Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Differentiate between series and parallel circuits.
• Construct and analyze circuits with series and parallel parts.
• Identify and explain the function of different types of switches.
• Design simple circuits with multiple switches.
• Calculate resistance, voltage, and current using Ohm's Law.

Lesson summary

Electrical circuits are more than just simple pathways for electrons. This week, we delve into more complex circuits and switches, learning how to control electricity in sophisticated ways. Understanding these concepts is crucial in South Africa, where access to reliable electricity is essential for economic development, education, and overall quality of life. From understanding how traffic lights function to designing efficient lighting systems for homes and businesses, the knowledge you gain this week is highly relevant. Many learners may aspire to become electricians, technicians, or engineers, contributing to the country's infrastructure and technological advancement.

Lesson notes

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 traffic must flow through each point sequentially.

Current: The current (I) is the same throughout the entire series circuit. It's like the flow of water in a pipe; the amount of water entering one end must equal the amount exiting the other.

Voltage: The total voltage (V T ) of the power supply is divided among the components in the series circuit. This means the voltage drop across each component adds up to the total voltage. V T = V 1 + V 2 + V 3 + ...

Resistance: The total resistance (R T ) of a series circuit is the sum of the individual resistances. R T = R 1 + R 2 + R 3 + ...

Example 1 (Series Circuit): Imagine three light bulbs connected in series: R 1 = 5 ohms, R 2 = 10 ohms, and R 3 = 15 ohms, powered by a 12V battery.

Calculate the total resistance: R T = 5 ohms + 10 ohms + 15 ohms = 30 ohms Calculate the total current: Using Ohm's Law (V = IR), I = V/R = 12V / 30 ohms = 0.4 amps Calculate the voltage drop across each bulb: V 1 = I R 1 = 0.4 amps * 5 ohms = 2V V 2 = I R 2 = 0.4 amps * 10 ohms = 4V V 3 = I R 3 = 0.4 amps * 15 ohms = 6V Notice that V 1 + V 2 + V 3 = 2V + 4V + 6V = 12V (which equals the battery voltage). If one bulb burns out, the entire circuit breaks, and all the bulbs go off because the current path is interrupted.

Parallel Circuits: In a parallel circuit, components are connected along multiple paths. This is like having multiple lanes on a highway; traffic can choose different routes.

Current: The total current (I T ) from the power supply is divided among the different branches of the parallel circuit. I T = I 1 + I 2 + I 3 + ...

Voltage: The voltage (V) is the same across all components in parallel. The full voltage of the battery is applied to each path.

Resistance: The total resistance (R T ) of a parallel circuit is calculated using the following formula: 1/R T = 1/R 1 + 1/R 2 + 1/R 3 + ... Alternatively, for only two resistors in parallel, you can use: R T = (R 1 R 2 ) / (R 1 + R 2 )

Example 2 (Parallel Circuit): Three light bulbs are connected in parallel: R 1 = 20 ohms, R 2 = 30 ohms, and R 3 = 60 ohms, powered by a 12V battery.

Calculate the total resistance: 1/R T = 1/20 + 1/30 + 1/60 = 3/60 + 2/60 + 1/60 = 6/60 = 1/10 Therefore, R T = 10 ohms Calculate the total current: I T = V/R T = 12V / 10 ohms = 1.2 amps Calculate the current through each bulb: I 1 = V/R 1 = 12V / 20 ohms = 0.6 amps I 2 = V/R 2 = 12V / 30 ohms = 0.4 amps I 3 = V/R 3 = 12V / 60 ohms = 0.2 amps Notice that I 1 + I 2 + I 3 = 0.6 amps + 0.4 amps + 0.2 amps = 1.2 amps (which equals the total current). If one bulb burns out, the other bulbs will continue to shine because the current can still flow through the other paths. This is why houses are wired in parallel.

Series-Parallel Circuits: These circuits combine series and parallel connections. To analyze them, you simplify the circuit step-by-step, calculating the equivalent resistance of the parallel sections first, and then adding the series resistances.

Switches: Switches are components used to control the flow of current in a circuit. They can be used to open or close a circuit.

SPST (Single Pole Single Throw): The simplest type of switch. It has one input (pole) and one output (throw). It's either ON or OF

F. Think of a simple light switch in a home.

SPDT (Single Pole Double Throw): One input (pole) and two outputs (throws). It can connect the input to either of the two outputs, but not both at the same time. Like a selector switch.

DPST (Double Pole Single Throw): Two independent SPST switches controlled by a single mechanism. It opens or closes two separate circuits simultaneously.

DPDT (Double Pole Double Throw): Two independent SPDT switches controlled by a single mechanism. Allows for more complex switching functions and reversing polarity in DC circuits.

Example 3: Light controlled by two switches (corridor lighting): This can be achieved using two SPDT switches. The switches are wired in a way that changing the state of either switch (flipping it) will either turn the light on or off. This is common in hallways or stairwells, where you want to control a light from either end. Guided Practice (With Solutions)

Question 1: Two resistors are connected in series across a 9V battery. R 1 = 20 ohms and R 2 = 25 ohms. Calculate the total resistance, the total current, and the voltage drop across each resistor.

Solution: Total Resistance: R T = R 1 + R 2 = 20 ohms + 25 ohms = 45 ohms Total Current: I = V/R T = 9V / 45 ohms = 0.2 amps Voltage Drop across R 1 : V 1 = I R 1 = 0.2 amps 20 ohms = 4V Voltage Drop across R 2 : V 2 = I R 2 = 0.2 amps 25 ohms = 5V

Commentary: We applied the series circuit rules directly. First, we summed the resistances. Second, we calculated the total current using Ohm's Law. Finally, we used Ohm's Law again to find the voltage drop across each resistor.