Russia 8 класс Physics (basic)

Chapters: 2

1. Thermal phenomena

Molecular kinetic theory · Models of states of matter · Temperature and internal energy · Heat transfer · Quantity of heat · Melting and solidification · Evaporation and boiling · Humidity · Fuel and heat engines · Energy conservation in thermal processes

The Particle Model of Matter – All matter is made of tiny particles that are always moving. In a solid they are close and only vibrate; in a liquid they are close but slide past each other; in a gas they are far apart and move fast in all directions. Heating gives particles more energy: either they move faster (temperature rises) or the forces between them are broken (the state changes while the temperature stays the same). Gas pressure comes from particles hitting the walls.
Internal Energy – Everything is made of tiny particles. They move (kinetic energy) and are held by forces between them (potential energy). The total of all these energies is the internal energy U. Heating a substance raises its internal energy: either the particles move faster (temperature rises, Q = mcΔT) or bonds are broken (state changes at constant temperature, Q = mL). Doing work on it, like rubbing or squashing, also raises U.
Thermal Expansion, Specific Heat, Calorimetry and Latent Heat – Temperature tells how hot a body is; heat is energy that flows because of a temperature difference. Most things expand when heated: ΔL = αLΔT, ΔA = βAΔT, ΔV = γVΔT, with β = 2α and γ = 3α. Water is an exception between 0 °C and 4 °C, where it shrinks on heating, so it is densest at 4 °C. The heat needed to warm a body is Q = mcΔT, where c is the specific heat; gases have two, Cp > Cv, with Cp − Cv = R per mole. In calorimetry, heat lost by hot bodies equals heat gained by cold ones. During melting or boiling the temperature stays constant and heat Q = mL goes into changing the state.
Specific Heat Capacity and Heat Balance – Different materials need different amounts of energy to warm up. The specific heat capacity c of a material is the energy needed to raise the temperature of 1 kg of it by 1 °C (or 1 K). Its unit is J/(kg·°C) or J/(kg·K). The energy needed to heat any amount is Q = m × c × ΔT. Water has a very high c (about 4200 J/(kg·°C)), so it heats and cools slowly. When a hot and a cold body touch and nothing is lost, the heat given by the hot body equals the heat taken by the cold body (heat balance). A calorimeter uses this idea to measure c.
Changes of State: Melting, Boiling, Evaporation and More – A substance can change between solid, liquid and gas when we heat it or cool it. Melting, boiling, evaporation and sublimation need heat. Freezing, condensation and deposition give heat out. While the state is changing, the temperature stays the same, because the heat is used to break (or is released by making) the pull between particles. This hidden heat is called latent heat. Evaporation happens at any temperature, only from the surface, and it cools things down. Changes of state are physical changes: no new substance forms and the mass stays the same.
Humidity: How Much Water Vapour Is in the Air – Air always carries some invisible water vapour. Absolute humidity is the mass of vapour in 1 m³ of air. Warm air can hold more vapour than cold air. Relative humidity is the vapour present divided by the most the air could hold, as a percent. When air cools to its dew point, it is full and the extra vapour turns into drops.
Heat Engines – A heat engine is a machine that turns heat into mechanical work. Fuel burns and releases heat Q = q × m, where q is the heat value (heat of combustion) of the fuel and m is its mass. Hot gas expands and pushes a piston or spins a turbine. A four-stroke engine repeats intake, compression, power and exhaust. Only part of the heat becomes useful work; the rest leaves as waste heat. Efficiency η = W ÷ Q × 100%. Engines also release CO₂ and other gases that harm the air and warm the planet.
Conservation of Energy – Energy cannot be created or destroyed. It only changes from one form to another, or moves from one object to another. The total energy of a closed system stays the same. When there is no friction, mechanical energy (potential + kinetic) stays constant: mgh + ½mv² = constant. With friction, some mechanical energy turns into heat (thermal energy), but the total is still the same. Efficiency = useful energy out ÷ total energy in × 100%.

2. Electric and magnetic phenomena

Electric charge · Electric field · Charge carriers and atom · Electric current · Circuit quantities and Ohm's law · Series and parallel connection · Work and power of current · Magnets and magnetic field · Magnetic field of current · Electric motor · Electromagnetic induction

Static Electricity – Rubbing two different materials moves electrons from one to the other. The one that gains electrons becomes negative; the one that loses them becomes positive. Charge is never made or destroyed, only moved. Like charges repel and unlike charges attract. A charged object can pull a neutral one by induction. Earthing lets extra charge flow safely away.
Electric Field and Field Strength – A charge changes the space around it so that any other charge placed there feels a force. This region is the electric field. Its strength at a point is the force on a small positive test charge divided by that charge: E = F/q, measured in N/C (same as V/m). For a point charge, E = kQ/r², with k = 9 × 10⁹ N m²/C². E is a vector: it points away from + charges and towards − charges. Field lines show its direction and, by how close they are, its strength. Fields from several charges add as vectors (superposition).
Structure of the Atom – An atom has a tiny, heavy, positive nucleus made of protons and neutrons. Electrons move around it in fixed shells K, L, M, N. The number of protons (Z) tells the element; protons + neutrons give the mass number (A). Outer electrons decide valency. Isotopes share Z; isobars share A.
Electric Current, Potential Difference and Electric Circuits – Electric current is the rate of flow of charge, I = Q/t, measured in amperes with an ammeter joined in series. Potential difference is the work done to move a unit charge between two points, V = W/Q, measured in volts with a voltmeter joined in parallel. Charge flows only in a closed circuit.
Ohm's Law – At constant temperature, the current through a conductor is directly proportional to the potential difference across it: V = I × R. R is the resistance, measured in ohms (Ω); 1 Ω = 1 V/1 A. The V–I graph of an ohmic conductor is a straight line through the origin.
Resistors in Series and Parallel – In series, resistors form one path: the same current flows through each, voltages add up and R_s = R₁ + R₂ + R₃. In parallel, each resistor gets its own branch: the voltage across each is the same, currents add up and 1/R_p = 1/R₁ + 1/R₂ + 1/R₃, so R_p is smaller than the smallest resistor.
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.
Magnetic Effect of Electric Current – A wire carrying electric current makes a magnetic field around itself. The field forms circles around a straight wire, becomes nearly straight at the centre of a loop, and is uniform inside a solenoid, which then acts like a bar magnet.
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.
Electromagnetic Induction: Making Electricity with Magnets – When the magnetic field through a coil changes, a voltage (potential difference) is made across the coil. This is electromagnetic induction. If the coil is part of a closed circuit, a current flows. A faster change, more turns or a stronger magnet give a bigger voltage. Generators, microphones and transformers all use this idea, and transformers let the power grid send electricity far with little waste.