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.
• Explain the function of different switches (SPST, SPDT, DPDT).
• Describe how a relay switch operates.

Lesson summary

Electrical circuits are all around us, from the lights in our homes to the appliances we use every day. Understanding how circuits work, especially more complex ones and the switches that control them, is crucial for anyone who wants to understand and interact with the technology in our modern world. In South Africa, a reliable electricity supply is essential for homes, businesses, and industries. Learning about circuits and switches will equip you with the foundational knowledge to troubleshoot simple electrical problems, appreciate the design of electrical devices, and potentially pursue careers in electrical engineering or related fields.

Lesson notes

2.1 Series and Parallel Circuits Series Circuit: In a series circuit, all components are connected along a single path. This means the current has only one route to flow. Imagine it as a single file of people walking down a narrow path. If one person stops, everyone behind them stops too.

Current (I): The current is the same at all points in a series circuit. I total = I 1 = I 2 = I 3 … Voltage (V): The voltage is divided among the components. The sum of the voltage drops across each component equals the total voltage supplied by the source. V total = V 1 + V 2 + V 3 … Resistance (R): The total resistance is the sum of the individual resistances. R total = R 1 + R 2 + R 3 …

Example: Imagine three resistors connected in series: R 1 = 10 ohms, R 2 = 20 ohms, and R 3 = 30 ohms. The total resistance is R total = 10 + 20 + 30 = 60 ohms. If a 12V battery is connected to this circuit, the current flowing through the circuit can be found using Ohm's Law (V = IR): 12V = I * 60 ohms, therefore I = 12/60 = 0.2 Amps.

Parallel Circuit: In a parallel circuit, components are connected along multiple paths. The current has several routes to flow. Think of it as a highway with multiple lanes. If one lane is blocked, traffic can still flow through the other lanes.

Current (I): The current is divided among the branches. The total current is the sum of the currents in each branch. I total = I 1 + I 2 + I 3 … Voltage (V): The voltage is the same across all components in parallel. V total = V 1 = V 2 = V 3 … Resistance (R): The total resistance is less than the smallest individual resistance. 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 … For two resistors in parallel, a shortcut formula can be used: R total = (R 1 * R 2 ) / (R 1 + R 2 )

Example: Imagine two resistors connected in parallel: R 1 = 10 ohms and R 2 = 20 ohms. The total resistance is R total = (10 * 20) / (10 + 20) = 200/30 = 6.67 ohms (approximately). If a 12V battery is connected to this circuit, the total current flowing from the battery can be calculated using Ohm's Law: I = V/R = 12/6.67 = 1.8 Amps (approximately). 2.2 Types of Switches Switches are used to control the flow of electricity in a circuit by either opening or closing the circuit.

Here are some common types: SPST (Single-Pole Single-Throw): This is the simplest type of switch. It has one input (pole) and one output (throw). It either connects the circuit (ON) or disconnects it (OFF). Think of a simple light switch in your home.

SPDT (Single-Pole Double-Throw): This switch has one input (pole) and two outputs (throws). It can connect the input to either one of the outputs, but not both at the same time. A good example is a three-way light switch arrangement.

DPST (Double-Pole Single-Throw): This switch has two inputs (poles) and two outputs (throws). It essentially acts like two SPST switches that are operated simultaneously. It's used to switch two different circuits on or off at the same time.

DPDT (Double-Pole Double-Throw): This switch has two inputs (poles) and four outputs (throws). It can connect each input to either one of its two corresponding outputs. It can be used for more complex switching arrangements, such as reversing the polarity of a motor. 2.3 Relays A relay is an electromechanical switch. It uses a small electrical current to control a larger electrical current. Inside a relay, a coil of wire is energized by a small current. This creates a magnetic field that attracts a metal arm (the armature). The armature moves, switching a set of contacts (like a regular switch) to connect or disconnect a separate, higher-power circuit. Why use a Relay?*: Relays are used to isolate a low-voltage control circuit from a high-voltage circuit. This is important for safety and to prevent damage to sensitive control components. For example, a low-voltage microcontroller can control a high-power motor using a relay. Relays are particularly important in South Africa where power fluctuations are common, helping to protect sensitive electronic equipment. Guided Practice (With Solutions)

Question 1: Three resistors are connected in series: R 1 = 5 ohms, R 2 = 15 ohms, and R 3 = 20 ohms. If a 9V battery is connected across the series combination, calculate the total resistance and the current flowing through the circuit.

Solution: Total Resistance (R total ): In a series circuit, R total = R 1 + R 2 + R 3 = 5 ohms + 15 ohms + 20 ohms = 40 ohms.

Current (I): Using Ohm's Law (V = IR), we can find the current: I = V/R = 9V / 40 ohms = 0.225 Amps.

Commentary: This question tests the understanding of series circuits and Ohm's Law. Remember the current is the same at all points in a series circuit.

Question 2: Two resistors are connected in parallel: R 1 = 30 ohms and R 2 = 60 ohms. If a 12V battery is connected across the parallel combination, calculate the total resistance and the total current flowing from the battery.