Atomic, nuclear and electronic physics
Models answer different questions
An contains a small positively charged and surrounding electrons. The contains protons and usually neutrons. Proton number identifies an element; mass number counts protons plus neutrons in one . Isotopes have the same proton number but different neutron numbers. An differs in electron count, not in the proton number that defines the element. Relative atomic mass is an isotopic weighted and need not be an integer. Keep atomic, nuclear and electrical descriptions separate: removing an outer electron does not convert one chemical element into another.
Scattering and the nuclear
In a thin-foil scattering experiment, most energetic alpha particles pass through with small deflections, while a small fraction undergo large deflections. This supports a tiny concentrated and an that is largely empty space on that scale. It does not the contains all the 's volume or that electrons are stationary planets in classical orbits. Historical models are useful when connected to the evidence they explain and their limits. A scattering observation concerns the distribution of mass and charge; an emission spectrum asks additional questions about allowed states.
levels and spectral lines
Electrons in a simple atomic model occupy discrete allowed energies. Absorption can promote an electron to a higher level when the photon matches a permitted difference. A downward transition can emit a photon with , where h is Planck's constant and f is . Using gives photon hc/λ. Shorter wavelengths correspond to higher photon energies. The electronvolt is an unit: 1 eV is approximately 1.60 × 10⁻¹⁹ J. It is not a unit of despite its name. Atomic line patterns help identify substances without requiring a copied picture of an .
Photoelectric emission
Light can eject electrons from a suitable surface when its exceeds a material-dependent threshold. The photon first supplies the work function φ; the maximum emitted kinetic is Kmax = hf − φ. Below threshold, increasing intensity alone does not produce the expected emission in the elementary one-photon model. Above threshold, increasing intensity generally increases emission rate rather than the maximum kinetic at fixed . Increasing raises that maximum . Stopping potential connects the maximum kinetic to eVs. State whether a question changes intensity, or material before applying a memorised conclusion.
Radiation comes from nuclear change
Unstable nuclei can transform spontaneously. Alpha radiation is a helium-4 , beta-minus radiation consists of electrons emitted in nuclear transformation, and gamma radiation is high- electromagnetic radiation. In beta-minus decay a neutron changes into a proton with an emitted electron and antineutrino; the electron was not simply stored as an orbital electron inside the . Alpha emission reduces mass number by four and proton number by two. Beta-minus leaves mass number unchanged and increases proton number by one. Gamma emission changes nuclear without changing those two numbers. Balance mass and charge numbers when writing a nuclear equation.
Penetration, ionisation and detection
Alpha particles are strongly ionising and have short penetration in ordinary matter; beta particles have intermediate behaviour, while gamma photons are more penetrating and indirectly ionise matter. Shielding depends on and circumstances, so introductory comparisons are not universal protection specifications. A Geiger–Müller instrument detects ionising events but does not alone identify every or measure every possible dose accurately. Background counts must be considered when analysing source measurements. Radiation is not detectable by smell or ordinary human senses. Use supervised educational measurements or provided data; no radioactive-source handling instructions are implied by a classroom calculation.
Half-life is statistical
Half-life is the time for the expected number of undecayed nuclei, or the activity of a fixed sample, to halve. Individual decay events are unpredictable, but large populations follow a statistical law. After n half-lives. Three half-lives leave one eighth, not one third. Activity is decay rate in becquerels, where one Bq is one decay per second. In the standard school model, nuclear decay is unaffected by ordinary changes of temperature, or chemical state. A measured count rate can include background, so subtract a known background before comparing source activity ratios.
Mass, binding and nuclear reactions
A bound has less mass than the separated nucleons used to assemble it. The mass difference corresponds to binding through. This does not violate mass- conservation: the full system includes released . Fission splits a heavy and can release neutrons that trigger further reactions. Fusion joins light nuclei under suitable conditions and can release for relevant reactions. Not every arbitrary split or joining releases useful ; compare binding energies. Nuclear reactions alter nuclei, while chemical reactions mostly rearrange electrons and bonds. Their scales and controlling mechanisms are therefore different.
Chain reactions and applications
A chain reaction continues when emitted neutrons cause further fissions. A reactor controls the process to produce heat for an conversion system. A moderator slows suitable neutrons, control materials absorb neutrons, and a coolant transports heat; these are distinct functions. Medical tracers, imaging and industrial thickness measurements use different radiation properties and require trained control. A short half-life can reduce long-term activity but may complicate and measurement. A suitable application answer explains why the radiation type, half-life and detection method fit the purpose rather than calling every useful for every task.
Semiconductors and rectification
Semiconductors have electrical behaviour that can be modified by temperature, light and controlled doping. In elementary descriptions, n-type material has electrons as majority carriers and p-type material has holes as majority carriers. Both are electrically neutral overall in their ordinary bulk state. A p–n junction diode conducts much more readily in forward bias than in reverse bias within its rated range. It can therefore rectify AC, producing a unidirectional output. A smoothing capacitor can reduce variation but does not make every output perfectly constant. Real diodes have drops, leakage and breakdown limits; the ideal one-way model is a simplification.
Use models with numerical checks
A transistor can control a larger circuit response through a smaller input and is used for switching or amplification. Identify the particular circuit before claiming its gain or logic state. Across modern-physics questions, begin with the relevant process: an electronic transition, photoelectric event, nuclear transformation or semiconductor response. Convert electronvolts and nanometres carefully. If 80 mg of a single radioactive has a half-life of six hours, the expected undecayed mass after eighteen hours is 10 mg. If a photon has 5 eV and a surface work function is 2 eV, maximum photoelectron kinetic is 3 eV. Explain what remains and what changes in each model.