Energy, Atomic Structure and Modern Materials

The physical sciences answer a single big question: what is the world made of, and what makes it move? Two threads run through any CSS-style discussion of physical science — the atom, which is the building block of matter, and energy, which is the currency that drives all physical change.

Atomic structure

Every atom has a tiny, dense nucleus containing protons (positively charged) and neutrons (neutral), surrounded by a cloud of electrons (negatively charged) in quantised orbitals.

Key historical milestones in our picture of the atom:

• John Dalton (1808) — atoms are indivisible building blocks of matter.
• J. J. Thomson (1897) — discovered the electron; proposed the "plum-pudding" model.
• Ernest Rutherford (1911) — gold-foil experiment showed the atom has a small dense nucleus.
• Niels Bohr (1913) — electrons orbit in fixed energy levels; only certain orbits are allowed.
• Erwin Schrödinger & Werner Heisenberg (1920s) — quantum mechanics: electrons occupy probability clouds (orbitals), not fixed paths.

Sources of energy

Energy comes in many forms — kinetic, potential, thermal, chemical, electrical, nuclear, radiant — but it always obeys the law of conservation: energy cannot be created or destroyed, only converted from one form to another.

Practical energy sources fall into two broad categories.

Non-renewable (fossil and nuclear)

• Coal, oil, natural gas — formed over millions of years from buried organic matter. High energy density but emit CO₂ on combustion.
• Nuclear fission — splitting heavy nuclei such as uranium-235 releases enormous energy per kilogram of fuel. No CO₂ emissions but produces long-lived radioactive waste.

Renewable

• Solar — photovoltaic cells convert sunlight directly to electricity; solar thermal heats fluids to drive turbines.
• Wind — kinetic energy of moving air turns turbine blades connected to generators.
• Hydroelectric — falling water turns turbines. Pakistan's Tarbela and Mangla dams are major examples.
• Geothermal — heat from inside the Earth, used for power and heating.
• Biomass and biofuels — plant material or its derivatives (ethanol, biodiesel) burned for energy.
• Tidal and wave — energy from ocean motion driven by lunar gravity and wind.

Weather and atmospheric hazards

The atmosphere is a thin shell of gases held by gravity: roughly 78% nitrogen, 21% oxygen, 0.93% argon, 0.04% carbon dioxide, plus traces. It is layered into the troposphere, stratosphere, mesosphere, thermosphere and exosphere.

Weather phenomena occur mostly in the troposphere (up to ~12 km). Major hazards include:

• Cyclones / typhoons / hurricanes — rotating storms over warm oceans, with sustained winds exceeding 119 km/h. Same phenomenon, different regional names.
• Tornadoes — narrow, violently rotating columns of air from severe thunderstorms.
• Floods — driven by intense rain, glacial melt or storm surges.
• Droughts — prolonged precipitation deficits leading to water and agricultural stress.
• Heatwaves — abnormally hot periods, increasingly frequent under climate change.

Modern materials

The 20th and 21st centuries have been shaped as much by new materials as by new theories. A few that frequently appear in CSS questions:

• Semiconductors (silicon, germanium) — the basis of all digital electronics; conduct partially and can be precisely doped.
• Polymers and plastics — long chains of repeating units; lightweight, mouldable, durable.
• Composites (fibre-reinforced plastics, carbon fibre) — combine the strengths of two materials.
• Superconductors — conduct electricity with zero resistance below a critical temperature.
• Nanomaterials (graphene, carbon nanotubes) — atomically thin sheets or tubes with extraordinary strength and conductivity. Graphene is a single layer of carbon atoms in a hexagonal lattice.
• Smart materials — change properties in response to stimuli (shape-memory alloys, piezoelectric crystals).