Particle Model of Matter - Density and States

Understanding density is simpler than you think - it's just how much stuff is packed into a space. The formula density = mass/volume $ρ = m/V$ tells you everything, with density measured in kg/m³.

Solid, liquid, and gas particles arrange themselves differently, which explains why ice floats on water. In solids, particles are tightly packed and vibrate in fixed positions. Liquids have particles that can slide past each other, whilst gas particles zoom around freely with loads of space between them.

When substances change state (melting, freezing, boiling, evaporating, condensing, sublimation), the mass stays exactly the same - only the arrangement changes. Internal energy is the total energy stored by all particles in a system, combining both their movement (kinetic) and position (potential) energy.

Top Tip: Remember that heating can either raise temperature OR cause a state change, but not both at the same time!

Energy Changes and Gas Behaviour

Specific heat capacity determines how much energy you need to heat something up - it's why water takes ages to boil compared to oil. The equation change in thermal energy = mass × specific heat capacity × temperature change helps you calculate exactly how much energy transfers occur.

Latent heat is the sneaky energy that changes states without changing temperature. Specific latent heat measures the energy needed to change 1kg of a substance's state. Fusion is solid to liquid, vaporisation is liquid to gas.

Gas particles are constantly moving randomly, and their temperature relates directly to their average kinetic energy. When you heat a gas in a sealed container, pressure increases because particles move faster and hit the walls harder.

For gas calculations, remember pressure × volume = constant $pV = constant$ when temperature stays the same. Squash the volume and pressure shoots up - think of a bike pump getting hot when you use it!

Quick Check: If you see a flat line on a heating graph, that's a state change happening with latent heat!

Atomic Structure and Models

Atoms are incredibly tiny - about 1 × 10⁻¹⁰ metres across - with an even tinier nucleus that's less than 1/10,000th the atom's radius. The plum pudding model imagined electrons dotted through positive "pudding", but Rutherford's alpha scattering experiment proved atoms have dense, positive nuclei.

Bohr then suggested electrons orbit in specific energy levels, and Chadwick discovered neutrons hiding in the nucleus. Atoms contain protons (positive), neutrons (neutral), and electrons (negative), with most mass concentrated in the nucleus.

Isotopes are atoms with the same number of protons but different numbers of neutrons. Radioactive decay happens randomly when unstable nuclei break down, releasing alpha particles (helium nuclei), beta particles $high-speed electrons$, or gamma rays (electromagnetic radiation).

Alpha particles are stopped by paper, beta particles by aluminium, and gamma rays need thick lead. Each type has different ionising power - alpha is strongest but least penetrating.

Memory Trick: Alpha = paper, Beta = aluminium, Gamma = lead - they get harder to stop as they get more penetrating!

Radioactive Decay and Nuclear Reactions

Half-life is the time it takes for half the radioactive nuclei to decay - it could be seconds or millions of years. You can calculate how much radiation remains after several half-lives by repeatedly halving the original amount.

Contamination means radioactive material gets inside you (dangerous), whilst irradiation means radiation hits you from outside (less dangerous). Background radiation is everywhere from rocks, cosmic rays, and human activities like nuclear weapons testing.

Nuclear fission splits large unstable nuclei like uranium, releasing energy and neutrons that can trigger chain reactions. Control the chain reaction and you get a power station; let it run wild and you get a nuclear explosion.

Nuclear fusion joins light nuclei to make heavier ones, converting some mass into enormous amounts of energy. This powers the Sun and creates all elements - you're literally made of star stuff!

Real World: Medical scans use gamma rays because they penetrate your body but don't stick around to cause long-term damage.

Basic Electricity and Circuits

Electric current is simply the flow of electrical charge, measured by charge flow = current × time $Q = It$. Current flows the same everywhere in a simple loop - imagine water flowing through a single pipe.

Ohm's Law is your best friend: potential difference = current × resistance $V = IR$. Higher resistance means less current flows for the same voltage - like a narrower pipe restricting water flow.

Ohmic conductors (like metal wires) have constant resistance, but filament lamps get hotter and more resistant as current increases. Diodes only let current flow one way, like electrical one-way valves.

Thermistors become less resistant when heated (useful for thermostats), whilst LDRs (Light Dependent Resistors) become less resistant in bright light (perfect for automatic street lights).

Circuit Building: Always draw your circuit diagrams first - it's like having a map before starting a journey!

Series and Parallel Circuits

Series circuits are like a single-lane road where current flows through each component in turn. Current stays the same throughout, but voltage gets shared between components. Total resistance equals the sum of individual resistances.

Parallel circuits are like multi-lane highways where current can split and take different paths. Each branch gets the full supply voltage, but current divides between branches. Adding parallel resistors actually decreases total resistance.

When you add resistors in series, you're making it harder for current to flow (more resistance). Add them in parallel and you're giving current more paths to take (less total resistance).

This explains why Christmas lights wired in series all go out when one bulb fails, but parallel-wired lights keep working even if several bulbs break.

Logic Check: Parallel circuits are safer for homes because if one appliance breaks, the others keep working!

Power and Mains Electricity

Power tells you how quickly energy transfers, calculated using Power = potential difference × current $P = VI$ or Power = current² × resistance $P = I²R$. Higher power means more energy transferred per second.

UK mains electricity supplies 230V at 50Hz alternating current. The live wire (brown) carries the dangerous voltage, neutral (blue) completes the circuit, and earth $green/yellow$ provides safety protection.

The live wire is always dangerous even when switches are off because it carries 230V relative to earth. Never provide connections between live and earth - that's how people get electrocuted!

Energy calculations use Energy = power × time $E = Pt$ or Energy = charge × voltage $E = QV$. Your electricity bill basically charges you for energy transferred over time.

Safety First: The earth wire only carries current when something goes wrong - it's your electrical safety net!

Energy Transfer and the National Grid

Electrical appliances transfer energy from the mains to useful forms like kinetic energy (motors), thermal energy (heaters), or light energy (lamps). The amount depends on power rating and usage time.

Work is done when charge flows through circuits. Calculate energy transfers using multiple equations: E = Pt, E = QV, or E = VIt depending on what information you have.

The National Grid efficiently transfers electrical power from power stations to homes using step-up transformers (increase voltage for transmission) and step-down transformers (reduce voltage for domestic use).

High voltage transmission reduces energy losses in cables - it's more efficient to send high voltage/low current than low voltage/high current over long distances.

Efficiency Insight: The National Grid uses high voltages not to be dangerous, but to waste less energy during transmission!

Static Electricity and Space Physics

Static electricity forms when insulators are rubbed together, transferring electrons and creating electric fields. Like charges repel, opposite charges attract - these are non-contact forces that work through electric fields.

Electric fields are strongest close to charged objects and weaken with distance. When fields become strong enough, they can cause sparking as charge jumps through the air.

Our solar system formed from a nebula (dust and gas cloud) that collapsed under gravity. The Sun ignited when gravitational compression triggered nuclear fusion reactions.

Star life cycles depend on mass. Sun-sized stars become red giants, then white dwarfs, then black dwarfs. Massive stars become red supergiants, explode as supernovae, then form neutron stars or black holes.

Cosmic Connection: Every element in your body except hydrogen was forged inside a star - you really are made of stardust!

Essential Equations and Exam Success

You must memorise these key equations: W = mg, W = Fd, s = vt, a = Δv/t, P = E/t, Q = It, V = IR, P = VI, P = I²R, E = Pt, and ρ = m/V.

The given equations include specific heat capacity (ΔE = mcΔθ), latent heat (E = mL), and gas laws (pV = constant). You'll need to select the right equation for each problem.

Practice identifying which equation fits each question type. Energy questions often need E = Pt, circuit problems typically use V = IR or P = VI, and heating problems require the thermal energy equations.

Remember units matter: power in watts, energy in joules, current in amps, voltage in volts, and resistance in ohms. Wrong units = wrong answers even if your calculation is perfect!

Exam Strategy: Write down the equation first, substitute values second, then calculate - this method prevents silly mistakes and earns method marks!