Lesson Notes By Weeks and Term v5 - Grade 8
Electrical systems: more complex circuits and switches – Week 8 focus
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Electrical systems: more complex circuits and switches – Week 8 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Technology
Class: Grade 8
Term: 2nd Term
Week: 8
Theme: General lesson support
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In Grade 7, you were introduced to simple electrical circuits. We learned about basic components like cells, bulbs, wires, and switches, and how they work together to create a complete circuit. Now, we're taking it a step further! We'll explore more complex circuits, specifically series and parallel circuits, and investigate different types of switches and their uses. Understanding these concepts is crucial because electricity is fundamental to modern life. From charging your phone to powering the lights in your home or school, electrical circuits are everywhere.
2.1 Series Circuits: In a series circuit, components are connected one after the other along a single path. Think of it like a single lane road – all the cars must follow the same route. This means that the current (the flow of electrical charge) is the same through every component in a series circuit.
However, the voltage (electrical potential difference) is divided amongst the components.
Current (I): The same throughout the circuit. I total = I 1 = I 2 = I 3 ...
Voltage (V): Divided amongst the components. V total = V 1 + V 2 + V 3 ...
Resistance (R): The total resistance is the sum of all individual resistances. R total = R 1 + R 2 + R 3 ...
Example 1: Imagine three light bulbs connected in series to a 9V battery. Each bulb has a resistance of 3 ohms (Ω). What is the total resistance, total current, and voltage drop across each bulb?
Total Resistance: R total = R 1 + R 2 + R 3 = 3Ω + 3Ω + 3Ω = 9Ω Total Current: Using Ohm's Law (V = IR), I total = V total / R total = 9V / 9Ω = 1A Voltage Drop across each bulb: Since the current is the same through each bulb (1A), and each bulb has a resistance of 3Ω, V 1 = V 2 = V 3 = I R = 1A * 3Ω = 3V. Notice that 3V + 3V + 3V = 9V (the total voltage). Why does the voltage drop across each bulb? The voltage represents the electrical potential energy. As the current flows through each bulb, some of this energy is converted into light and heat, causing a drop in the electrical potential. 2.2 Parallel Circuits: In a parallel circuit, components are connected across each other, creating multiple paths for the current to flow. Think of it like a multi-lane highway – cars can choose different routes. The voltage is the same across all components in a parallel circuit.
However, the current is divided amongst the branches.
Current (I): Divided amongst the branches. I total = I 1 + I 2 + I 3 ...
Voltage (V): The same across all branches. V total = V 1 = V 2 = V 3 ...
Resistance (R): The reciprocal of the total resistance is the sum of the reciprocals of the individual resistances. 1/R total = 1/R 1 + 1/R 2 + 1/R 3 ...
Example 2: Two resistors, 4Ω and 8Ω, are connected in parallel to a 12V battery. Calculate the total resistance, the current through each resistor, and the total current.
Total Resistance: 1/R total = 1/4Ω + 1/8Ω = 2/8Ω + 1/8Ω = 3/8Ω.
Therefore, R total = 8/3 Ω ≈ 2.67Ω Current through the 4Ω resistor: I 1 = V / R 1 = 12V / 4Ω = 3A Current through the 8Ω resistor: I 2 = V / R 2 = 12V / 8Ω = 1.5A Total Current: I total = I 1 + I 2 = 3A + 1.5A = 4.5A Why is the total resistance in a parallel circuit less than the smallest individual resistance? Adding more parallel paths allows the current to flow more easily, effectively reducing the overall resistance the battery "sees". 2.3 Switches: Switches are used to control the flow of current in a circuit. They act as a bridge, either completing the circuit (allowing current to flow) or breaking the circuit (stopping the flow). Different types of switches exist, each with different functionalities: SPST (Single Pole Single Throw): The simplest type of switch. It has one input terminal and one output terminal.
It can only be in one of two states: ON (closed) or OFF (open). Think of a simple light switch in your house.
SPDT (Single Pole Double Throw): This switch has one input terminal and two output terminals. It can connect the input to either one of the two outputs. Imagine a railway switch that diverts a train onto one of two tracks.
DPST (Double Pole Single Throw): This switch is essentially two SPST switches that are controlled by the same mechanism. It has two input terminals and two output terminals, and it switches both circuits simultaneously. This is useful for switching both the live and neutral wires of an appliance.
DPDT (Double Pole Double Throw): This is the most versatile type of switch. It has two input terminals and four output terminals. Each input can be connected to either of its two corresponding outputs. It can be used for reversing polarity or creating more complex control circuits.
Example 3: Imagine you want to design a system where one light can be turned on and off from two different locations (like at the top and bottom of a staircase). You would need to use two SPDT switches. One switch would be at the top of the stairs and the other at the bottom. This configuration allows you to control the light from either location, regardless of the position of the other switch. Guided Practice (With Solutions)
Question 1: Two light bulbs, one with a resistance of 6Ω and another with a resistance of 12Ω, are connected in series to a 18V battery. Calculate the total resistance of the circuit.
Solution: In a series circuit, the total resistance is the sum of the individual resistances. R total = R 1 + R 2 = 6Ω + 12Ω = 18Ω Question 2: In the circuit from Question 1, calculate the current flowing through the circuit.
Solution: Using Ohm's Law (V = IR), we can find the current.