HookThe mountain that catches a nation switching on its kettle
On the evening of 4 July 1990, England lost a World Cup semi-final to West Germany on penalties, and at the final whistle roughly a million people got up and switched on a kettle. The National Grid recorded a surge in demand of around 2800 megawatts in a few minutes — the biggest "TV pickup" in its history. A gas power station cannot wake up that fast. What answered the call was a hollowed-out slate mountain above Llanberis in Snowdonia: Dinorwig, the pumped-storage station opened in 1984, which can push out \(1{,}728\) MW and reach full power in about sixteen seconds by letting water fall half a kilometre through six turbines.
Every idea in P1 is hiding inside that mountain. Overnight, when demand is low, Dinorwig runs its turbines backwards to pump water uphill, filling a gravitational potential energy store. At peak demand it releases that water, transferring the energy to a kinetic store and then, through generators, to the electrical grid. Energy is never created or destroyed — it is only shifted between stores — yet the round trip is only about 75% efficient, because friction, turbulence and electrical resistance dissipate the rest as heat. Stores, transfers, the rate of transfer (power), and the fraction that ends up useful (efficiency): learn to read any situation as a flow between stores and most of this topic is already yours.
ModelEnergy stores and systems
Physicists no longer talk about "heat energy" or "electrical energy" as if they were different substances. Instead there is one quantity, energy, measured in joules (J), held in a small set of stores: kinetic (movement), gravitational potential (height in a field), elastic potential (stretched or squashed), thermal (the jiggling of particles), chemical (fuels, food, batteries), nuclear, electrostatic and magnetic. Nothing else. When you describe a change, you say which store fell and which store rose.
A system is simply the object, or group of objects, you have chosen to study. When the system changes — a ball is thrown, a kettle switched on, a car brakes — energy is transferred between stores by one of four pathways: mechanically (a force doing work), electrically (charge moved by a potential difference), by heating, or by radiation (light, sound, other waves). The exam wants the transfer named precisely: from the chemical store of the petrol to the kinetic store of the car, by the mechanical work of the expanding gases.
In a closed system — one where no energy enters or leaves — the total energy stays exactly the same, however it is shuffled around. That single sentence, the law of conservation of energy, is the backbone of the whole topic and the thing examiners return to again and again.
ModelThe three energy equations
Three formulae let you put numbers on the stores. The kinetic energy of a moving mass is \(E_k = \tfrac{1}{2}mv^2\) (joules, from kilograms and metres per second). The gravitational potential energy gained by lifting is \(E_p = mgh\), where \(g\) is the gravitational field strength, about \(9.8\) N/kg at the Earth's surface. The elastic potential energy stored in a spring stretched within its limit is \(E_e = \tfrac{1}{2}ke^2\), where \(k\) is the spring constant and \(e\) the extension.
The examiner's favourite trap lives in that little squared symbol. Kinetic energy depends on the square of speed, so doubling the speed does not double the energy — it quadruples it. Gravitational potential energy, by contrast, is linear in height, so doubling the height simply doubles the store. Mixing these up is the single most common calculation error in P1.
Because energy is conserved, these equations link together. A falling object converts \(mgh\) into \(\tfrac{1}{2}mv^2\); set them equal and you can find the landing speed without ever mentioning time. A trampolinist converts kinetic energy into elastic potential in the stretched mat and back again. Reading a problem as "which store empties into which" tells you which two equations to set equal to each other.
A 900 kg car travels at 20 m/s. Its kinetic store holds \[E_k = \tfrac{1}{2}mv^2 = \tfrac{1}{2}\times 900 \times 20^2 = 180000\ \text{J} = 180\ \text{kJ}.\] Brake to a stop and all 180 kJ is transferred to the brakes, tyres and road as thermal energy. Now double the speed to 40 m/s: \(E_k = \tfrac{1}{2}\times 900 \times 40^2 = 720000\) J — four times as much, because \(E_k \propto v^2\). That squared term is exactly why braking distance roughly quadruples when speed doubles, and why speed limits matter so much. Lifting is gentler: raising one cubic metre (1000 kg) of water to Dinorwig's upper reservoir, 500 m up, stores \(E_p = mgh = 1000 \times 9.8 \times 500 = 4.9\times 10^{6}\) J = 4.9 MJ, and doubling the height would simply double it.
MechanismSpecific heat capacity — why water is so hard to heat
Some materials soak up a lot of energy for a small temperature rise; others heat quickly. The specific heat capacity \(c\) of a material is the energy needed to raise the temperature of 1 kg of it by 1°C. The equation is \(\Delta E = mc\Delta\theta\), where \(m\) is mass in kg, \(\Delta\theta\) is the temperature change, and \(\Delta E\) is the energy transferred in joules.
Water has an unusually large specific heat capacity, about 4200 J/kg°C — roughly five times that of most metals. This is why the sea warms and cools far more slowly than the land, why a hot-water bottle stays warm for hours, and why coolant in a car engine is mostly water: it can carry away a lot of energy without its own temperature soaring. A high specific heat capacity means a material is a good energy reservoir but slow to respond.
The equation also runs backwards. If you know how much energy you put into a block and by how much its temperature rose, you can find the material's specific heat capacity — which is exactly what Required practical 1 does. The habit that earns marks is to state the equation, substitute with consistent units, and only then rearrange.
In Required practical 1, a 1.0 kg aluminium block is heated by an immersion heater running at 12 V and 4.0 A for 5 minutes (300 s). The electrical energy supplied is \[E = VIt = 12 \times 4.0 \times 300 = 14400\ \text{J}.\] The block's temperature rises from 20°C to 35°C, so \(\Delta\theta = 15\). Rearranging \(\Delta E = mc\Delta\theta\): \[c = \dfrac{E}{m\,\Delta\theta} = \dfrac{14400}{1.0 \times 15} = 960.\] So the measured specific heat capacity is 960 J/kg°C. The accepted value for aluminium is about 900, an overshoot of roughly 7%. That direction is not luck: some of the 14400 J leaked to the room instead of heating the block, so the energy you supplied overstates the energy the block absorbed, and \(c\) comes out too high. Wrapping the block in insulation and adding a lid shrinks the gap.
ModelPower — the rate, not the amount
Two appliances can transfer the same energy while feeling utterly different, because one takes seconds and the other takes hours. Power is the rate of energy transfer, measured in watts (W), where one watt is one joule per second. Both spellings of the equation appear in P1: \(P = \dfrac{E}{t}\) when energy is transferred, and \(P = \dfrac{W}{t}\) when work is done — the two are the same idea, since work done is energy transferred.
Power is what makes Dinorwig special. Its energy store is large but not extraordinary; what no gas plant can match is the rate at which it delivers, 1728 MW in sixteen seconds. A 2 kW kettle and a 100 W laptop charger might eventually transfer the same total energy over an afternoon, but the kettle does it in seconds because it draws twenty times the power.
Watch the prefixes, because this is where marks vanish. A kilowatt (kW) is 1000 W, a megawatt (MW) is one million watts, and a gigawatt (GW) is a thousand million. Convert everything to joules and seconds before substituting, and convert back to sensible units at the end.
A 2.4 kW kettle transfers energy at 2400 J every second. Heating 0.50 kg of water from 20°C to 100°C needs \[\Delta E = mc\Delta\theta = 0.50 \times 4200 \times 80 = 168000\ \text{J}.\] Ignoring losses, the time to do this is \[t = \dfrac{E}{P} = \dfrac{168000}{2400} = 70\ \text{s}.\] In reality it takes a little longer, because energy also heats the kettle body, the flex and the escaping steam — which is the same reason its efficiency is below 100%. Notice how power stitches the topic together: the specific-heat-capacity equation gives the energy, and \(P = E/t\) turns it into a time you can actually check with a stopwatch.
MechanismConservation, dissipation and efficiency
Energy is always conserved, yet devices still "waste" it — because some of every transfer ends up in stores we cannot use, almost always the thermal store of the surroundings. This spreading-out of energy into low-grade heat is called dissipation. A phone that grows warm, brakes that glow, a bulb hot to the touch: all are dissipating energy that was meant for something else.
Engineers fight dissipation in two main ways. Lubrication reduces the friction between moving surfaces, so less kinetic energy is dissipated as heat and sound — oil in an engine, grease on a bearing. Thermal insulation slows the unwanted transfer of energy by heating; the thicker the insulator and the lower its thermal conductivity, the more slowly a building or a hot drink cools. Trapped air, as in loft insulation or double glazing, works because gases conduct heat poorly.
Efficiency puts a number on how well a device does its job: the fraction of the input energy that ends up in the useful store. \[\text{efficiency} = \dfrac{\text{useful output energy transfer}}{\text{total input energy transfer}}\] The same ratio works with power in place of energy. Efficiency is always between 0 and 1 (or 0% and 100%) — a device can never be more than 100% efficient, because that would mean creating energy from nothing.
An old filament bulb draws 60 W but emits only 6 W as light: \[\text{efficiency} = \dfrac{6}{60} = 0.10 = 10\%.\] The other 54 W is dissipated as heat. Swap in an LED that produces the same 6 W of light from just 8 W of input and efficiency leaps to \(6 \div 8 = 0.75 = 75\%\) — which is why the UK began phasing out incandescent bulbs in 2009. That same 75% is, not by coincidence, Dinorwig's round-trip efficiency: pump 100 units of energy uphill overnight and you recover about 75 at peak, the missing 25 lost to friction, turbulence and electrical resistance. Storing energy is never free.
DataRequired practicals 1 and 2 — measuring heat and testing insulation
Required practical 1 finds the specific heat capacity of a material (the aluminium block above is the standard version). An immersion heater and a joulemeter, or a voltmeter and ammeter with a timer, measure the energy supplied; a thermometer records the temperature rise. The independent variable is energy supplied, the dependent variable is temperature, and you control the mass and material of the block. The biggest systematic error is energy escaping to the room, which makes the measured \(c\) too high — so you insulate the block, add a lid, and use a small drop of water or oil in the thermometer hole to improve thermal contact.
Required practical 2 (physics only) tests how well different materials work as thermal insulators. Pour equal volumes of hot water into identical beakers, wrap each in a different material — bubble wrap, newspaper, cotton wool — leave one bare as a control, put a lid on every one, and record the temperature every minute. The independent variable is the insulating material (or the number of layers); the dependent variable is the temperature, or the temperature drop over a fixed time; and you must control the starting temperature, the volume of water, the beaker, the room temperature and the time.
The skill the examiner rewards is fair comparison. Because the beakers start at the same temperature, the material that leaves the water hottest — the smallest temperature fall — is the best insulator. Reporting a temperature drop rather than a final temperature makes the comparison cleaner still.
In Required practical 2, a bare control beaker of 100 cm³ of water cools from 80°C to 62°C in ten minutes — a fall of \(80 - 62 = 18\) °C. An identical beaker wrapped in bubble wrap cools only from 80°C to 74°C, a fall of \(80 - 74 = 6\) °C. The bubble wrap has cut the temperature drop to one-third of the control's, because the pockets of trapped air conduct heat poorly. To compare three materials fairly you plot temperature against time for each on the same axes: the highest curve, with the shallowest gradient, is the best insulator. Quoting the difference in temperature fall — 18°C versus 6°C — is worth more than saying one "stayed warmer", because it is a number the examiner can credit.
CaseNational and global energy resources
Energy resources split into non-renewable — coal, oil, natural gas and nuclear fuel, which will run out on human timescales — and renewable — wind, solar, hydroelectric, tidal, wave, geothermal and biofuel, which are replenished as fast as they are used. The exam asks you to weigh three things for each: reliability, environmental impact and the job it is suited to.
The UK has changed faster than almost anywhere. In 1990 coal generated most of Britain's electricity; the last coal power station, Ratcliffe-on-Soar, closed in September 2024, and wind now routinely supplies more than a third of the grid. But renewables bring a new problem: the wind does not always blow and the sun does not shine at 7 p.m. in December, so output is intermittent. This is where pumped storage like Dinorwig, and increasingly grid-scale batteries, earn their keep — storing surplus renewable energy and releasing it on demand to keep supply reliable.
Every source has costs. Fossil fuels release carbon dioxide, driving climate change, and burning coal also produces sulfur dioxide and acid rain. Nuclear power is low-carbon and reliable but produces radioactive waste and carries a small risk of serious accident. Renewables are low-carbon in use, yet wind farms affect landscapes and habitats, large hydro schemes flood valleys, and manufacturing any of them has a footprint. "Renewable" is not the same as "no impact" — the honest answer to an energy question always names a trade-off, not a winner.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Every calculation follows the same disciplined path: write the equation, substitute with consistent units (kilograms, metres, seconds, joules), rearrange only after substituting, then give the answer with a sensible unit and prefix. Marks are lost far more often to unit slips and forgotten kilo/mega conversions than to not knowing the formula. When kinetic energy is involved, slow down at the \(v^2\) — squaring 20 to get 400 before multiplying is where the working stays honest.
Description questions want the store-to-store language: name the store that empties, the store that fills, and the pathway (mechanical, electrical, heating or radiation). 'Energy is transferred' on its own earns little; 'the chemical store of the fuel is transferred to the kinetic store of the car by mechanical work' earns full marks. For efficiency, always show useful over total as a fraction before converting to a percentage, and sanity-check that your answer is below 100%.
Required-practical marks are for method detail and error reasoning. Know which variable is independent, which is dependent and which are controlled, and be ready to state that heat loss to the surroundings makes a measured specific heat capacity too high — then say how insulation and a lid reduce it. On resources, refuse to crown a winner: examiners reward a balanced comparison of reliability against environmental impact, with a real UK example such as the 2024 closure of the last coal plant or the role of pumped storage in backing up wind.