Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Differentiate between series and parallel circuits.
• Calculate total resistance in both circuit types.
• Describe the function of at least three types of switches.
• Design a simple circuit with series and parallel parts.
• Explain how complex circuits are used in everyday devices.

Lesson summary

This week, we delve deeper into electrical systems, focusing on more complex circuits and switches. Building on what you already know about simple circuits, we'll explore series and parallel circuits and different types of switches that control electricity in more sophisticated ways. Understanding these concepts is crucial because electrical systems are integral to our daily lives, from the lights in our homes during Eskom load shedding to the appliances we use every day. In South Africa, with the constant challenge of power outages, understanding how circuits work and how switches can be used to control electricity becomes even more important.

Lesson notes

Series Circuits: In a series circuit, components are connected one after another along a single path. This means that the current has only one route to flow. Think of it like a single lane road – all the cars (electrons) must follow the same path.

Key Characteristics: The current is the same throughout the entire circuit. This is because all electrons must flow through each component. The total resistance is the sum of individual resistances. `R_total = R_1 + R_2 + R_3 + ...` The total voltage is divided across each component. `V_total = V_1 + V_2 + V_3 + ...` If one component fails (e.g., a bulb burns out), the entire circuit is broken, and everything stops working.

Example: Imagine three light bulbs (R1, R2, and R3) connected in series to a 6V battery. If R1 = 2 ohms, R2 = 3 ohms, and R3 = 1 ohm: `R_total = 2 ohms + 3 ohms + 1 ohm = 6 ohms` To find the current (I) using Ohm's Law (V = IR, therefore I = V/R): `I = 6V / 6 ohms = 1 Ampere` The voltage across each bulb can be calculated using Ohm's Law: `V_1 = 1 Ampere 2 ohms = 2V` `V_2 = 1 Ampere 3 ohms = 3V` `V_3 = 1 Ampere 1 ohms = 1V` Notice that `2V + 3V + 1V = 6V` (the total voltage).

Why it works this way: Electrons flow from the negative terminal of the battery, through each resistor in turn, and back to the positive terminal. The resistors impede the flow of electrons, and the total impedance (resistance) is simply the sum of each resistor's individual impedance. The battery's voltage is the total energy delivered per electron. That energy gets used up as electrons travel through the resistors.

Parallel Circuits: In a parallel circuit, components are connected across each other, providing multiple paths for the current to flow. This is like a multi-lane highway - cars (electrons) have multiple routes to reach their destination.

Key Characteristics: The voltage is the same across each branch of the circuit. The total current is the sum of the currents in each branch. `I_total = I_1 + I_2 + I_3 + ...` The total resistance is less than the resistance of the smallest resistor. The formula to calculate total resistance is: `1/R_total = 1/R_1 + 1/R_2 + 1/R_3 + ...` If one component fails (e.g., a bulb burns out), the other components continue to work. This is because the current can still flow through the other paths.

Example: Imagine three light bulbs (R1, R2, and R3) connected in parallel to a 12V battery. If R1 = 4 ohms, R2 = 6 ohms, and R3 = 12 ohms: `1/R_total = 1/4 + 1/6 + 1/12 = 3/12 + 2/12 + 1/12 = 6/12 = 1/2` Therefore, `R_total = 2 ohms` To find the total current (I) using Ohm's Law (V = IR, therefore I = V/R): `I = 12V / 2 ohms = 6 Amperes` The current through each bulb can be calculated using Ohm's Law: `I_1 = 12V / 4 ohms = 3 Amperes` `I_2 = 12V / 6 ohms = 2 Amperes` `I_3 = 12V / 12 ohms = 1 Ampere` Notice that `3A + 2A + 1A = 6A` (the total current).

Why it works this way: Electrons flowing from the battery reach a junction where the circuit splits into branches. The electrons then choose which path to take depending on the resistance of each path. Paths with lower resistance will have more electrons flowing through them. Then at the next junction the electrons rejoin. The voltage across each path is the same because that potential difference is what drives the electrons through each path.

Switches: Switches are devices used to control the flow of electricity in a circuit. They essentially "open" or "close" the circuit.

Here are a few common types: SPST (Single Pole Single Throw): This is the simplest type of switch. It has one input terminal (pole) and one output terminal (throw). It's like a simple on/off switch. Imagine the light switch in your house – it either connects the circuit (on) or disconnects it (off).

SPDT (Single Pole Double Throw): This switch has one input terminal (pole) and two output terminals (throws). It can connect the input to either of the two outputs. A good example is a selector switch that chooses between two different settings on a device.

DPST (Double Pole Single Throw): This switch has two input terminals (poles) and two output terminals (throws). It simultaneously controls two separate circuits. This is used when you need to switch two things on or off at the same time with one switch.

DPDT (Double Pole Double Throw): This is the most versatile switch. It has two input terminals (poles) and four output terminals (throws). Each input can be connected to either of its two outputs.