IGCSE Physics Block 2、4、5

这是一篇关于Physics的思维导图，主要内容包括：Atomic physics，Electricity and magnetism，Thermal Physics。

Physics

• Electricity and magnetism
  • Magnetism
    • Type of magnetic material
      • Hard; retains magnetism well, but difficult to magnetise in the firstplace; hard steel; permanent magnets, compass needles, loudspeaker magnets
      • Soft; easy to magnetise, but readily loses its magnetism; soft iron; cores for electromagnets (discussed later in the chapter), transformers and radio aerials
    • magnetised: when a magnetic material has been made magnetic
    • unmagnetised: when a magnetic material has not been made magnetic
    • induced magnetism: when a magnetic material is only magnetised when placed in a magnetic field
    • Magnetic field
      • electromagnet
  • Static electricity
    • Coulomb (C): the Sl unit for electric charge proton: a positively charged particle found in the atomic nucleus
    • Electron charge: the electric charge of a single electron = -1.6 × 10-19 C
    • Proton charge: the electric charge of a single proton = +1.6 × 10-19 C
    • Electrostatic charge: a property of an object that causes it to attract or repel other objects with charge
    • Positive charge: the type of electric charge carried in the nucleus of an atom
    • Negative charge: the type of electric charge carried by electrons
    • Static electricity: electric charge held by a charged insulator
      • Friction and charging
        • • An object which gains electrons becomes negative. • An object which loses electrons becomes positive.
      • Induced
  • Electrical quantities
    • Current
      • current: the rate at which electric charge passes a point in a circuit
      • cell: a device that provides an electromotive force (e.m.f.) in a circuit by means of a chemical reaction
      • battery: two or more electrical cells connected together in series
      • direct current(d.c.): electric current that lows in the same direction all the time
      • alternating current (a.c.): electric current that (periodically) changes direction
      • conductor
        • conductor: a material that allows an electric current to flow through it
        • insulator: a material that makes it very difficult for an electrical current to flow through it
        • charge: carried around a circuit by the current; negative charge is carried by electrons
      • current and electrons
    • Voltage
      • voltage: the energy transferred or work done per unit charge; it can be imagined as the push of a battery or power supply in a circuit
      • potential difference (p.d.): the work done by (a unit) charge passing through an electrical component; another name for the voltage between two points
      • voltmeter: a meter for measuring the p.d. (voltage) between two points
      • electromotive force (e.m.f.): the electrical work done by a source (cell, battery, etc.) in moving (a unit) charge around a circuit; the voltage across the terminals of a source
    • Resistance
      • resistance: a measure of how difficult it is for an electric current to flow through a device or a component in a circuit; it is the p.d. across a component divided by the current through it
      • Resistance and thickness
        • • The longer a wire, the greater its resistance. • The greater the diameter of a wire, the less its resistance.
    • Power
      • electrical power: power = current × p.d (P = VI)
    • Energy
  • Electrical Circuit
    • variable resistor : a resistor whose resistance can be changed, for example by turning a knob or moving a slider
    • light-dependent resistor (LDR): a device whose resistance decreases when light shines on it
    • NTC thermistor: a resistor whose resistance decreases with increasing temperature
    • relay: a switch controlled by an electromagnet
    • Diodes
    • Combinations of resistors
      • Resistors in series
        • • the combined resistance is equal to the sum of the resistances • the current is the same at all points around the circuit • the bigger the resistance, the bigger the p.d. across it.
      • Resistors in parallel
        • • the effective resistance is less than the resistance of either resistor • the current from the source is greater than the current through either resistor.
  • Electromagnetic force
    • right-hand grip rule: a rule which gives the direction of field lines around a straight wire when a current flows through it
    • the relay
      • armature: the moving part of an electromagnetic device such as a relay or bell
    • Motor
      • motor effect: when current flows in a wire in a magnetic field which is not parallel to the current, a force is exerted on the wire
      • basic requirements
        • • a magnetic field • a current flowing which cuts across the magnetic field lines.
      • reversed
        • • reversing the direction of the current • reversing the direction of the field of the permanent magnet by turning it round.
      • commutator: a device used to allow current to flow to and from the coil of a d.c. motor or generator
    • Fleming's left-hand rule: a rule that gives the relationship between the directions of force, field and current when a current flows across a magnetic field
  • Electromagnetic induction
    • induced e.m.f.: (or induced voltage) the e.m.f. created in a conductor when it cuts through magnetic field lines
    • electromagnetic induction: the production of an e.m.f across an electrical conductor when there is relative movement between the conductor and a magnetic field
    • Increasing the induced e.m.f.
      • • use a stronger magnet • move the wire or coil more quickly relative to the magnet • use a coil with more turns of wire. Each turn of wire will have an e.m.f. induced in it, and these all add together to give a bigger e.m.f.
    • a.c. generator
      • a.c. generator: a device such as a dynamo used to generate alternating current
      • slip rings: a device used to allow current to flow to and from the coil of an a.c. generator
    • Fleming's right-hand rule: a rule that gives the relationship between the directions of force, field and current when a current flows across a magnetic field
    • Direction of the induced e.m.f
      • Lenz's law: the direction of an induced current always opposes the change in the circuit or the magnetic field that produces it
    • transformers
      • • A primary coil: the incoming voltage V, is connected across this coil. • A secondary coil: this provides the voltage V, to the external circuit. • An iron core: this links the two coils.
      • • A step-up transformer increases the voltage. There are more turns on the secondary coil than on the primary coil. • A step-down transformer reduces the voltage. There are fewer turns on the secondary coil than on the primary coil.
• Atomic physics
  • the nuclear atom
    • ionisation: when a particle (atom or molecule) becomes electrically charged by losing or gaining electrons
    • plum pudding model : a disproved model of the atom which imagined it to consist of a positive 'pudding' with electrons dotted through it
    • alpha particle (a-particle): a particle made up of two protons and two neutrons; it is emitted by an atomic nucleus during radioactive decay
    • nucleus: small, dense, positively charged region at the centre of an atom
    • Rutherford experiment
      • • most of the mass of a n atom is concentrated in the central nucleus • the nucleus is positively charged • the nucleus is tiny compared to the atom; an atom is mainly empty space
    • Protons, neutrons and electrons
    • Nuclear fission and fusion
  • radioactivity
    • radioactive substance: a substance that decays by emitting radiation from its atomic nuclei
    • radiation: energy spreading out from a source carried by particles or waves
    • contaminated: when an object has acquired some unwanted radioactive substance
    • irradiated: when an object has been exposed to radiation
    • background radiation: the radiation from the environment to which we are exposed all the time
    • count rate: the number of decaying radioactive atoms detected each second (or minute, or hour)
    • Radioactive decay
      • radioactive decay: the emission of alpha, beta or gamma radiation from an unstable nucleus
      • random process: a process that happens at a random rate and in random directions; the timing and direction of the next emission cannot be predicted
      • beta particle (ß-particle): a high speed electron that is emitted by an atomic nucleus during radioactive decay
      • gamma ray (γ-ray): electromagnetic radiation emitted by an atomic nucleus during radioactive decay
      • Penetrating power
        • • a-particles are absorbed most easily. They can travel about 5cm in air before they are absorbed. They are absorbed by a thin sheet of paper. a-particles cannot penetrate skin. • ß-particles can travel fairly easily through air or paper. But they are absorbed by a few millimetres of metal such as aluminium. • γ-radiation is the most penetrating. It takes several centimetres of a dense metal like lead, or several metres of concrete, to absorb most of the gamma radiation.
      • • α-decay: two protons a n d two neutrons are emitted. The proton number decreases b y 2 and t h e nucleon number decreases by 4 . • ß-decay: a neutron splits into a proton and an electron; the electron is emitted. The proton number increases by 1 and the nucleon number remains the same. • y-decay : this is the emission of energy from the nucleus and it does not change the particles in the nucleus .
      • Deflecting
    • Safety precaution
    • Activity and half-life
      • activity: the rate at which nuclei decay in a sample of a radioactive substance
      • half-life: the average time taken for half the atoms in a sample of a radioactive material to decay
• Thermal Physics
  • The kinetic particle model of matter
    • States
      • Solid
        • rigid, fixed shape, fixed volume; cannot be squashed
        • takes the shape of its container
        • The particles are packed closely together, in a regular pattern. Notice that each particle is in close contact with all its neighbours.
        • Because the particles are so tightly packed, they cannot move around. However, they do move a bit. They can vibrate about a fixed position. The hotter the solid, the more they vibrate.
      • Liquid
        • not rigid, no fixed shape, fixed volume; cannot be squashed
        • takes the shape of its container
        • The particles are packed slightly less closely together than in a solid. The particles are arranged randomly rather than in a fixed pattern.
        • Because the particles are slightly less tightly packed than in a solid, they can move around. So the particles are both vibrating and moving from place to place. The hotter the liquid is, the faster its molecules move.
      • Gas
        • not rigid, no fixed shape, no fixed volume; can be squashed
        • expands to fill its container
        • The particles are widely separated from one another. They are no longer in contact, unless they collide with each other. In air, the average separation between the particles is about ten times their diameter.
        • The particles move freely about, bouncing off one another and off the walls of the container. In air at room temperature, the average speed of the particles is about 500 m/s and this increases with temperature
    • Absolute zero: the temperature at which particles have no kinetic energy
    • Brownian motion: the motion of small particles suspended in a liquid or gas, caused by molecular bombardment
    • Evaporation
      • Particles of the water are moving around, and some are moving faster than others. Some may be moving fast enough to escape from the surface of the water. The particles that are escaping from the water are the fastest-moving ones. They are the particles with the most energy. This means that the particles that remain are those with less energy, and so the water is colder.
      • Observation & Explaination
        • A liquid evaporates more rapidly when it is hotter.
          • At a higher temperature, more of the particles of the liquid are moving fast enough to escape from the surface.
        • A liquid evaporates more quickly when it is spread out, so that it has a greater surface area. A liquid evaporates more quickly when it is spread out, so that it has a greater surface area.
          • With a greater surface area, more of the particles are close to the surface, and so they can escape more easily.
        • A liquid evaporates more quickly when a draught blows across its surface.
          • A draught is moving air. When particles escape from the water, they are blown away so that they cannot fall back in to the water.
    • Celsius scale
      • T(K) = θ(°C) + 273
      • Fixed points
        • 0 °C: the melting point of pure ice at atmospheric pressure
        • 100 °C: the boiling point of pure water at atmospheric pressure
      • Thermal capacity
        • Metal objects heat up easily - their thermal capacities are low. Objects made of non-metals and liquids have higher thermal capacities.
      • Mercury-in-glass (and alcohol-in-glass)thermometers
        • Mercury expands at a steady rate as it is heated. This means that the marks on the scale are evenly spaced. We say that the scale is linear.
        • The thermometer can be made very sensitive, by making the tube up which the mercury expands very narrow. Then a small change in temperature will push the mercury a long way up the tube.
        • A mercury thermometer can have a wide range, because mercury is liquid between -39°C and +350°C. Some domestic ovens have mercury thermometers that read up to 250°C.
      • Thermistors
        • The resistance of a thermistor changes in a non-linear way, so that the intervals on a scale will not all be equal in size.
        • The range of such a thermometer will be narrow, because the resistance only changes significantly over a narrow range of temperatures.
      • Thermocouples
        • Thermocouples can be used to measure high temperatures (up to just below the melting point of the metal used).
        • Because they are small, they can heat up and cool down quickly, so they are useful for measuring rapidly varying temperatures.
  • Thermal properties of matter
    • Boyle' s Law
      • pV = constant
      • The volume of a fixed mass of gas is inversely proportional to its pressure, provided its temperature remains constant.
    • Thermal Properties of Matter
      • Specific heat capacity = energy required / (mass × temperature increase)
    • Specific latent heat
      • Specific latent heat of vaporisation - the energy per kilogram required to cause a substance to change state from liquid to gas at its boiling point.
      • Specific latent heat of fusion - the energy per kilogram required to cause a substance to change state from solid to liquid at its melting point.
      • energy required = mass × specific latent heat
        • E = mL
  • Thermal heat energy transfered
    • Conduction
      • Conduction in non-metals
        • At the hot end of the rod ，the atoms are vibrating much more than they are at the cold end. As the atoms vibrate, they collide with their neighbour. This process results in each atom sharing its energy with its neighbouring atoms.
    • Convection
      • Convection in liquid
        • Expansion means an increase in volume while mass stays constant - hence, density decreases. A less dense material is lighter, and is pushed upwards by the surrounding denser material. Convection results from the expansion of a fluid when it is heated.
    • Radiation
      • infrared radiation: electromagnetic radiation with a wavelength greater than that of visible light; sometimes known as thermal energy radiation
      • Characteristics
        • • is produced by warm or hot objects • is a form of electromagnetic radiation • travels through empty space (and through air) in the form of waves • travels in straight lines warms the object that absorbs it • is invisible to the naked eye • can be detected by nerve cells in the skin.
      • good absorber & good emitter
        • • Shiny or white surfaces are the best reflectors (the worst absorbers). • Matt black surfaces are the best absorbers (the worst reflectors). • Matt black surfaces are the best emitters.
        • • the temperature of the surface - a hotter surface radiates energy at a greater rate • the surface area - energy is radiated at a greater rate from a bigger surface area.
    • Comparing evaporation and boiling
      • • Boiling only happens at the boiling point of the substance. Evaporation occurs at all temperatures. • For a liquid to boil, it has to be heated-the kinetic energy of its particles must be increased. Evaporation happens when the most energetic particles escape, so evaporation takes energy from the substance • Boiling happens throughout the liquid. Evaporation only happens at the surface. • A boiling liquid bubbles. A liquid can evaporate without bubbles.