South 중학교 3학년 Science 3
Chapters: 8
1. Chemical reactions: laws and energy
Physical vs chemical change · Chemical equations · Conservation of mass · Law of definite proportions · Gay-Lussac's law of combining volumes · Energy in reactions
- Chemical Reactions and Balancing Equations – In a chemical reaction atoms are not made or destroyed; they only change partners. So a chemical equation must have the same number of each kind of atom on both sides. We balance it by changing the numbers in front of formulas, never the formulas themselves.
- The Law of Conservation of Mass – In a chemical reaction, mass is not made and not destroyed. The total mass of the substances before the reaction equals the total mass after it. This is true because atoms are only rearranged: the same atoms, in the same numbers, are there before and after. If a gas leaves or joins from the air, the mass on a balance seems to change, but when we count every substance, the total stays the same. A related rule, the law of definite proportions, says a pure compound always has its elements in the same mass ratio (water is always 1 g hydrogen to 8 g oxygen).
- Gas Laws: How Pressure, Volume and Temperature Are Linked – Gas pressure comes from particles hitting the walls. For a fixed amount of gas: Boyle's law P₁V₁ = P₂V₂ (constant T); Charles's law V₁/T₁ = V₂/T₂ (constant P); pressure (Gay-Lussac's) law P₁/T₁ = P₂/T₂ (constant V). Temperature must be in kelvin. Combined: P₁V₁/T₁ = P₂V₂/T₂. Dalton's law: in a mixture, total pressure = sum of partial pressures, and each partial pressure = mole fraction × total pressure.
- Types of Chemical Reactions – Most reactions fit a few patterns. Combination: A + B → AB. Decomposition: AB → A + B. Displacement: A + BC → AC + B, where the more reactive A pushes out B. Double displacement: AB + CD → AD + CB, where partners swap; if an insoluble solid forms it is a precipitation reaction. Reactions that give out heat are exothermic; those that take in heat are endothermic.
2. Atmosphere and weather
Layers of the atmosphere and greenhouse effect · Humidity, clouds and precipitation · Air pressure and wind · Air masses, fronts, weather maps
- Atmospheric Circulation and Weather Systems – Air has weight, so it presses down: this is air pressure (about 1013 mb at sea level). Wind blows from high pressure to low pressure, pushed by the pressure gradient force, turned by the Coriolis force (right in the north, left in the south) and slowed by friction near the ground. Uneven heating makes pressure belts (equatorial low, subtropical highs, subpolar lows, polar highs) and three circulation cells, giving planetary winds: trade winds, westerlies and polar easterlies. Belts shift with the seasons, giving seasonal winds like the monsoon; local winds include land and sea breezes and mountain and valley winds. Big bodies of air with the same temperature and moisture are air masses; where two meet is a front. Cyclones are low-pressure storms: tropical cyclones form over warm seas; extratropical ones form along fronts. Thunderstorms and tornadoes are small but violent storms.
- Atmospheric Pressure – Air has weight. The whole column of air above a surface pushes down on it; this push per square metre is atmospheric pressure. At sea level it is about 101 kPa (101 300 Pa), the same as a 760 mm column of mercury or about 10 m of water. Higher up there is less air above you, so pressure falls. In still air, the upward push of pressure on a layer balances the layer's weight (hydrostatic balance). Air flows from high pressure to low pressure, which is wind.
3. Motion and energy
Uniform motion graphs · Free fall · Work, potential and kinetic energy
- Motion: Distance, Speed, Velocity, Acceleration and Graphs – An object is in motion when its position changes with time. Distance is the full path length (a scalar); displacement is the straight gap from start to finish with a direction (a vector). Speed = distance ÷ time; velocity = displacement ÷ time. Acceleration = change in velocity ÷ time. The slope of an s–t graph gives velocity, the slope of a v–t graph gives acceleration, and the area under a v–t graph gives the distance. For uniform acceleration: v = u + at, s = ut + ½at², v² = u² + 2as.
- Work, Energy and Power – Work is done when a force moves an object: W = F × s, measured in joules (J). Energy is the ability to do work. A moving body has kinetic energy ½mv²; a raised body has potential energy mgh. Energy is never made or destroyed, only changed from one form to another. Power is how fast work is done: P = W ÷ t, in watts. Simple machines like levers and pulleys let a small effort move a big load.
4. Stimulus and response
Sense organs · Neurons and the nervous system · Hormones
- Sense Organs – Sense organs let us pick up changes around us (stimuli). The eye senses light, the ear sound and balance, the nose smell, the tongue taste, and the skin touch, pressure, heat, cold and pain. Each has receptor cells that turn the stimulus into nerve signals. The signal goes along a sensory nerve to the brain or spinal cord, which sends an order to a muscle: the response.
- Nervous System, Reflex Action and the Human Brain – Nerve cells (neurons) carry messages as electric impulses and pass them to the next cell with chemicals across a gap called a synapse. Quick safety responses (reflexes) are handled by the spinal cord; thinking and control come from the brain, which is guarded by the skull and a fluid cushion.
- Hormones in Animals: Endocrine Glands and Feedback – Endocrine glands release chemical messengers called hormones straight into the blood. The blood carries them all over the body, but they act only on target cells. Hormones control growth, metabolism, emergency response, blood sugar and puberty, and their amount is kept right by feedback.
5. Reproduction and heredity
Cell division and growth · Chromosomes, mitosis, meiosis · Development from a fertilised egg · Mendel's laws · Human inheritance and pedigrees
- Cell Division: How One Cell Becomes Two (or Four) – All living things grow because their cells divide. Every cell comes from another cell (cell theory). Before dividing, a cell copies its chromosomes. Mitosis gives 2 identical cells with the same chromosome number, used for growth and repair. Meiosis divides twice and gives 4 cells with half the chromosomes, used to make sex cells.
- Development: From One Cell to a Whole Body – Development (ontogenesis) is the life story of one organism. The zygote divides by cleavage into a ball of cells, the blastula. In gastrulation cells move inwards and make three germ layers: ectoderm, mesoderm and endoderm. Cells then differentiate: every cell has the same genes, but each type switches on a different set. Signals between cells (induction) and master genes such as Hox genes decide where each part forms. After birth or hatching the organism grows, develops directly or by metamorphosis, ages and can sometimes regenerate.
- Heredity and Mendel's Laws – Heredity is the passing of traits from parents to children through genes. Each parent gives one copy of every gene. A dominant copy hides a recessive one, which is why Mendel saw 3:1 in a monohybrid cross and 9:3:3:1 in a dihybrid cross. Genes are pieces of DNA that make proteins, and proteins build the trait.
6. Energy conversion and conservation
Mechanical energy conservation · Electromagnetic induction · Electrical energy at home
- Work, Energy and Power – Work is done when a force moves an object: W = F × s, measured in joules (J). Energy is the ability to do work. A moving body has kinetic energy ½mv²; a raised body has potential energy mgh. Energy is never made or destroyed, only changed from one form to another. Power is how fast work is done: P = W ÷ t, in watts. Simple machines like levers and pulleys let a small effort move a big load.
- Force on a Current-Carrying Conductor, Motor and Induction – A wire carrying current inside a magnetic field feels a push. The push is biggest when the wire is at 90° to the field and zero when it is parallel. Fleming's left-hand rule gives its direction. A motor uses this push to spin a coil; a generator does the reverse and makes current by moving a coil or magnet.
- Electric Power and Electrical Energy – Electric power is the rate of using electrical energy: P = VI = I²R = V²/R, in watts. Energy used = power × time. At home energy is measured in kilowatt-hours: 1 kWh = 1 unit = 3.6 × 10⁶ J. Bill = units × rate.
7. Stars and universe
Stellar distance and temperature · Our galaxy · Expanding universe · Space exploration
- Stars: Colour, Brightness, Distance and the H-R Diagram – A star is a huge ball of hot gas that shines by nuclear fusion. Its colour shows its surface temperature: red stars are cool, blue stars are hot (Wien's law: λmax × T = 2.9 × 10⁻³ m K). Stars are sorted into spectral classes O B A F G K M, hottest to coolest. How bright a star looks (apparent magnitude) depends on its real power (luminosity) and its distance, because light weakens as 1/d². Nearby distances are found by parallax: d (pc) = 1/p (arcsec). Luminosity depends on size and temperature (L = 4πR²σT⁴). The H-R diagram plots luminosity against temperature and shows the main sequence, giants and white dwarfs.
- Stars and Galaxies: From Starlight to the Big Bang – Everything we know about stars comes from their light. Splitting starlight into a spectrum shows dark lines that tell us which elements a star has; the colour tells us its temperature (blue = hot, red = cool) and stars are sorted into classes O B A F G K M. The H-R diagram plots true brightness against temperature: most stars, including the Sun, lie on the main sequence, with giants top right and white dwarfs bottom left. A star is born in a nebula and its mass decides its life: a Sun-like star becomes a red giant and then a white dwarf; a star of more than about 8 solar masses becomes a supergiant, explodes as a supernova and leaves a neutron star or black hole. Stars gather in galaxies, which are spiral, elliptical or irregular. Distances are huge, so we use the astronomical unit, the light-year and the parsec. Galaxies are moving apart, faster when farther (Hubble's law), which together with the cosmic microwave background supports the Big Bang about 13.8 billion years ago.
- Cosmology: The Expanding Universe and the Big Bang – Cosmology is the study of the whole universe: its structure, history and future. Galaxies gather in groups, clusters and filaments around huge voids. Distant galaxies are moving away from us, faster the farther they are (Hubble's law, v = H₀d), because space itself is expanding. Running the expansion backwards leads to a hot, dense beginning about 13.8 billion years ago, the Big Bang. The main evidence is redshift, the cosmic microwave background and the amounts of hydrogen and helium. Most of the universe is dark matter and dark energy.
- Space Exploration – Space exploration means sending rockets, satellites, probes and people beyond Earth's air. Rockets rise by pushing gas down (action–reaction). At about 7.9 km/s sideways a craft keeps falling around Earth in orbit; at 11.2 km/s it escapes. Since 1957 space has been a place of rivalry and cooperation, and it gives us weather forecasts, navigation, communication and new science.
8. Science and civilisation
Science and human civilisation · Designing with science
- Science, Technology and Society: How Discoveries Change Our Lives – Science finds out how nature works; technology uses that knowledge to make tools and solve problems. Society shapes science too, by asking questions, paying for research and making rules. Big changes such as the printing press, the steam engine, electricity, vaccines and the internet changed how people work, live and think. Every technology has benefits and risks. To decide wisely we look at evidence, weigh benefits against risks, ask who gains and who loses, think about ethics and the future, and use safety rules.
- The Design Process: From Problem to Product – The design process is a loop of steps designers use to solve a real problem for real people: investigate the need, define it in a brief and a measurable specification, generate many ideas, build a prototype, then test and evaluate it against the specification. Whatever fails sends you back round the loop. This repeating is called iteration, and it is how almost every product, app, building and artwork is improved.