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.
Energy transfers happen by four pathways: mechanically (a force moving something), electrically (a current doing work), by heating, or by radiation (light or sound).
Total energy before an event always equals total energy after it. In real systems some energy is always wasted, usually as heat from friction or air resistance, spreading out into the surroundings and becoming harder to use. This wasted energy is called dissipation. No device is ever 100% efficient because of this.
Lagging, insulation and lubrication all reduce unwanted energy transfers. Thermal conductivity describes how quickly a material transfers energy by heating. Thicker insulation and materials with low thermal conductivity reduce the rate of energy transfer, which is why cavity wall insulation and loft insulation cut heating bills.
Non-renewables (coal, oil, gas, nuclear) will run out and mostly release greenhouse gases. Renewables (wind, solar, hydro, geothermal, biofuel, tidal, wave) will not run out but can be unreliable or have limited output. Exam answers often need you to weigh up reliability, cost, and environmental impact rather than just naming one as 'better'.
Current (I) is the flow of charge, measured in amps (A) with an ammeter in series.
Voltage or potential difference (V) is the energy transferred per unit charge, measured in volts with a voltmeter in parallel.
Resistance (R) in ohms opposes the flow of current.
The key equation is V = I x R, sometimes written I = V / R.
In series circuits: current is the same everywhere, voltages across each component add up to the supply voltage, and total resistance is the sum of each resistance.
In parallel circuits: voltage is the same across each branch, current splits between branches and adds back up to the total, and total resistance is lower than the smallest individual resistor.
Common mistake: students think adding a parallel branch increases total resistance. It actually decreases it because you have given the current more paths to flow.
Know the standard symbols: cell, battery, switch, ammeter, voltmeter, resistor, variable resistor, diode, LED, thermistor, LDR, fuse and lamp.
A thermistor's resistance falls as temperature rises. An LDR's resistance falls as light intensity rises.
Diodes only let current flow one way, which matters for rectifying AC.
Charge Q = I x t, measured in coulombs.
Energy transferred E = Q x V, also E = I x V x t.
Power P = V x I, also P = I squared x R, also P = V squared / R.
Mains electricity in the UK is 230V AC at 50Hz. Domestic sockets are typically wired at 230V.
Rubbing insulating materials together transfers electrons, leaving one object positively charged and the other negatively charged.
Like charges repel, opposite charges attract, all without touching.
Static build-up can cause sparks; earthing provides a safe path for charge to flow away.
Live wire is brown and carries the alternating potential difference from the supply.
Neutral wire is blue and completes the circuit, near 0V.
Earth wire is green and yellow stripes, a safety wire that only carries current in a fault.
Fuses and circuit breakers protect circuits: a fuse melts if current exceeds its rating, breaking the circuit.
Always check fuse rating matches the appliance current, not the wire colour, to avoid exam trick questions.
The National Grid transmits electricity at high voltage and low current to minimise energy loss as heat in the cables.
Step-up transformers increase voltage for transmission; step-down transformers reduce it for safe domestic use.
Remember: higher voltage means lower current for the same power, which is why transmission losses are reduced.
Solids, liquids and gases are the three states of matter. Particles in a solid are close together in a fixed, regular arrangement and can only vibrate about a fixed position. Particles in a liquid are close together but can move past each other. Particles in a gas are far apart and move randomly at high speed.
Density tells you how much mass is packed into a given volume. The equation is density = mass / volume, or p = m / V. Density is measured in kg/m3 or g/cm3. Solids are usually densest because particles are packed tightly; gases are least dense because particles are spread out. To find the density of a regular solid, measure mass with a balance and calculate volume from its dimensions. For an irregular solid, use displacement in a eureka can to find volume.
Melting is solid to liquid, freezing is liquid to solid, boiling or evaporating is liquid to gas, and condensing is gas to liquid. Sublimation is solid straight to gas. Changes of state are physical changes, not chemical ones — the substance can change back and no new substance is made, so mass is conserved.
Internal energy is the total kinetic energy and potential energy of all the particles in a system. Heating a system transfers energy to its particles, increasing internal energy, which raises temperature or changes state. Specific heat capacity is the energy needed to raise the temperature of 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, or Q = m c (change in theta). Units: Q in joules, m in kg, c in J/kg C.
During a change of state, temperature stays constant even though energy is still being transferred, because the energy is changing the arrangement of particles, not their speed. Specific latent heat is the energy needed to change the state of 1 kg of a substance with no change in temperature. The equation is energy = mass x specific latent heat, or E = m L. Latent heat of fusion applies to melting/freezing; latent heat of vaporisation applies to boiling/condensing.
Gas particles move randomly and collide with the walls of their container, creating pressure. Increasing the temperature of a gas increases the average kinetic energy of its particles, so they move faster and collide more often and more forcefully, increasing pressure (at constant volume). Doing work on a gas, such as compressing it, can increase its temperature.
An atom has a tiny, dense, positive nucleus (protons + neutrons) surrounded by electrons in shells at relatively large distances. The radius of an atom is about 1 x 10^-10 m, and the nucleus is about 1/10,000 of that (around 1 x 10^-14 m) - almost all the mass is in the nucleus but it takes up almost none of the volume.
Early scientists thought the atom was a ball of positive charge with electrons dotted through it (the 'plum pudding' model). Rutherford's alpha scattering experiment (fired alpha particles at thin gold foil) changed this: most alpha particles passed straight through, some deflected slightly, and a very few bounced almost straight back. This showed most of the atom is empty space, the nucleus is tiny, dense and positively charged, and most of the mass is concentrated there. Niels Bohr then proposed electrons orbit the nucleus at specific, fixed distances (energy levels/shells), which fixed a flaw in the nuclear model where electrons would otherwise spiral into the nucleus.
Some nuclei are unstable and decay randomly, emitting radiation to become more stable. You cannot predict when a particular nucleus will decay - it's a random process.
Half-life is the time taken for the number of unstable nuclei in a sample to halve, or for the count rate (or activity) to fall to half its original value. Activity is measured in becquerels (Bq), where 1 Bq = 1 decay per second.
Speed is how fast something moves, measured in m/s. Velocity is speed in a given direction, so it is a vector.
Acceleration is the rate of change of velocity, measured in m/s squared.
Stopping distance = thinking distance + braking distance.
Momentum (kg m/s) = mass (kg) x velocity (m/s). It is a vector.
Waves transfer energy and information without transferring matter.
Transverse waves (eg light, all EM waves, water waves) vibrate at right angles to the direction of energy transfer.
Longitudinal waves (eg sound, P-waves) vibrate parallel to the direction of energy transfer, forming compressions and rarefactions.
Wave speed (m/s) = frequency (Hz) x wavelength (m), written v = f x lambda.
Frequency is measured in hertz (Hz), one wave per second.
Amplitude is the maximum displacement from the rest position; period (s) = 1 divided by frequency.
Sound in air travels at roughly 340 m/s; light and all EM waves travel at 300,000,000 m/s (3 x 10^8 m/s) in a vacuum.
Order of increasing frequency and energy, decreasing wavelength: radio, microwave, infrared, visible light, ultraviolet, X-ray, gamma.
All EM waves travel at the same speed in a vacuum but have different wavelengths and frequencies.
Uses: radio - broadcasting; microwaves - satellite comms and cooking; infrared - thermal imaging and remote controls; visible light - vision and fibre optics; ultraviolet - sun tanning and sterilising; X-rays - medical imaging; gamma - sterilising equipment and treating cancer.
Hazards increase with frequency: UV can damage skin cells causing cancer; X-rays and gamma rays are ionising and can mutate or kill cells.
Magnetic fields point from north to south outside a magnet; field is strongest at the poles.
A current-carrying wire creates a magnetic field around it; the strength increases with current.
A solenoid (coil of wire) creates a strong, uniform field inside, like a bar magnet; adding an iron core makes an electromagnet stronger.
The motor effect: a current-carrying wire in a magnetic field experiences a force, given by F = B x I x L (force = magnetic flux density x current x length), used in electric motors and loudspeakers.
The life cycle of a star depends on its mass, formed from clouds of dust and gas (nebula) pulled together by gravity.
Small to medium stars (like the Sun): main sequence, red giant, white dwarf, black dwarf.
Massive stars: main sequence, red supergiant, supernova, then either a neutron star or a black hole.
Orbits are kept circular by gravity providing the centripetal force; a satellite orbits at constant speed but changing velocity (direction).
Redshift: light from distant galaxies is shifted towards the red end of the spectrum, showing they are moving away from us; more distant galaxies show greater redshift, evidence for an expanding universe and the Big Bang theory.
Don't confuse frequency and wavelength in the wave equation, check units carefully.
Remember EM waves do NOT need a medium to travel through, but sound does need a medium.
Don't say gravity 'pulls' a satellite off course, it provides the centripetal force that keeps it in orbit.