Powering Earth — OCR GCSE Physics

Earth's main energy resources are fossil fuels (coal, oil, gas), nuclear fuel, biofuel, wind, hydroelectricity, tides and the Sun. Each is used in a particular way: fossil and nuclear fuels heat water to make steam that drives turbines; wind turns turbine blades directly; falling water in a hydroelectric scheme spins turbines; tidal schemes use the rise and fall of the sea; the Sun supplies direct heating, daylight and electricity through solar cells; biofuel is burned or fermented. Renewable resources (biofuel, wind, hydroelectricity, tides, the Sun) are replenished naturally, while fossil and nuclear fuels are non-renewable because their supplies are finite. A multiple-choice question tests whether you can match each resource to its use and classify it correctly.

Energy use on Earth has changed over time, and an extended answer should explain why. Early reliance on wood and other biofuels gave way to coal during industrialisation, then to oil and gas as transport and electricity demand grew, and more recently to nuclear fuel and renewable resources such as wind and solar. Trends include a rising total demand as populations and industry grow, a shift towards electricity, and growing use of renewables driven by concerns about finite fossil fuel supplies and environmental impact. A strong answer links each change to a cause, such as cost, availability, technology or policy, and uses the specified resources to illustrate the pattern rather than listing them.

The national grid is the network of cables and transformers that carries electrical power from power stations to homes and businesses. Power leaves a power station at a high voltage, set by a step-up transformer, and travels along transmission lines at that high voltage. A high voltage means a smaller current is needed to transfer the same power, because power = voltage × current, and a smaller current wastes less energy as heat in the cables. Near each locality a step-down transformer reduces the voltage to a lower, safer value suitable for domestic use. So the grid transfers power at high voltage over the long-distance network and at lower voltage locally. Recall means remembering this two-stage pattern: high voltage from power stations, lower voltage in each locality for domestic use.

A transformer changes the size of an alternating potential difference and works only with alternating current, because it relies on a changing magnetic field linking two coils wound on a shared iron core. A step-up transformer has more turns on its secondary coil than on its primary coil, so it raises the potential difference; a step-down transformer has fewer secondary turns and lowers it. In the National Grid, a step-up transformer near a power station raises the potential difference to a very high value for transmission, which reduces the current for the same transmitted power and so reduces energy wasted as heat in the cables. Step-down transformers then lower the potential difference in stages towards the safe level used in homes.

The National Grid moves electrical energy from power stations to homes, schools and industry. A learner should explain efficiency by tracing the chain: a step-up transformer raises the potential difference for transmission, so for the same power a smaller current flows, and because heating in cables depends on current, less energy is wasted as thermal energy in the wires. Step-down transformers then lower the potential difference to safe domestic levels near users. Efficiency also comes from keeping transmission cables thick and short where possible and from balancing supply with demand. A strong answer names each stage, links high voltage to low current and low wasted heating, and states that the grid lets energy be generated far from where it is used and shared across regions.

This Higher tier, separate-science statement asks a learner to connect three ideas. First, the turns ratio of a transformer sets the ratio of potential differences: the ratio of secondary to primary turns equals the ratio of secondary to primary potential difference. Second, an ideal transformer transfers the same power, so if potential difference rises the current falls by the same factor, because power equals potential difference multiplied by current. Third, transmitting at high voltage therefore means a low current, and since heating in cables depends on current, less energy is wasted. A correct multiple-choice response selects the option that keeps power constant while linking turns, potential differences and current, and identifies high-voltage transmission as reducing wasted heating.

The mains electricity delivered to UK homes is an alternating supply. Its voltage repeatedly reverses direction, so the potential difference swings positive and negative many times each second. The frequency of that reversal is 50 Hz, meaning 50 complete cycles every second, so the direction changes 100 times per second. The stated voltage is about 230 V; the word 'about' matters because the actual value drifts slightly around that figure. A learner should be able to pick out all three facts together: alternating current, 50 Hz and roughly 230 V. A useful check is to reject any option that gives a direct (d.c.) supply, a frequency such as 60 Hz, or a voltage far from 230 V, since each of those contradicts one clause of the specification statement.

A direct voltage keeps the same polarity, so the potential difference stays on one side of zero and current flows one way around the circuit; a battery gives this kind of supply. An alternating voltage repeatedly reverses polarity, so the potential difference swings between positive and negative values and the current changes direction each cycle; the UK mains at 50 Hz is an example. The key contrast is direction: d.c. is unidirectional and steady in sign, while a.c. changes direction periodically. A good answer links the voltage behaviour to the resulting current direction and can name a source of each type. When choosing an option, check that it captures the reversal of an alternating supply and the one-way nature of a direct supply, and reject options that confuse voltage with frequency or claim a.c. never changes value.

In UK domestic mains supply the three wires have distinct jobs. The live wire carries the alternating potential difference from the supply and alternates between a positive and a negative potential difference relative to the neutral wire; its root-mean-square value is about 230 V, with a peak of roughly 325 V. The neutral wire completes the circuit and is held at about zero potential difference, close to earth potential. The earth wire is a safety wire connected to the metal case of an appliance; it carries no current in normal use but provides a low-resistance path to earth if a fault connects the live wire to the case, so the case cannot stay at a dangerous potential difference. The potential difference between live and neutral is the full mains value, while the potential difference between live and earth is also about the mains value; between neutral and earth it is close to zero.

A switch placed in the live wire stops current flowing when open, but the live wire on the supply side of the switch stays at a high potential relative to earth. Touching that section, or a fault that connects it to an earthed metal case, completes a path to earth and a large current can flow through a person, causing a shock. A direct live-to-earth connection also risks a large current, overheating and fire. Insulation around wires and the casing of devices prevents contact between the live conductor and a person or earthed metal, so intact insulation is a key protection; damaged insulation removes that protection and makes the danger real.