China 九年级（初三） Physics
Chapters: 11
1. Ch.13 Internal energy
Heat and specific heat capacity · Kinetic molecular theory basics · Internal energy
- 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.
- 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.
2. Ch.14 Using internal energy
Energy transformation and conservation · Heat engines · Efficiency; heat value of fuels · Interdisciplinary: simple heat-engine model
- 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.
- 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.
- Build a Simple Heat-Engine Model – A heat engine turns heat into movement. In a simple air engine, a flame heats the air in a flask. The hot air expands and pushes a piston up, lifting a load. This is work. Cooling brings the piston back. Only a part of the heat becomes work; the rest escapes. Efficiency = work out ÷ heat in.
3. Ch.15 Current and circuits
Two kinds of charge · Current and circuits · Series and parallel circuits · Measuring current; current rules in series/parallel
- 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 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.
- 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.
4. Ch.16 Voltage and resistance
Voltage; rules in series/parallel · Resistance · Rheostats · Interdisciplinary: dimmable desk lamp
- 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.
- Factors Affecting Resistance and Resistivity – A wire's resistance grows with its length and falls as it gets thicker: R = ρL/A. The constant ρ (rho) is the resistivity of the material, in Ω m. It depends only on the material and its temperature. Metals have low resistivity, alloys higher, insulators very high.
- Build a Dimmable Desk Lamp – A dimmer is a variable resistor in series with a lamp. Sliding a contact along a graphite rod changes how much rod the current must pass. A longer path means more resistance, less current and a dimmer lamp. A thicker rod has less resistance. A separate switch is needed to turn the lamp fully off.
5. Ch.17 Ohm's law
Current vs voltage and resistance · Ohm's law · Measuring resistance · Ohm's law in series/parallel
- 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.
6. Ch.18 Electric power
Electrical energy and work · Electric power (rated/actual) · Interdisciplinary: saving electricity at home · Joule's law
- 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.
- Save Electricity at Home: Audit and Saving Plan – A home electricity audit lists every appliance with its power in watts and the hours it runs per day. Energy = power × time, shown in kilowatt-hours (units). The tallest energy bars show where to save first. Switch to efficient devices, run them for less time and cut standby use, then work out the new bill to see the saving.
- Heating Effect of Electric Current – When current flows through a resistor, electrical energy turns into heat. Joule's law: H = I²Rt, so heat grows with the square of current, with resistance and with time. Heaters, irons, toasters, bulbs and fuses all use this effect.
7. Ch.19 Electricity in daily life
Electrical safety · Household circuits · Interdisciplinary: design a home circuit
- Domestic Electric Circuits – A house gets 220 V, 50 Hz AC through a live and a neutral wire; appliances are joined in parallel on separate 5 A and 15 A circuits, and a fuse or MCB in the live wire plus an earth wire keep the house safe.
8. Ch.20 Electricity and magnetism
Magnets and fields; Earth's magnetism · Magnetic effect of current · Electromagnets and relays · Electric motor · Interdisciplinary: simple DC motor · Electromagnetic induction; generator
- 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.
- Build a Simple DC Motor – A DC motor turns electrical energy into spinning motion. A coil carrying current sits in a magnetic field and feels a force on its two sides, one up and one down, so it turns. A split ring (commutator) flips the current every half turn so the coil keeps turning the same way. More voltage, more turns or a stronger magnet make it spin faster.
9. Ch.21 Electromagnetic waves
Electromagnetic waves (wavelength, frequency, speed) · Broadcasting, TV and mobile communication · Satellite and optical-fibre communication
- The Electromagnetic Spectrum – Electromagnetic (EM) waves are transverse waves of changing electric and magnetic fields. They need no medium and all travel at 3 × 10⁸ m/s in a vacuum. In order of falling wavelength (rising frequency and energy) they are radio, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Speed = frequency × wavelength (c = fλ). Each type has uses; the high-energy ones (UV, X-rays, gamma) are ionising and can harm living cells.
- Radio, TV and Mobile Phones: How Signals Travel – Radio, TV and mobile phones send information with radio waves. Sound and pictures are turned into a small electric signal. The signal rides on a strong carrier wave from a tower. A receiver tunes to one frequency and turns the signal back into sound and pictures. Mobile networks split land into cells, each with its own tower.
- Satellites and Optical Fibre: Long-Distance Links – Radio waves cannot bend round the round Earth, so far links use a relay in space or a glass thread. A satellite receives microwaves from the ground (uplink) and sends them down again (downlink). An optical fibre guides pulses of light by total internal reflection and carries huge amounts of data with little loss.
10. Ch.22 Energy and sustainability
Energy use and challenges · New energy (nuclear, solar) · Interdisciplinary: energy-saving plan
- Minerals and Energy Resources – Minerals are natural substances found in rocks. They are metallic (ferrous or non-ferrous), non-metallic, or energy minerals. Energy comes from conventional sources like coal, petroleum, natural gas and electricity, and from non-conventional sources like sun, wind, nuclear, biogas, tides and heat from the Earth. Both minerals and energy must be conserved.
- Renewable Energy Sources – Non-renewable sources (coal, oil, gas, uranium) are used faster than nature makes them, and fossil fuels release CO₂. Renewable sources are refilled by nature: solar (photovoltaic cells and solar heating), wind (P ∝ v³), hydroelectric (P = η ρ g h Q), tidal, geothermal and biomass. Hydrogen fuel cells turn hydrogen and oxygen into electricity and water. Because sun and wind change, we need storage (batteries, pumped hydro) and a mix of sources. Every source has costs: land, materials, cost and effect on nature.
- Energy-Saving Plan: Audit Your Home and Save kWh – An energy-saving plan has five steps: list the appliances, measure their energy with E = P × t (in kWh), find the biggest user, change habits (use for fewer hours), and change devices (use more efficient ones such as LEDs). Then check the new total and the cost saved.
11. Structure of matter (standard 1.3)
Molecules, atoms, nuclear model; scales of the universe
- 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.