Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Explain how a basic electric motor works.
• Describe how a transistor can be used as a switch.
• Construct and test a simple transistor circuit.
• Analyse gear and lever systems to find mechanical advantage.
• Identify different types of sensors and their uses.

Lesson summary

This week, we delve into more advanced mechanical and electrical systems, building upon the foundational knowledge you gained previously. Understanding these systems is crucial for anyone interested in how things work, from everyday appliances to complex industrial machinery. In South Africa, a strong understanding of these principles is essential for aspiring engineers, technicians, and even entrepreneurs looking to develop innovative solutions to local challenges, such as improving access to electricity in rural areas or designing more efficient farming equipment. These systems form the backbone of many South African industries, including mining, manufacturing, and agriculture.

Lesson notes

2.1 Electric Motors An electric motor converts electrical energy into mechanical energy. The fundamental principle behind its operation is the interaction between magnetic fields. A current-carrying conductor placed within a magnetic field experiences a force. The direction of this force can be determined using Fleming's Left-Hand Rule (although it's not explicitly stated by name in CAPS at this level, the concept should be understood).

Components: Stator: The stationary part of the motor, usually containing permanent magnets or electromagnets.

Rotor (Armature): The rotating part of the motor, which contains coils of wire.

Commutator: A rotating electrical switch that reverses the current flow in the rotor coils, ensuring continuous rotation.

Brushes: Conductors that make contact with the commutator to provide the current to the rotor.

Operation: Current flows through the rotor coils, creating a magnetic field. This magnetic field interacts with the magnetic field of the stator. The interaction creates a force that causes the rotor to rotate. The commutator reverses the current flow at the appropriate moment, keeping the rotor spinning in the same direction.

Example: Imagine building a simple DC motor using a battery, wire, magnets, and a paper clip. The current flowing through the wire (rotor) creates a magnetic field. This field interacts with the magnets (stator), causing the paper clip to spin. 2.2 Transistors as Switches A transistor is a semiconductor device that can amplify or switch electronic signals and electrical power. For our purposes, we'll focus on its use as a switch.

Types: NPN and PNP transistors are the two main types. We'll focus on NPN for simplicity.

Terminals: Base (B): Controls the flow of current between the collector and emitter.

Collector (C): The positive terminal (in most circuits we are using) from which current flows when the transistor is "on".

Emitter (E): The negative terminal that current flows through to ground when the transistor is "on".

Operation as a Switch: When a small current flows into the base (B) of an NPN transistor, it allows a larger current to flow from the collector (C) to the emitter (E). In this state, the transistor is "on" or "saturated." When no current flows into the base (B), no current flows from the collector (C) to the emitter (E). The transistor is "off" or "cut-off."

Example: Consider a simple circuit with a battery, an LED, a resistor, a transistor (NPN), and a push-button switch. The LED is connected to the collector of the transistor. When the push-button is pressed, a small current flows into the base of the transistor, turning it "on." This allows current to flow from the battery, through the LED, and through the transistor, causing the LED to light up. When the button is released, the base current stops, the transistor turns "off," and the LED turns off. Why use a transistor as a switch? Transistors allow a small current to control a larger current, which is useful for applications where a weak signal needs to control a powerful device (e.g., using a sensor to switch on a motor). 2.3 Mechanical Advantage Mechanical advantage (MA) is a measure of how much a mechanical system multiplies the force applied to it. It is calculated as the ratio of the output force (load) to the input force (effort). MA = Load / Effort Gears: Gears are toothed wheels that transmit rotational motion and torque. When two gears are meshed together, the smaller gear rotates faster than the larger gear, but the larger gear exerts more torque. Gear Ratio (GR) = Number of teeth on driven gear / Number of teeth on driving gear MA = GR (ideal, neglecting friction)

Levers: A lever is a rigid object that pivots around a fixed point called a fulcrum. Levers can be used to multiply force. MA = Distance from fulcrum to effort / Distance from fulcrum to load Example 1 (Gears): A gear system has two gears. The driving gear has 20 teeth, and the driven gear has 60 teeth.

Gear Ratio: 60 / 20 = 3 Mechanical Advantage: 3 This means the system multiplies the input force by 3, but the output speed is reduced by a factor of

3. Example 2 (Levers): A lever is used to lift a rock. The distance from the fulcrum to the effort (where you push) is 1.5 meters, and the distance from the fulcrum to the load (the rock) is 0.5 meters.

Mechanical Advantage: 1.5 / 0.5 = 3 This means you only need to apply one-third of the force required to lift the rock directly. 2.4 Sensors A sensor is a device that detects a physical quantity (e.g., light, temperature, pressure) and converts it into an electrical signal that can be used by a control system.

Types of Sensors: Light Sensor (Photoresistor): Its resistance changes with the amount of light falling on it. More light, lower resistance.

Temperature Sensor (Thermistor): Its resistance changes with temperature. Some thermistors have decreasing resistance with increasing temperature (NTC), while others increase resistance (PTC).