Chapter 13: Electronics and Modern Physics
Semiconductors and diodes
In a pure semiconductor (silicon, germanium) a few electrons are freed by thermal energy. Doping adds impurities: n-type (extra electrons, donors) or p-type (extra holes, acceptors). A p–n junction forms a depletion region and acts as a diode: it conducts when forward-biased (above about 0.7 V for silicon) and blocks when reverse-biased. Uses: rectification (half-wave and full-wave with a bridge), LEDs, photodiodes, solar cells, and Zener diodes for voltage regulation.
Transistors
A bipolar junction transistor has three terminals (base, collector, emitter) with and current gain . It acts as a switch (fully on or off, for logic and relays) or an amplifier (small base current controls a large collector current). Typical A/L analysis uses a biasing resistor with .
Operational amplifiers
An ideal op-amp has infinite gain and input resistance and zero output resistance. With negative feedback, the inputs are at the same potential (virtual earth). Inverting amplifier: (for and , gain ). Non-inverting: . Without feedback, an op-amp acts as a comparator. Logic gates (AND, OR, NOT, NAND, NOR, XOR) are covered in Chapter 19.
The photoelectric effect and quantum ideas
Light of frequency consists of photons with energy . In the photoelectric effect, an electron is emitted only if (the work function), and . The stopping potential satisfies . Because eV nm, a photon of wavelength 400 nm has energy eV; if eV the maximum kinetic energy is 1.1 eV. The effect cannot be explained by wave theory (no delay, a threshold frequency, and energy that depends on frequency, not intensity).
De Broglie wavelength: . For an electron accelerated through 100 V, and m, comparable with atomic spacings, which is why electrons diffract in crystals.
Atomic structure and spectra
In the Bohr model of hydrogen, energy levels are , and a photon emitted in a transition from to has energy eV. The line spectra of atoms are evidence of discrete energy levels. X-rays are produced when fast electrons strike a metal target; lasers use stimulated emission.
Nuclear physics
The nucleus has protons and neutrons (mass number ). Radioactive decay is random but obeys , with half-life . Example: 80 g of a substance with half-life 5 days leaves g after 15 days. Alpha particles (helium nuclei) are stopped by paper; beta (electrons or positrons) by thin metal; gamma (photons) are attenuated by thick lead.
Mass–energy equivalence: . The binding energy of a nucleus equals the energy equivalent of its mass defect. Fission (splitting a heavy nucleus) and fusion (joining light nuclei) release energy because the binding energy per nucleon is higher in the products. Uses: nuclear power, medicine (imaging, radiotherapy), and radiocarbon dating. Safety rests on three principles: time, distance, and shielding.
Common mistakes
Using the wrong sign in the op-amp inverting gain.
Mixing electron-volts and joules (1 eV J).
Treating radioactive decay as if each nucleus has a fixed lifetime; half-life is statistical.
Practice questions
Find the photon energy (in eV) for light of wavelength 620 nm. [2.0 eV]
A sample’s activity falls from 800 Bq to 100 Bq in 12 h. Find the half-life. [4 h]
An inverting op-amp has and with input 0.3 V. Find the output. [−2.7 V]
Part Four: Chemistry
Chemistry studies the composition, structure, properties, and changes of matter. The A/L course brings together physical chemistry (energy, equilibrium, rates), inorganic chemistry (the elements and their compounds), and organic chemistry (carbon compounds), together with laboratory and analytical skills. Standard references include Chang and Goldsby (2016), Atkins et al. (2018), Housecroft and Sharpe (2018), Clayden et al. (2012), and Harris and Lucy (2020). Chemical terms follow the IUPAC conventions (International Union of Pure and Applied Chemistry [IUPAC], n.d.). Chemistry is a third-subject choice alongside ICT in the Physical Science stream, so students who take ICT can use this part as an introduction to the subject.