Lesson Notes By Weeks and Term v5 - Grade 8
Electrical systems: more complex circuits and switches – Week 9 focus
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Electrical systems: more complex circuits and switches – Week 9 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: 9
Theme: General lesson support
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This week, we're diving deeper into the fascinating world of electrical circuits, moving beyond simple series and parallel arrangements to explore more complex circuits and the switches that control them. Understanding these concepts is crucial because electrical systems are everywhere in our modern South African lives, from the lights in our homes during load shedding to the appliances we use every day and the complex control systems in cars and factories. Knowing how these systems work allows us to troubleshoot problems, understand energy consumption, and even design our own simple electronic devices.
2.1 Types of Switches: Switches are essential components in electrical circuits that allow us to control the flow of electricity. They act as gatekeepers, either allowing current to pass through (closed position) or blocking the current (open position). Different types of switches exist, each with specific functionalities. SPST (Single Pole, Single Throw): This is the simplest type of switch. It has one input (pole) and one output (throw). It's essentially an on/off switch. Think of a simple light switch in your room. When the switch is "on," the circuit is closed, and the light turns on. When the switch is "off," the circuit is open, and the light turns off.
Symbol:* A line with a break and a lever that can swing to close the gap. SPDT (Single Pole, Double Throw): This switch has one input (pole) and two outputs (throws). The input can be connected to either one output or the other. A common use is to switch between two different light bulbs. Imagine you have a desk lamp that can switch between a bright bulb and a dimmer one.
Symbol:* A line with a break and a lever that can swing to connect to one of two different lines. DPST (Double Pole, Single Throw): This switch is essentially two SPST switches controlled by a single mechanism. It has two inputs (poles) and two outputs (throws). Both circuits are switched on or off simultaneously. This is used for safety when switching mains AC electricity, as it cuts both the live and neutral wires at the same time.
Symbol:* Two SPST switch symbols joined together by a dotted line (representing the mechanical linkage). DPDT (Double Pole, Double Throw): This is the most versatile type of switch we will consider. It has two inputs (poles) and two outputs (throws) for each input. Each input can be switched to either of its two outputs. This type of switch can be used for complex switching arrangements, such as reversing the polarity of a DC motor.
Symbol:* Two SPDT switch symbols joined together by a dotted line. 2.2 Complex Circuit Design: Controlling a Light from Two Locations A classic example of a complex circuit uses two SPDT switches to control a light from two different locations (e.g., at the top and bottom of a staircase).
How it works: The power source is connected to the common terminal (pole) of one SPDT switch. The two "throw" terminals of the first switch are connected to the two "throw" terminals of the second switch. The common terminal (pole) of the second switch is connected to the light bulb. The other side of the light bulb is connected back to the power source. When both switches are in the same position (both up or both down), the circuit is complete, and the light is on. When either switch is flipped to the opposite position, the circuit is broken, and the light turns off. If the light is off, flipping either switch will turn it on.
Why this works: The switches are wired such that they provide alternative paths for the current to flow. By flipping either switch, we can either complete or break one of those paths.
Real-world example: This type of circuit is frequently used in houses and buildings across South Africa and internationally to allow you to turn a light on or off regardless of the position of the other switch. 2.3 Relays: Electrically Controlled Switches A relay is an electrically operated switch. Instead of physically flipping a switch, a small current is used to activate an electromagnet, which then pulls a contact to close or open a circuit.
Components of a Relay: Coil: A coil of wire that creates a magnetic field when current flows through it.
Armature: A movable arm that is attracted to the electromagnet when the coil is energized.
Contacts: A set of electrical contacts that are either normally open (NO) or normally closed (NC). When the relay is energized, the armature moves, changing the state of the contacts.
How it Works: A small current is applied to the relay coil. The coil generates a magnetic field. The magnetic field attracts the armature. The armature moves, closing the normally open (NO) contact or opening the normally closed (NC) contact.
Application: Relays are used to control high-power circuits with a low-power signal. For example, in a car, a small signal from the car's computer can activate a relay to turn on the headlights, which require a much higher current. Relays are also used in many industrial control systems. Consider the use of relays in controlling the large electric motors used in mining operations. A small control signal sent through a cable can activate the appropriate relays to turn the motor on and off safely. 2.4 Worked Examples Example 1: Controlling a buzzer with two SPST switches in series. Imagine you want to make a simple alarm system where a buzzer will sound only if both Switch A and Switch B are closed (turned on).
Circuit Diagram: Draw a circuit with a battery, Switch A, Switch B, and a buzzer, all connected in series.