Lesson Notes By Weeks and Term v5 - Grade 9
Systems and control: more advanced mechanical and electrical systems – Week 3 focus
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Systems and control: more advanced mechanical and electrical systems – Week 3 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Technology
Class: Grade 9
Term: 2nd Term
Week: 3
Theme: General lesson support
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This week, we delve into more advanced mechanical and electrical systems. Building upon what we've already learned about simple machines, circuits, and control, we'll explore how these elements combine to create more complex and useful technologies. This is crucial because understanding these systems empowers you to not only appreciate the technology around you but also to potentially contribute to solving real-world problems in South Africa, from optimizing agricultural practices to developing innovative energy solutions. Imagine designing a smart irrigation system to conserve water in drought-stricken areas or improving the efficiency of public transportation using electric vehicles.
2.1 Mechanical Systems: Gear Ratios and Mechanical Advantage Let's consider a bicycle gearing system as a more advanced mechanical system. A bicycle uses gears to change the relationship between the speed at which you pedal (input) and the speed at which the wheels turn (output). This is about mechanical advantage and gear ratio.
Gear Ratio: The gear ratio is the ratio of the number of teeth on the driven gear (the gear being turned) to the number of teeth on the driving gear (the gear that's doing the turning). It can also be calculated as the ratio of the diameter of the driven gear to the diameter of the driving gear. `Gear Ratio = (Number of Teeth on Driven Gear) / (Number of Teeth on Driving Gear)` `Gear Ratio = (Diameter of Driven Gear) / (Diameter of Driving Gear)` Mechanical Advantage (MA): The mechanical advantage of a gear system is related to the gear ratio. A gear ratio greater than 1 means you have increased torque (turning force) but reduced speed. A gear ratio less than 1 means you have increased speed but reduced torque. If Gear Ratio > 1: Lower speed, higher torque (good for climbing hills) If Gear Ratio < 1: Higher speed, lower torque (good for flat roads)
Example 1: Bicycle Gear Ratio Imagine your bicycle has a front gear (driving gear) with 48 teeth and a rear gear (driven gear) with 12 teeth. Gear Ratio = (12 teeth) / (48 teeth) = 0.25 This means the rear wheel turns four times for every one turn of the pedals. This is a high-speed, low-torque setup, ideal for flat roads or going downhill.
Example 2: Bicycle Gear Ratio (Hill Climbing) Now imagine you shift to a smaller front gear with 24 teeth and a larger rear gear with 24 teeth. Gear Ratio = (24 teeth) / (24 teeth) = 1.0 This gear has equal number of teeth and will output the same torque and speed. 2.2 Electrical Systems: Series and Parallel Circuits An electrical circuit is a closed path that allows electrons to flow, creating an electric current. We'll focus on series and parallel circuits.
Series Circuit: In a series circuit, components are connected one after another along a single path.
Current (I): The current is the same through all components.
Voltage (V): The voltage is divided among the components. The sum of the voltage drops across each component equals the total voltage supplied.
Resistance (R): The total resistance is the sum of the individual resistances. `R_total = R1 + R2 + R3 + ...` Disadvantage: If one component fails, the entire circuit breaks. This is problematic; imagine if one street light failing in Soweto caused the whole street to go dark.
Parallel Circuit: In a parallel circuit, components are connected along multiple paths.
Current (I): The current is divided among the branches. The total current is the sum of the currents in each branch.
Voltage (V): The voltage is the same across all branches.
Resistance (R): The total resistance is calculated using the following formula: `1/R_total = 1/R1 + 1/R2 + 1/R3 + ...` or `R_total = 1/(1/R1 + 1/R2 + 1/R3 + ...)` Advantage: If one component fails, the other components continue to function. This is why houses are wired in parallel; if one light bulb blows, the others still work. This is crucial for reliability in South African homes and businesses.
Ohm's Law: This is fundamental to understanding circuits: `V = I R` (Voltage = Current Resistance).
Example 3: Series Circuit Calculation Consider a series circuit with a 12V battery and two resistors: R1 = 4 ohms and R2 = 2 ohms.
Calculate Total Resistance: `R_total = R1 + R2 = 4 ohms + 2 ohms = 6 ohms` Calculate Current: `I = V / R_total = 12V / 6 ohms = 2 Amps` Calculate Voltage Drop across R1: `V1 = I R1 = 2 Amps 4 ohms = 8V` Calculate Voltage Drop across R2: `V2 = I R2 = 2 Amps 2 ohms = 4V` Notice that `V1 + V2 = 8V + 4V = 12V`, which equals the total voltage supplied.
Example 4: Parallel Circuit Calculation Consider a parallel circuit with a 12V battery and two resistors: R1 = 4 ohms and R2 = 2 ohms. Calculate the reciprocal of each resistance: `1/R1 = 1/4` and `1/R2 = 1/2` Calculate the reciprocal of the total resistance: `1/R_total = 1/4 + 1/2 = 3/4` Calculate the total resistance: `R_total = 1/(3/4) = 4/3 = 1.33 ohms` Calculate the total current `I = V / R_total = 12V / (4/3 ohms) = 9 Amps` Calculate the current through R1: `I1 = V/R1 = 12V / 4 ohms = 3 Amps` Calculate the current through R2: `I2 = V/R2 = 12V / 2 ohms = 6 Amps` Notice that `I1 + I2 = 3 A + 6 A = 9 A`, which equals the total current. 2.3 Control Systems: Sensors and Actuators A control system uses sensors to detect changes in the environment and actuators to respond to those changes, creating a closed-loop system.
Sensor: A sensor detects a physical quantity (e.g., temperature, light, pressure) and converts it into an electrical signal.
Examples: Temperature sensor: Used in refrigerators to maintain a constant temperature. Important for preserving food in areas with unreliable electricity supply.