Energy is stored in different ways: kinetic, gravitational potential, elastic potential, thermal (internal), chemical, magnetic, electrostatic and nuclear. Energy is never created or destroyed, only transferred between stores or by heating, working (mechanically, electrically) or by radiation (light, sound).
The total energy before and after a transfer is always the same. In real systems some energy is always 'wasted', usually dissipated as heat to the surroundings, spreading out and becoming less useful. This is why no machine is 100% efficient.
Lubrication reduces energy lost to friction between moving parts. Insulation (like thicker walls, double glazing, loft insulation) reduces the rate of energy transfer by heating between objects and their surroundings. Streamlining reduces energy lost to air resistance.
Materials with higher specific heat capacity need more energy to raise their temperature by 1 degree C. In building insulation questions, remember thicker insulation and lower thermal conductivity both reduce the rate of energy transfer.
Renewable resources (wind, solar, hydro, geothermal, tidal, wave, biofuel) will not run out; non-renewable resources (coal, oil, gas, nuclear fuel) will. Exam questions often ask you to evaluate resources using reliability, cost, environmental impact and carbon emissions - always give a balanced answer using specifics from the question, not generic statements.
Current (I) is the flow of charge, measured in amps (A), using an ammeter connected in series.
Potential difference (V) is the energy transferred per unit charge, measured in volts (V), using a voltmeter connected in parallel across a component.
Resistance (R) opposes current flow, measured in ohms (Ω).
The key equation is V = I x R, rearranged as I = V / R or R = V / I.
Charge equation: Q = I x t, where Q is charge in coulombs, I is current in amps, t is time in seconds.
Energy transferred: E = Q x V and also E = I x V x t.
In series circuits: current is the same everywhere, voltage splits across components, and resistances add up (R total = R1 + R2).
In parallel circuits: voltage is the same across each branch, current splits between branches, and total resistance is less than the smallest individual resistance.
A common mistake is forgetting that adding a resistor in parallel DECREASES total resistance, not increases it.
A resistor at constant temperature gives a straight line I-V graph through the origin (obeys Ohm's law).
A filament lamp curves as it heats up, resistance increases with temperature, so the graph flattens.
A diode only allows current one way; almost zero current in reverse, then a sharp rise once forward voltage exceeds about 0.6-0.7V (exact value not required, just the shape).
Thermistors: resistance DECREASES as temperature increases.
LDRs (light dependent resistors): resistance DECREASES as light intensity increases.
Power equations: P = V x I, P = I^2 x R, P = V^2 / R.
UK mains supply is 230V, alternating current (AC), at 50Hz.
A three-pin plug: live wire (brown) carries alternating potential difference, neutral (blue) completes the circuit near 0V, earth (green and yellow) is a safety wire that stops the case becoming live.
Fuses melt and break the circuit if current is too high, protecting the wiring and reducing fire risk.
Static electricity builds up when insulating materials are rubbed together, transferring electrons; like charges repel, opposite charges attract.
Mixing up series and parallel voltmeter/ammeter placement loses easy marks.
Forgetting units or not rearranging V = IR correctly under exam pressure.
Saying resistance 'increases' for LDRs and thermistors when it should decrease.
Not showing working in calculation questions, even simple substitution steps score marks.
Solids, liquids and gases are all made of the same particles arranged differently. In a solid, particles are close together in a fixed, regular pattern, held by strong forces, so they only vibrate about fixed positions. In a liquid, particles are close together but arranged randomly and can move past each other, so liquids flow but keep their volume. In a gas, particles are far apart, move randomly at high speed, and have almost no forces between them, so gases spread out to fill their container.
Density tells you how much mass is packed into a given volume. The equation is density = mass / volume, with units kg/m3 (or g/cm3). Solids are usually densest, gases least dense, because of how tightly the particles are packed. To find density of a regular solid, measure mass with a balance and volume with a ruler (length x width x height). For an irregular solid, use displacement: lower it into a measuring cylinder of water and read the volume of water displaced. For liquids, weigh an empty measuring cylinder, add liquid, weigh again, and read the volume directly.
Melting (solid to liquid), freezing (liquid to solid), boiling/evaporating (liquid to gas), condensing (gas to liquid), and sublimating (solid straight to gas, e.g. dry ice) are all physical changes. The mass is conserved: no atoms are created or destroyed, so total mass before equals total mass after. These changes are reversible, unlike chemical changes.
Internal energy is the total kinetic energy and potential energy of all the particles in a system. Heating a system increases internal energy either by raising temperature or by changing state, but never both at once. Specific heat capacity is the energy needed to raise 1 kg of a substance by 1 degree C. The equation is change in thermal energy = mass x specific heat capacity x change in temperature (E = mcθ), with energy in joules, mass in kg, specific heat capacity in J/kg°C.
Specific latent heat is the energy needed to change the state of 1 kg of a substance without changing its temperature. The equation is energy = mass x specific latent heat (E = mL). Latent heat of fusion applies to melting/freezing; latent heat of vaporisation applies to boiling/condensing. On a heating graph, flat sections show a change of state (energy goes into breaking bonds, not raising temperature), while sloped sections show temperature rising within one state.
Gas particles move randomly and collide with the walls of their container, creating pressure. Increasing temperature increases the average speed of particles, so they hit the walls harder and more often, increasing pressure (if volume is fixed). Common mistake: students often think particles expand when heated — actually the particles themselves stay the same size; they just move faster and spread further apart.
An atom has a tiny, dense, positive nucleus (protons and neutrons) surrounded by electrons in shells at relative distance. Radius of an atom is about 1 x 10^-10 m, and the nucleus is about 1/10,000 the size of the atom, around 1 x 10^-14 m. Almost all the mass is in the nucleus.
The plum pudding model (positive ball with electrons embedded) was replaced after Rutherford's alpha scattering experiment. Most alpha particles went straight through gold foil, some deflected, and a very few bounced straight back. This showed the atom is mostly empty space, with a small charged nucleus containing most of the mass. Niels Bohr then proposed electrons orbit at fixed distances (energy levels/shells), later refined further with the discovery of the neutron by Chadwick.
Some nuclei are unstable and decay randomly, emitting radiation. You cannot predict which nucleus decays next or when, only the probability over time. This is why decay is described as random and spontaneous, not affected by temperature, pressure or chemical state.
Half-life is the time for half the radioactive nuclei in a sample to decay, or for the activity to halve. Activity is measured in becquerels (Bq), where 1 Bq = 1 decay per second. Use repeated halving to solve problems: after n half-lives, the remaining fraction is (1/2)^n.
Speed is distance divided by time (m/s). Velocity is speed in a given direction — a vector, so it can be negative.
Acceleration: a = (v - u) / t, where v is final velocity, u is initial velocity.
The key equation for uniform acceleration: v^2 = u^2 + 2as. This one is on the equation sheet but you should still know when to use it — usually when time isn't given.
Weight = mass x gravitational field strength (W = m g). On Earth g is about 9.8 N/kg (sometimes rounded to 10 N/kg in questions).
Resultant force is the overall force after adding up all forces acting on an object, taking direction into account. If forces balance, resultant force is zero and there's no change in motion.
Stopping distance = thinking distance + braking distance.
Momentum = mass x velocity (kg m/s), a vector quantity.
In a closed system, momentum is conserved: total momentum before a collision equals total momentum after.
Waves transfer energy without transferring matter. Transverse waves (like light and all EM waves) have oscillations at right angles to the direction of travel. Longitudinal waves (like sound) have oscillations parallel to the direction of travel, made of compressions and rarefactions.
Order by increasing frequency (decreasing wavelength): radio, microwave, infrared, visible light, ultraviolet, X-ray, gamma. All travel at the same speed in a vacuum, 3 x 10^8 m/s. Higher frequency means higher energy and more ionising danger.
Magnets have a north and south pole; like poles repel, unlike poles attract. Magnetic field lines run from north to south outside the magnet, and are closer together where the field is stronger. A current-carrying wire creates a magnetic field around it, and this can be strengthened by coiling the wire into a solenoid (an electromagnet), which can be switched on and off.
Our Solar System has the Sun, 8 planets, dwarf planets, moons, asteroids and comets, held in orbit by gravity. Orbits are roughly circular, and gravity provides the centripetal force. Stars form from clouds of dust and gas (nebulae) pulled together by gravity; a main sequence star like our Sun eventually becomes a red giant, then a white dwarf.