Class 8 ICSE Physics Electricity Notes PDF

Electricity Class-8 Household Electricity Household electricity refers to the electrical energy supplied to homes for powering appliances, lighting, and other devices. This electricity is usually delivered as alternating current (AC), which f...

Electricity Class-8 Household Electricity Household electricity refers to the electrical energy supplied to homes for powering appliances, lighting, and other devices. This electricity is usually delivered as alternating current (AC), which flows back and forth in cycles. The standard voltage for household electricity varies by country but is typically around 220 to 240 volts. In homes, electricity is distributed through a network of wires, switches, and sockets. It enters through the main fuse box or circuit breaker, which protects the household circuits from overloads. The power is then divided into various circuits, supplying electricity to different parts of the house. Common electrical devices, such as lights, fans, and televisions, convert this electrical energy into other forms like light, heat, and sound. Safety devices like fuses and circuit breakers are essential to prevent electrical hazards, ensuring the safe use of household electricity. Types of Current 1. Direct Current (D.C.) Class 8 Physics ICSE | Electricity | Notes Current remains constant with time E.g. cell or battery 2. Alternating Current (A.C.) Magnitude and polarity changes with time E.g. Mains & Electric generator Sources of Electricity The electric cell and battery The mains (electricity supplied by local electricity board) The generator (or dynamo) The solar cells Electrical Energy and Power Consumed in a Circuit In an electrical circuit, electrical energy is the energy transferred by electric current from a power source to a device. The amount of electrical energy consumed depends on the voltage, current, and the time for which the device operates. It is measured in joules (J) or kilowatt-hours (kWh). Suppose current I is flowing through a resistance R. Current is the rate of flow of charges in a unit time. Unit – ampere (A) Class 8 Physics ICSE | Electricity | Notes Potential difference is the work done in moving a unit charge, so work needed to move a charge Q through a potential difference V is Power in a circuit refers to the rate at which electrical energy is consumed or converted into other forms of energy, such as heat or light. The formula for electrical power is: Power (P) = Voltage (V) × Current (I) 1W=1Vx1A 1 watt = 1 volt x 1 ampere Power is measured in watts (W). A device with higher power consumption uses more energy over time. For example, a 100-watt bulb consumes more power and energy compared to a 60-watt bulb when both are used for the same duration. Transmission of Power from the Power generating station to the consumer Electric power is transmitted from a power generating station to consumers through a series of steps involving high-voltage transmission lines and transformers. Here’s how the process works: Power Generation: Electricity is generated at power plants using various energy sources like coal, water (hydroelectricity), wind, or solar. The electricity generated is typically at a lower voltage. Step-up Transformers: To minimize energy loss during transmission, the voltage is increased using step-up transformers. High voltage reduces the current and, therefore, minimizes heat loss in the transmission wires. Class 8 Physics ICSE | Electricity | Notes Transmission Lines: The high-voltage electricity is then transmitted over long distances through transmission lines, which are often supported by tall towers called pylons. Step-down Transformers: As the power reaches closer to the consumers, stepdown transformers reduce the voltage to a safer level suitable for household or industrial use, usually around 220-240 volts for homes. Distribution: Finally, the electricity is distributed to homes and industries through local distribution lines and enters through the main fuse box or circuit breaker before being used by various appliances. Colour Coding of Wires In electrical systems, wires are color-coded to identify their specific function, ensuring safety and preventing confusion during installation or maintenance. The most commonly used colors in household wiring are: Live (or Phase) Wire: This wire carries the current from the power source to the appliance. It is typically red or brown. Neutral Wire: This wire completes the circuit by carrying the current back to the power source. It is usually black or blue. Earth (or Ground) Wire: This wire provides safety by connecting the appliance to the ground, preventing electric shocks. It is green or yellow-green striped. Class 8 Physics ICSE | Electricity | Notes Commercial Unit of Electrical Energy The commercial unit of electrical energy is known as the kilowatt-hour (kWh). It is commonly used by electricity providers to measure and bill consumers for their energy consumption. One kilowatt-hour represents the amount of energy consumed when a device with a power rating of one kilowatt (1 kW) operates for one hour. The energy consumed in kilowatt-hours can be calculated using the formula: Energy (kWh) = Power (kW) × Time (h) For example, if a 100-watt bulb is used for 10 hours, the energy consumed is: Energy = 0.1 kW × 10 h = 1 kWh The kilowatt-hour is used on electricity bills to show how much energy has been consumed over a specific period. Understanding this unit helps consumers monitor their energy usage and manage their electricity costs effectively. Electric meter An electric meter, also known as a utility meter or energy meter, is a device used to measure the amount of electrical energy consumed by a household or business. It plays a crucial role in tracking electricity usage for billing purposes. There are mainly two types of electric meters: Class 8 Physics ICSE | Electricity | Notes Analog Meters: These have dials that display the energy consumption in kilowatt-hours (kWh). The dials turn as electricity is used, and the readings are taken manually. Digital Meters: These provide a digital display of energy consumption, often showing additional information such as peak usage times and voltage levels. They can also send data to the utility company automatically. Working Principle: The electric meter measures the current flowing through the circuit and the voltage across the load. By multiplying these two values, it calculates the power consumed and keeps track of the total energy used over time. Importance: Electric meters are essential for utility companies to determine how much electricity to bill consumers. They help households and businesses monitor their energy consumption, encouraging energy conservation and efficient usage. Electric Fuse (A Safety Device) An electric fuse is a safety device used in electrical circuits to protect against excessive current flow, which can cause overheating and potentially lead to fires or damage to appliances. It is an essential component in ensuring electrical safety in homes and industries. Construction: A fuse consists of a thin wire or metal strip enclosed in a protective casing, often made of glass or ceramic. The wire is designed to melt when the current exceeds a certain level. Class 8 Physics ICSE | Electricity | Notes Working Principle: When the current flowing through the circuit exceeds the fuse's rated capacity (due to a short circuit or overload), the heat generated by the current causes the wire in the fuse to melt. This breaks the circuit, stopping the flow of electricity and protecting the rest of the electrical system. A fuse wire is a short and thin piece of wire of a material of low melting point. Made up of an alloy of lead and tin, having 50% of each metal. Importance: Fuses are critical for preventing electrical hazards. By disconnecting the circuit in the event of excessive current, they protect electrical devices, reduce the risk of fire, and ensure the safety of people using electrical appliances. Characteristics of a Fuse: Short wire with a low melting point Fuse wire is connected in series with live wire. Thickness of fuse wire depends on its current rating. Miniature Circuit Breaker A Miniature Circuit Breaker (MCB) is an automatic electrical switch designed to protect electrical circuits from overloads and short circuits. The primary function of an MCB is to interrupt the flow of electricity when it detects a fault condition, such as an overload (when too many devices are drawing power) or a short circuit (when there is an unintended path for the current). Class 8 Physics ICSE | Electricity | Notes Household Electrical Circuits Household electrical circuits are the networks of wires, switches, outlets, and appliances that deliver electricity to various parts of a home. Basic Components: Power Source: This is usually the main electrical supply coming from the utility company, providing alternating current (AC) at a standard voltage (typically 220-240 volts). Wires: Conductive wires carry electricity from the power source to different parts of the house. They are often color-coded for safety (e.g., live, neutral, and earth wires). Switches: These control the flow of electricity to appliances. When a switch is turned on, it completes the circuit, allowing current to flow. Outlets: These are sockets where electrical devices can be plugged in. They provide a point of access for electrical appliances. Types of Circuits: Series Circuits: In these circuits, components are connected one after another. If one component fails, the entire circuit stops working. Parallel Circuits: In these circuits, components are connected alongside each other. This allows each device to operate independently; if one device fails, the others continue to work. Power rating of appliances The power rating of an electrical appliance indicates the amount of electrical energy it consumes when operating. It is an essential factor for understanding Class 8 Physics ICSE | Electricity | Notes how much energy an appliance will use and its impact on electricity bills. Energy (in kWh) = Power (in KW) x time (in h) For example, if a 200-watt fan runs for 5 hours, the energy consumed would be: Energy = (200/1000) × 5 = 1 kWh Knowing the power rating helps consumers make informed decisions about energy use, allowing them to choose energy-efficient appliances and manage their electricity bills effectively. Additionally, it is crucial for ensuring that the electrical circuit can handle the total power load without overheating or causing failures. Hazards of Electricity House circuit overloaded may cause short circuit. Poor insulation can cause short circuit or excess flow of current. Electric shock to a person coming in contact with live wire. Precautions to taken Do not touch switch with wet hands. Connecting wires to the plugs, sockets and switches should be tightly joined. Appliances should be properly earthed. Do not repair appliance while in use. Ensure that MCB or fuse is connected to live wire. Proper insulation of wires. Class 8 Physics ICSE | Electricity | Notes Ordinary copper wire not to be used as fuse wire. Switching off the gadgets by taking out socket from plug. Static Electricity Static electricity refers to the accumulation of electric charge on the surface of objects. Unlike current electricity, which involves the flow of electric charge through conductors, static electricity occurs when there is an imbalance of charges within or on the surface of a material. This can happen due to processes conduction, or induction. Object can be charged by rubbing. Two charged objects having like charges repel each other while two charged objects having unlike charges always attract each other. Kinds of Electric Charges Positive charges Negative charges Like charges repel and unlike charges attract each other. Conservation of Charge Before rubbing, both the objects are uncharged. On rubbing both the objects get equal and opposite charge. Class 8 Physics ICSE | Electricity | Notes Conductors and Insulators Conductors Conductors are materials that allow electric current to flow through them easily. This is due to the presence of free electrons that can move throughout the material. Common examples of conductors include metals such as copper, aluminum, gold, and silver. These materials are widely used in electrical wiring and circuits because they efficiently transmit electricity. Conductors have low resistance to electric current, which means they can carry large amounts of current without significant energy loss. This makes them ideal for applications in power distribution and electrical devices. Insulators Insulators are materials that do not allow electric current to flow easily. They have very few free electrons, which restricts the movement of electric charge. Common insulators include rubber, glass, plastic, and wood. These materials are used to coat or encase electrical wires and components to prevent accidental electric shocks and short circuits. Insulators have high resistance to electric current, which means they can effectively prevent the flow of electricity. This property is crucial for ensuring safety in electrical systems. Methods of Charging a Conductor Charging a conductor involves creating an imbalance of electric charge on its surface. There are two methods to charge a conductor: by conduction, and by induction. Class 8 Physics ICSE | Electricity | Notes Charging by Conduction In this method, a charged object is brought into contact with a neutral conductor. The excess charge from the charged object spreads over the surface of the conductor, resulting in the conductor becoming charged. If a negatively charged rod touches a neutral metal sphere, electrons will flow from the rod to the sphere. As a result, the sphere becomes negatively charged. Charging by Induction Charging by induction involves bringing a charged object close to a neutral conductor without direct contact. The presence of the charged object causes a redistribution of charges within the conductor. When the conductor is grounded (connected to the Earth), some charges may leave or enter the conductor, resulting in a net charge. If a positively charged rod is brought near a neutral metal sphere, the electrons in the sphere will be attracted towards the rod, causing the side of the sphere closest to the rod to become negatively charged. If the sphere is then grounded, some electrons will leave the sphere, and when the grounding is removed, the sphere retains a net positive charge. Electroscope An electroscope is a simple device used to detect the presence and magnitude of electric charge on an object. It is an essential tool in the study of electrostatics and helps demonstrate fundamental concepts of electric charge. Class 8 Physics ICSE | Electricity | Notes Types of Electroscope: Pith ball electroscope A pith ball electroscope is a simple electrostatic device used to detect electric charge and demonstrate electrostatic principles. It consists of lightweight pith balls suspended from a thin thread or wire, making it sensitive to electric charge. When a charged object is brought close to the pith balls, it induces a charge on the balls. If the charged object is positively charged, it attracts electrons from the pith balls, making them positively charged. Class 8 Physics ICSE | Electricity | Notes If the object is negatively charged, it repels electrons, resulting in negatively charged pith balls. The induced charges cause the pith balls to either attract or repel each other, depending on the type of charge they acquire. If both pith balls acquire the same type of charge (either both positive or both negative), they will repel each other and move apart. If they acquire opposite charges, they will be attracted to each other. Gold leaf electroscope A gold leaf electroscope is an electrostatic device used to detect and measure electric charge. It is a more advanced version of the electroscope and provides a visual indication of the presence and magnitude of electric charge. When a charged object is brought close to the metal rod, it induces a charge in the gold leaves. If the object is positively charged, it attracts electrons from the leaves, making them positively charged. If the object is negatively charged, it repels electrons, resulting in negatively charged leaves. Class 8 Physics ICSE | Electricity | Notes As the gold leaves acquire the same type of charge (either both positive or both negative), they repel each other and spread apart. The greater the charge on the object, the farther the leaves will separate, providing a visual indication of the charge's magnitude. Atmospheric Electricity Atmospheric electricity refers to the electrical phenomena that occur in the Earth's atmosphere, including the presence and behavior of electric charges. It plays a significant role in various natural processes and is essential for understanding weather patterns and lightning. Atmospheric electricity is crucial for understanding various natural phenomena, including thunderstorms, lightning, and the Earth's climate system. Studying atmospheric electricity helps scientists predict weather patterns and understand the processes that affect the environment. Lightning Lightning is a powerful and spectacular natural phenomenon that occurs during thunderstorms. It is a sudden electrostatic discharge that creates a bright flash of light and is accompanied by thunder. Safety Measures to be taken: Do not stand under a tree or near a tall building. Class 8 Physics ICSE | Electricity | Notes Rush to a safe place such as low house/building. Sit inside a car/bus and keep windows/doors closed. Do not carry an umbrella. Take out plugs of TV, computer, etc. Do not use wired phone. Do not take bath during thunderstorms, avoid contact with running water/metallic tap. Do not stand near the windows or in balcony having metallic railings. Do not stand near the windows or in balcony having metallic railings. Lightning Conductor A device which is used to protect buildings from being damaged due to lightning. When lightning strikes a building, the lightning conductor provides a direct path for the electrical charge to travel safely to the ground. This prevents the electrical current from passing through the building's structure, which could cause fires, explosions, or electrical failures. Class 8 Physics ICSE | Electricity | Notes Benefits Protection from Damage: By directing the electrical energy of a lightning strike safely into the ground, a lightning conductor helps protect buildings from structural damage and fire hazards. Safety for Occupants: Lightning conductors minimize the risk of injury to people inside or near the building during a lightning storm, making them an essential safety feature. Insurance and Building Codes: Many insurance companies require lightning protection systems for high-risk structures, and building codes in some areas mandate their installatio. Class 8 Physics ICSE | Electricity | Revise Notes