Energy is never created or destroyed, only transferred between stores. The main stores are: kinetic, gravitational potential, elastic potential, thermal (internal), chemical, magnetic, electrostatic and nuclear. Transfers happen by four pathways: mechanically (a force acting), electrically (a current), by heating, or by radiation (light or sound).
In any closed system the total energy before equals the total energy after. In real systems some energy is always 'wasted', usually dissipated to the thermal store of the surroundings through friction or air resistance, which is why no machine is 100% efficient. Insulation and lubrication reduce unwanted transfers.
Energy is measured in joules (J), power in watts (W), where 1 watt = 1 joule per second. Household energy bills use kilowatt-hours (kWh): energy (kWh) = power (kW) x time (hours). Cost = energy (kWh) x cost per kWh.
Non-renewable resources (fossil fuels, nuclear) will run out and most release carbon dioxide; renewables (wind, solar, hydro, geothermal, tidal, biofuel) won't run out but are often less reliable and lower power output. Know at least one advantage and one disadvantage of each for exam questions.
Electric current is the rate of flow of charge, measured in amps (A). Use Q = I × t, where Q is charge in coulombs, I is current in amps, t is time in seconds.
Voltage (potential difference) is the energy transferred per unit charge, measured in volts (V). Use W = Q × V, where W is energy in joules.
Current is the same at every point in a series circuit. In a parallel circuit, current splits between branches but total current in equals total current out.
Ohm's Law: V = I × R (voltage = current × resistance), resistance measured in ohms (Ω).
A resistor at constant temperature gives a straight-line graph through the origin on a current-voltage graph (ohmic conductor).
A filament lamp's resistance increases as it heats up, so its I-V graph curves and flattens.
A diode only lets current flow easily in one direction — almost no current flows in reverse, shown by a flat line near zero on the reverse side of its graph.
Electrical power: P = V × I (power = voltage × current), measured in watts (W).
Also P = I² × R and P = V²/R — useful when you're only given current or voltage plus resistance.
Energy transferred: E = P × t (joules = watts × seconds), or E = Q × V.
UK mains supply is 230 V, alternating current (a.c.), at 50 Hz.
Live wire (brown) carries the alternating potential difference; neutral wire (blue) completes the circuit near 0 V; earth wire (green and yellow) is a safety wire that only carries current if there's a fault, stopping the appliance casing becoming live.
Fuses contain a thin wire that melts and breaks the circuit if current gets too high, protecting the appliance and wiring from overheating. Circuit breakers do the same job but can be reset.
A common mistake is thinking the fuse protects the person — it actually protects the wiring and appliance from fire risk; the earth wire plus fuse together protect the user from electric shock.
The National Grid transmits electricity at high voltage and low current to reduce energy losses from heating in the cables (since power loss = I² × R, lower current means much less wasted heat).
Step-up transformers increase voltage for transmission; step-down transformers reduce it again for safe use in homes.
Solids, liquids and gases are the three states of matter. Particles are closest together and most ordered in solids, and furthest apart with the most energy in gases.
Density is mass per unit volume: density (kg/m3) = mass (kg) / volume (m3). Solids are usually densest, gases least dense, because particle spacing (not particle size) changes between states.
To find density of a regular solid, measure mass on a balance and calculate volume from length measurements (e.g. cuboid: 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.
Melting, freezing, boiling, evaporating and condensing are physical changes: the substance's mass is conserved and the change is reversible, unlike a chemical change which produces a new substance.
Internal energy is the total kinetic and potential energy of all the particles in a system. Heating a system transfers energy to its particles, which either raises temperature (kinetic energy increases) or changes state (potential energy increases) — not both at once.
Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 degC. Equation: change in thermal energy (J) = mass (kg) x specific heat capacity (J/kg degC) x change in temperature (degC).
Specific latent heat is the energy needed to change the state of 1 kg of a substance with no change in temperature. Equation: energy (J) = mass (kg) x specific latent heat (J/kg). Latent heat of fusion is for melting/freezing; latent heat of vaporisation is for boiling/condensing — vaporisation is always larger than fusion for the same substance.
On a heating graph, flat sections show a change of state (energy going into breaking bonds, not raising temperature); sloped sections show temperature rising within one state.
Gas particles move randomly and rapidly, colliding with each other and the container walls; these collisions cause gas pressure.
Increasing temperature increases the average kinetic energy of particles (absolute temperature in kelvin is directly proportional to average kinetic energy).
For a fixed mass of gas at constant volume, increasing temperature increases pressure (particles hit walls harder and more often).
For a fixed mass of gas at constant temperature, doing work on a gas (compressing it) increases its pressure, described by: pressure x volume = constant (Boyle's law).
Kelvin conversion: kelvin = degC + 273. Absolute zero is 0 K (-273 degC), where particles have minimal kinetic energy.
An atom has a tiny, dense, positively charged nucleus (protons and neutrons) surrounded by electrons in shells (energy levels). Almost all the mass sits in the nucleus, but almost all the volume is empty space taken up by the electron shells. Atom radius is about 1 x 10^-10 m; the nucleus is about 1/10,000th of that, around 1 x 10^-14 m.
Atomic number (proton number, bottom-left or written as Z) = number of protons, which equals the number of electrons in a neutral atom. Mass number (top-left or A) = protons + neutrons. Number of neutrons = mass number minus atomic number. Isotopes are atoms of the same element (same proton number) with different numbers of neutrons, so different mass numbers.
Old idea (plum pudding): atom is a ball of positive charge with electrons dotted through it, like fruit in a pudding. Rutherford's alpha scattering experiment fired alpha particles at thin gold foil. Most passed straight through (atom is mostly empty space), some deflected slightly (nucleus is positive), and a few bounced straight back (nucleus is small, dense and positive). This led to the nuclear model. Bohr later showed electrons must orbit at fixed distances (shells), not spiral into the nucleus, matching experimental results.
Radioactive decay is random and spontaneous — you cannot predict which atom decays next or when, and it is not affected by temperature, pressure or chemical bonding. Unstable nuclei emit radiation to become more stable.
Half-life is the time for the number of unstable nuclei (or the activity, measured in becquerels, Bq) to halve. It is a fixed property of the isotope, unaffected by external conditions. Use it to work out how much activity remains after a given number of half-lives, or count half-lives to find how long has passed.
Don't confuse mass number with atomic mass, don't say alpha particles are the most penetrating (they are the least, gamma is most), and don't think decay can be sped up or slowed down artificially. Remember electrons have negligible mass but carry the negative charge that balances the protons.
Speed tells you how fast something moves. The equation is speed = distance / time, measured in m/s.
Acceleration is the change in velocity per second, in m/s squared.
These three laws explain how forces change motion.
When forces on an object are balanced, the resultant force is zero and the object moves at constant velocity or stays still. When forces are unbalanced, the object accelerates in the direction of the bigger force.
Momentum = mass x velocity, measured in kg m/s. Momentum is conserved in a closed system, meaning total momentum before a collision equals total momentum after, as long as no external forces act.
Waves transfer energy and information without transferring matter.
All EM waves travel at the same speed through a vacuum, about 3 x 10^8 m/s, and form a continuous spectrum.