Grade 8 General Science: Unit 6 and 7 Comprehensive Study Notes

Unit 6: Family of the Solar System and Its Formation

The solar system is defined as the Sun and all celestial bodies that revolve around it. It includes the Sun, eight planets, their moons, dwarf planets, and smaller bodies such as asteroids, comets, and meteors.
The Sun is a star located at the center of the solar system and is the closest star to Earth. It consists primarily of hydrogen gas (approximately $71\%$ ), helium gas (approximately $27\%$ ), and other gases ( $2\%$ ). The surface temperature of the Sun is approximately $5500\,^\circ{C}$ , while its center reaches about $15 \times 10^6\,^\circ{C}$ . The Sun is the most massive object in the system, accounting for $98\%$ (elsewhere noted as $99.8\%\text{ in formation studies}$ ) of the total mass of the solar system.
Planets are celestial bodies that revolve around the Sun in well-defined paths called orbits. In 2006, the International Astronomical Union (IAU) declared there to be eight planets and five recognized dwarf planets as of 2014.

Characteristics of the Eight Planets

Mercury: The closest planet to the Sun ( $57.9 \times 10^6\,km$ ). It is the smallest planet and has no satellites. It experiences extreme temperatures: $426\,^\circ{C}$ during the day and $-173\,^\circ{C}$ at night. One revolution takes $88 \text{ days}$ , and its rotation period is $59 \text{ days}$ .
Venus: The nearest planet to Earth, located $108.2 \times 10^6\,km$ from the Sun. It has a thick atmosphere of carbon dioxide, making it the hottest planet ( $464\,^\circ{C}$ ) due to the greenhouse effect. It rotates in the opposite direction (East to West) compared to most planets. One revolution takes $243 \text{ days}$ , and its rotation period is also $243 \text{ days}$ .
Earth: Our home planet, located $149.6 \times 10^6\,km$ from the Sun. It appears blue-green from space due to water and land. It has one moon and a revolution period of $365.25 \text{ days}$ . Its rotation period is $23.9345 \text{ hours}$ .
Mars: Known as the "Red Planet" due to its reddish color, located $227.9 \times 10^6\,km$ from the Sun. It is the second smallest planet and has two moons. It features the largest volcano in the solar system, Olympus Mons (three times taller than Mt. Everest), and the valley Valles Marineris. Revolution takes $687 \text{ days}$ , and rotation takes $24 \text{ hours } 37 \text{ min}$ .
Jupiter: The largest planet, located $778.3 \times 10^6\,km$ from the Sun. it has $79 \text{ moons}$ and faint rings. Its revolution takes $11.89 \text{ years}$ , and rotation is very fast at $9 \text{ hours } 50 \text{ min}$ .
Saturn: Located $1432 \times 10^6\,km$ from the Sun, it has the most moons at $82$ . It is the least dense planet (less dense than water) and is famous for its visible rings. Revolution takes $29.37 \text{ years}$ , and rotation takes $10 \text{ hours } 13 \text{ min}$ .
Uranus: The seventh planet, located $2871 \times 10^6\,km$ from the Sun. It has $27 \text{ moons}$ , a blue-green color, and a highly tilted rotational axis. It rotates from East to West. Revolution takes $84.099 \text{ years}$ , and rotation takes $17.24 \text{ hours}$ .
Neptune: The farthest planet, located $4498 \times 10^6\,km$ from the Sun. It is the coldest planet and the most massive of the outer planets. It has $14 \text{ moons}$ (recorded as $13$ in text tables). Revolution takes $165 \text{ years}$ , and rotation takes $16.1 \text{ hours}$ .

Classification of Planets

Terrestrial (Inner) Planets: Mercury, Venus, Earth, and Mars. Characteristics include low mass, high densities ( $4000 \text{ to } 5500\,kg/m^3$ ), solid rocky surfaces, and metallic cores.
Jovian (Outer) Planets: Jupiter, Saturn, Uranus, and Neptune. Characteristics include large diameters ( $4 \text{ to } 11 \text{ times Earth's size}$ ), high mass ( $14 \text{ to } 318 \text{ times Earth's mass}$ ), low densities ( $700 \text{ to } 1700\,kg/m^3$ ), compositions of hydrogen and helium without solid surfaces, and many moons.
Dwarf Planets: Bodies too small to be full planets but too large to be categorized as small bodies. The five accepted by the IAU are Pluto, Eris, Ceres, Haumea, and Makemake.

Other Members of the Solar System

Asteroids: Small objects revolving around the Sun, mostly in the Asteroid Belt between Mars and Jupiter. Inner asteroids are metallic; outer ones are rocky.
Comets: Icy and dusty objects with highly elliptical orbits. They feature a bright head and a tail that always points away from the Sun. Halley’s Comet appears every $76 \text{ years}$ (last seen $1986$ , next predicted $2061$ ).
Meteors and Meteorites: Meteors (shooting stars) are small objects that burn up in Earth's atmosphere due to friction. If they reach the surface, they are called meteorites.
Satellites: Celestial bodies revolving around planets. The Moon is Earth's natural satellite. Artificial satellites are man-made and used for weather, communication, and remote sensing.
Ethiopian Satellites: ETRSS-1 (launched Dec 20, 2019) used for remote sensing and natural resource management. ET-SMART-RSS (launched Dec 22, 2020) has higher resolution.

Formation of the Solar System

Scientific theory suggests the system formed $4.6 \times 10^9 \text{ years}$ ago from a rotating cloud of dust and gas (hydrogen and helium).
A supernova (star explosion) disturbed the cloud, causing contraction and increased density.
The gravity-led contraction formed the solar nebula. Most mass ( $99.8\%\text{ of the system}$ ) gathered at the center to form the Sun.
Planet formation occurred through the mutual gravity of leftover particles clumping together into larger objects.
Near the Sun, temperatures were too hot for elements to condense into solids, leading to rocky inner planets with iron cores. Farthest from the Sun, planets formed from lighter elements.

Earth’s Suitability for Life

Water: Approximately $71\%$ of the surface is covered in liquid water.
Distance from Sun: Positioned such that energy allows liquid water to exist without freezing or evaporating.
Temperature: Average of $15\,^\circ{C}$ , providing a stable environment.
Moon: Provides axial tilt, creating seasons. Without it, the rotation would result in $4\text{-hour days}$ .
Core and Magnetosphere: The magnetic field generated by the inner and outer core deflects solar wind, protecting the atmosphere.
Jupiter's Protection: Its massive gravity attracts asteroids away from Earth.
Atmosphere: Rich in Oxygen ( $O_2$ ), Nitrogen ( $N_2$ ), and Carbon Dioxide ( $CO_2$ ) for respiration and plant growth, plus an ozone layer to block UV rays.

Unit 7: Phenomena of Light

Light is a form of energy that enables vision. Luminous objects (the Sun, stars, bulbs) emit their own light, while non-luminous objects (the Moon, mirrors) are seen by reflection.
Reflection: The return of light into its original medium. Regular (specular) reflection occurs on smooth surfaces like mirrors, forming clear images. Diffuse reflection occurs on rough surfaces, forming blurry or no images.
Refraction: The bending of light rays as they travel from one medium (e.g., air) to another (e.g., water or glass).
Dispersion: The splitting of white light into its constituent colors (the spectrum). Sun light is white light consisting of Red, Orange, Yellow, Green, Blue, Indigo, and Violet (ROYGBIV). In a prism, red light bends the least and violet light bends the most.
Transmission of Light:
- Transparent: Allows light to pass completely (Glass, clean water, air).
- Translucent: Allows light to pass partially; objects appear blurry (Oiled paper, frosted glass, fog).
- Opaque: Completely blocks light (Metal, wood, stone).

Vision and Imaging

Vision requires both a light source and the eye. The human eye forms an inverted real image on the retina, which the brain corrects.
Plane Mirrors: Form images that are virtual, erect, the same size as the object, and laterally inverted (left appears right).
Medical Imaging: Tools like X-rays, CT scans, and MRI (Magnetic Resonance Imaging) use imaging technology to diagnose injuries.

Sound

Sound is energy produced by vibrating objects (back-and-forth motion). It requires a medium (solid, liquid, or gas) and cannot travel through a vacuum.
Propagation: Sound travels fastest in solids (closest molecules), slower in liquids, and slowest in gases.
Hearing: The eardrum senses vibrations and sends signals to the brain.
Sound Pollution: Excessive, unwanted noise. Loudness is measured in decibels ( $dB$ ). Normal conversation is $60\,dB$ . Exposure to $80\,dB$ continuously can cause hearing problems, and noise above $120\,dB$ (e.g., sirens) causes immediate pain.
Echo: A repeated sound caused by reflection from hard surfaces. Practical applications include hearing aids, soundboards (auditoriums), and stethoscopes.

Heat

Heat is a form of energy flowing from a higher temperature body to a lower temperature body. Transfer stops when temperatures equalize.
Conduction: Heat transfer in solids through successive particle collisions.
Convection: Heat transfer in fluids (liquids/gases) through the movement of the material itself (hotter liquid rises as it is less dense).
Radiation: Heat transfer through a vacuum or transparent medium without needing a material medium (e.g., Sun's heat reaching Earth).
Conductors vs. Insulators: Conductors (metals) allow heat transfer; insulators (wool, plastic, wood) reduce heat loss or gain.

Simple Circuits and Magnetism

Electric Circuit: A complete path for electricity. Components include a source (cell/battery), conductors (wires), a switch (breaking/starting flow), and a resistor (bulb). A closed circuit allows flow; an open circuit has a gap.
Magnetism: The property of attracting metals like iron, cobalt, and nickel. Natural magnets include Lodestone. Artificial magnets (bars, U-shapes) are man-made.
Magnetic Field: The region where magnetic influence is felt. Field lines point from North to South outside the magnet and never intersect. Strength is highest at the poles.
Uses of Magnets: Refrigerator stickers, HDD data storage, MRI machines, maglev trains (reaching speeds of $500\,km/h$ ), and speakers.