Science test Revision sheet.docx

**🔴 Topic 1: Electricity and Magnetism** ---------------------------------------- **1. Electrical Circuits** -------------------------- An electrical circuit is a closed path that allows electric current to flow. It consists of a **power source** (such as a battery or mains electricity), **wires*...

**🔴 Topic 1: Electricity and Magnetism** ---------------------------------------- **1. Electrical Circuits** -------------------------- An electrical circuit is a closed path that allows electric current to flow. It consists of a **power source** (such as a battery or mains electricity), **wires** to carry the current, and **components** like bulbs, resistors, or switches. Circuits can be either **series** (where all components are in a single loop and the same current flows throughout) or **parallel** (where current splits between different paths, and each branch gets full voltage). To complete a circuit, all connections must be **closed**. If there's a **break** in the circuit, the flow of current stops. Electrical circuits are the foundation of all electrical devices, from household appliances to advanced computers. **2. Current** -------------- Electric current is the **flow of electric charge (electrons)** through a conductor, such as a wire. It is measured in **amperes (A)** using an **ammeter**, which is always placed in **series** within the circuit. Current can be **direct (DC)**, where electrons move in one direction (e.g., from a battery), or **alternating (AC)**, where electrons change direction periodically (e.g., mains electricity). Good **conductors** (e.g., copper, aluminum) allow current to flow easily, while **insulators** (e.g., rubber, plastic) resist it. The movement of charge in a circuit is driven by **potential difference** (voltage), which is provided by the power source. **3. Potential Difference and Resistance** ------------------------------------------ Potential difference, commonly called **voltage**, is the amount of **energy transferred per unit charge**. It is measured in **volts (V)** using a **voltmeter**, which is always placed **in parallel** across a component. Resistance () is a measure of how much a component opposes current flow. It is measured in **ohms (Ω)**. **Ohm's Law** states that voltage, current, and resistance are related by: where **V** is voltage, **I** is current, and **R** is resistance. High resistance in a circuit reduces the current. Factors affecting resistance include **wire length (longer = more resistance), thickness (thinner = more resistance), and temperature (higher = more resistance in most materials).** **4. Linking Voltage, Current & Resistance** -------------------------------------------- The relationship between **voltage, current, and resistance** is crucial for designing circuits. If the voltage is increased while resistance remains constant, the current increases. Conversely, increasing the resistance while keeping voltage constant will **reduce** the current. In **series circuits**, resistance **adds up**, causing lower current. In **parallel circuits**, the total resistance **decreases**, allowing more current to flow. Understanding this relationship is essential in designing electrical devices and managing power efficiently. **5. Static Electricity** ------------------------- Static electricity occurs when **electrons are transferred** between objects due to **friction**, leading to an imbalance of charge. For example, rubbing a balloon against hair transfers electrons, making the balloon negatively charged and your hair positively charged, causing them to attract. Objects with **opposite charges attract**, while **like charges repel**. This principle is used in applications such as **paint sprayers (charged paint particles stick to surfaces evenly) and photocopiers**. Lightning is a large-scale example, where static charge builds up in clouds and is suddenly discharged to the ground. **6. How Magnets Behave** ------------------------- Magnets create an invisible force field around them called a **magnetic field**. Every magnet has a **North and South pole**, and the rule for magnetic attraction is **opposites attract, like poles repel**. Some materials, such as **iron, nickel, and cobalt**, are naturally magnetic. Permanent magnets retain their magnetism, whereas **induced magnets** (such as paperclips) become magnetized when placed in a magnetic field but lose their magnetism when removed. Magnets are widely used in **generators, motors, and compasses**. **7. Magnetic Fields** ---------------------- A **magnetic field** is the area around a magnet where magnetic forces are exerted. Field lines show the direction of the force and always travel **from North to South**. The field is strongest at the poles, where lines are closest together. Magnetic fields are used in technologies such as **MRI scanners, electric motors, and maglev trains**. Scientists study magnetic fields to understand planetary dynamics and develop efficient energy solutions. **8. Earth\'s Magnetic Field** ------------------------------ The Earth acts like a giant bar magnet due to the movement of **molten iron in the core**, creating a **magnetic field** that extends into space. This field protects the Earth from harmful **solar radiation and charged particles from the Sun**. The **magnetic poles shift over time** due to changes in the Earth\'s core. This field plays a vital role in **navigation systems and animal migration**. **9. How a Compass Works** -------------------------- A compass is a small, freely rotating magnet that aligns with Earth\'s magnetic field. The needle inside always points toward **Earth's magnetic North Pole**, helping with **navigation**. Historically, compasses were essential for explorers and sailors. Today, they are still used in **navigation systems, hiking, and military operations**. **10. Electromagnets** ---------------------- An **electromagnet** is a magnet created when electric current flows through a coil of wire wrapped around an iron core. Unlike permanent magnets, electromagnets can be **turned on and off** by controlling the current. They are used in **MRI machines, electric bells, relays, and scrapyards for lifting heavy metal objects**. The strength of an electromagnet increases when **more coils are added, the current is increased, or a soft iron core is used**. **11. Magnetic Effect of a Current** ------------------------------------ A **current-carrying wire produces a magnetic field** around it. This effect is stronger when the wire is coiled into a **solenoid**. The **Right-Hand Grip Rule** helps determine the direction of the magnetic field: if you grip the wire with your right hand, your thumb points in the direction of current flow, and your fingers curl in the direction of the field lines. This principle is used in **electromagnets, relays, and transformers**. **12. Motor Effect** -------------------- When a current-carrying wire is placed in a magnetic field, it experiences a **force**. This is called the **Motor Effect** and is used in **electric motors and loudspeakers**. The **Fleming's Left-Hand Rule** determines the force's direction: - **Thumb = Force direction** - **First finger = Magnetic field direction** - **Second finger = Current direction** By reversing the current or changing the magnetic field, the direction of motion can be altered, which is the basis of **electric motors**. 🔴 **Topic 2: Chemical Reactions** ================================= - **Exothermic reactions** release energy, typically in the form of heat, light, or sound. Combustion reactions and neutralisation reactions are common examples. The temperature of the surroundings increases during exothermic reactions. - **Endothermic reactions** absorb energy from their surroundings, often resulting in a temperature drop. Photosynthesis and the dissolution of ammonium nitrate in water are examples of endothermic reactions. These reactions require a constant input of energy to proceed. **🔴 Topic 3: Forces and Motion** -------------------------------- - **Balanced forces** occur when two or more forces acting on an object are equal in size but opposite in direction, resulting in no change in the object's motion. For example, a book resting on a table experiences a balanced force between the gravitational force pulling it downward and the normal force from the table pushing it upward. - **Unbalanced forces** occur when the forces acting on an object are not equal, causing a change in the object\'s motion. An example is a car accelerating down the road, where the force from the engine is greater than the resistance forces like friction and air resistance. **4. Difference Between Mass and Weight** ----------------------------------------- - **Mass** is the measure of the amount of matter in an object and is measured in kilograms (kg). Mass does not change regardless of location. - **Weight** is the force exerted by gravity on an object and is measured in newtons (N). Weight depends on the gravitational field strength, so it varies depending on where you are in the universe. The weight of an object can be calculated using the formula:\ Weight (N) = Mass (kg) × Gravitational Field Strength (N/kg). - - **Stars**, which are massive celestial bodies that emit light and heat, like our Sun. - **Planets**, which orbit stars and may have moons or rings. - **Asteroids and comets**, which are smaller bodies that move through space. - **Nebulae**, which are large clouds of gas and dust where new stars are born. - **Galaxies**, which are large collections of stars, dust, gas, and dark matter, bound together by gravity.