components. The total voltage of the power source is shared among the resistors or loads in the circuit. Think of it like the energy the cars have to expend climbing hills. Each hill (resistor) takes some energy.

Resistance (R): The total resistance (R T ) is the SUM of the individual resistances. R T = R 1 + R 2 + R 3 + ... Imagine adding the sizes of the hills on the road; the total size is just the sum of the individual sizes.

Example: Two resistors, R 1 = 10 Ω and R 2 = 20 Ω, are connected in series to a 9V battery.

Calculate the total resistance (R T ): R T = R 1 + R 2 = 10 Ω + 20 Ω = 30 Ω Calculate the total current (I) using Ohm's Law (V = IR): I = V / R T = 9V / 30 Ω = 0.3A Calculate the voltage drop across each resistor: V 1 (across R 1 ) = I R 1 = 0.3A * 10 Ω = 3V V 2 (across R 2 ) = I R 2 = 0.3A * 20 Ω = 6V Notice that V 1 + V 2 = 3V + 6V = 9V, which equals the total voltage of the battery. 2.2 Parallel Circuits: In a parallel circuit, components are connected along multiple paths. Think of it like a multi-lane highway – the cars (electrons) can choose different routes.

Current (I): The current is DIVIDED among the different branches. The total current entering the parallel section equals the sum of the currents in each branch.

Voltage (V): The voltage is the SAME across all branches in a parallel circuit. Each path experiences the full voltage of the power source. Imagine all lanes on the highway are flat and experience the same "push".

Resistance (R): The total resistance (R T ) is calculated using the following formula: 1/R T = 1/R 1 + 1/R 2 + 1/R 3 + ... Then, you take the reciprocal of the result to find R T . This means the overall resistance in a parallel circuit is always less than the smallest individual resistance. Think of adding more lanes to a highway. It's easier to get cars through!

Example: Two resistors, R 1 = 4 Ω and R 2 = 12 Ω, are connected in parallel to a 12V battery.

Calculate the total resistance (R T ): 1/R T = 1/4 Ω + 1/12 Ω = 3/12 + 1/12 = 4/12 = 1/3 R T = 3 Ω (reciprocal of 1/3) Calculate the total current (I) using Ohm's Law: I = V / R T = 12V / 3 Ω = 4A Calculate the current through each resistor: I 1 (through R 1 ) = V / R 1 = 12V / 4 Ω = 3A I 2 (through R 2 ) = V / R 2 = 12V / 12 Ω = 1A Notice that I 1 + I 2 = 3A + 1A = 4A, which equals the total current. 2.3 Combination Circuits: These circuits contain both series and parallel components. To analyze them, you need to simplify the circuit by combining the series and parallel sections step-by-step.

Example: A circuit has a 5 Ω resistor (R 1 ) in series with a parallel combination of a 10 Ω resistor (R 2 ) and a 15 Ω resistor (R 3 ). The circuit is connected to a 10V battery. Calculate the equivalent resistance of the parallel section (R 2 and R 3 ): 1/R parallel = 1/10 Ω + 1/15 Ω = 3/30 + 2/30 = 5/30 = 1/6 R parallel = 6 Ω Calculate the total resistance of the circuit (R T ): R T = R 1 + R parallel = 5 Ω + 6 Ω = 11 Ω Calculate the total current (I): I = V / R T = 10V / 11 Ω ≈ 0.91A 2.4 Switches: Switches control the flow of electricity in a circuit. They can be used to open or close a circuit, turning devices on or off.

SPST (Single-Pole Single-Throw): The simplest type of switch. It has one input and one output. It's like a simple on/off switch – flipping it completes or breaks the circuit.

Example: Light switch.

SPDT (Single-Pole Double-Throw): This switch has one input and two outputs. It can connect the input to either of the two outputs, but not both at the same time.

Example: Switching a light on from two different locations (requires two SPDT switches and some extra wiring).

DPDT (Double-Pole Double-Throw): This switch is like two SPDT switches combined. It has two inputs and four outputs. It can connect each input to either of its two corresponding outputs simultaneously.

Example: Reversing the polarity of a DC motor. Visual Representations (Drawing are very helpful here in a classroom setting). Draw schematic diagrams for series, parallel, and combination circuits, and different types of switches. Use standard symbols for resistors, batteries, switches, and lamps. Guided Practice (With Solutions)

Question 1: A 6V battery is connected to two resistors in series: a 2 Ω resistor and a 4 Ω resistor. Calculate the total resistance, the total current, and the voltage drop across each resistor.