8.2.1 Renewable and non-renewable energy resources

Energy resources

1. An energy resource is anything that holds a store of energy which people can transfer usefully, either by burning a fuel or by capturing a natural flow of energy such as wind, moving water or sunlight.
2. Energy resources are put to three main uses on Earth: generating electricity, heating homes and workplaces, and powering transport.
3. Every resource is placed in one of two groups, depending on whether it can be replaced as fast as it is used.
4. A renewable resource is replenished naturally, so the wind keeps blowing, the tides keep turning and the Sun keeps shining however much energy is taken from them.
5. A non-renewable resource exists in a fixed amount, so every tonne of coal or uranium that is used leaves less for the future.
6. The fossil fuels (coal, oil and natural gas) and nuclear fuel are the non-renewable resources you need to know.
7. Biofuel, wind, hydroelectricity, tides and the Sun are the renewable resources you need to know, and waves and geothermal energy also belong to this group.
8. The deciding question is how fast the resource is replaced, not whether it is clean or whether it can be used twice.

Fossil fuels

1. Fossil fuels formed over millions of years from the buried remains of plants and sea creatures, which is far too slowly for them to be replaced on a human timescale.
2. Energy is released from their chemical energy store when they are burned in oxygen.
3. In a fossil-fuel power station, the fuel is burned to heat water in a boiler and turn it into high-pressure steam.
4. The steam spins a turbine, the turbine turns a generator, and the generator transfers energy electrically to the national grid.
5. Coal was burned mainly in power stations, though the last coal-fired power station in the UK, at Ratcliffe-on-Soar in Nottinghamshire, closed in 2024.
6. Oil is refined into petrol, diesel and jet fuel, which makes it the main fuel for cars, lorries, ships and aircraft.
7. Natural gas is burned in most UK home boilers for heating and cooking, and gas-fired power stations generate a large share of the country's electricity.
8. Fossil fuels are reliable, because fuel can be stockpiled and burned whenever it is needed.
9. A gas-fired power station can also be started up within minutes, so it can respond quickly when demand rises.
10. Burning any fossil fuel releases carbon dioxide, a greenhouse gas that adds to climate change.
11. Burning coal and oil also releases sulfur dioxide, which causes acid rain, and burning coal produces smoke and ash.

Nuclear fuel

1. A nuclear power station uses uranium as its fuel.
2. Inside the reactor, uranium nuclei split, and energy from the nuclear store heats water into steam.
3. From that point on, the steam drives a turbine and generator in the same way as in a fossil-fuel power station.
4. A small mass of uranium releases an enormous amount of energy, so a reactor can run for months between refuelling.
5. Nuclear power stations release no carbon dioxide while they are operating, which makes them attractive for cutting emissions.
6. Nuclear fuel is still non-renewable, because the uranium ore mined from the ground is finite.
7. The used fuel is radioactive waste that stays dangerous for thousands of years, so it must be stored securely for a very long time.
8. Building a nuclear power station is very expensive and slow, and closing one safely at the end of its life (decommissioning) takes decades.
9. Reactors take a long time to start up, so they are used to supply a steady output all day and night rather than to meet sudden rises in demand.

Biofuel

1. Biofuels are made from material that was alive recently, such as wood pellets, sugar cane, oilseed rape or methane gas from rotting food and animal waste.
2. Like fossil fuels, they release energy from a chemical store when burned, so they can heat water to drive turbines, fuel boilers or be blended into petrol and diesel.
3. They count as renewable because new crops can be grown each year to replace what is burned.
4. Burning a biofuel releases carbon dioxide, but the growing plants absorbed a similar amount of carbon dioxide from the air, so biofuels are described as close to carbon neutral.
5. Biofuels are reliable, because the fuel can be stored and burned on demand.
6. Growing fuel crops takes up large areas of farmland that could otherwise grow food, and forests are sometimes cleared to make room for them.

Wind, water and the Sun

1. A wind turbine has large blades that are turned by moving air, and the rotating shaft drives a generator directly without any fuel or steam.
2. Wind farms are built on hills and out at sea, where winds are strongest, and the UK now has some of the largest offshore wind farms in the world.
3. Wind is unreliable, because turbines generate nothing on a still day and must be shut down when winds are dangerously strong.
4. Some people object to wind farms because they change the look of the landscape and make noise.
5. A hydroelectric power station traps a river behind a dam, and water released from the high reservoir flows down through pipes to spin turbines.
6. Hydroelectricity is reliable and can start generating within seconds, because the water stored behind the dam can be released whenever it is needed.
7. Building a dam floods a valley, destroying habitats and sometimes homes, and only mountainous areas such as Scotland and Wales have suitable sites.
8. Tidal power uses the rise and fall of the sea, either with a barrage across an estuary or with underwater turbines turned by tidal currents.
9. Tides happen twice a day at times that can be predicted years ahead, so tidal power is predictable even though its output rises and falls during each day.
10. A tidal barrage blocks an estuary, which can harm the mudflats where wading birds feed and can obstruct shipping.
11. The Sun is used in two ways: solar cells transfer energy from sunlight directly into an electrical supply, and solar heating panels use sunlight to warm water for a building.
12. Solar power needs no fuel and has very low running costs, but it produces nothing at night and much less on cloudy winter days.
13. Wave power uses floating generators that are moved up and down by waves, and geothermal power uses heat from hot rocks underground, though few places in the UK have rocks hot enough.

Comparing energy resources

1. When you compare two resources, judge them against the same factors so that each point sets one resource directly against the other.
2. Reliability is whether the resource can supply energy at any time: fossil fuels, nuclear fuel, biofuel and hydroelectricity can, while wind and solar depend on the weather.
3. Response time is how quickly output can be raised: hydroelectric and gas power stations respond in seconds or minutes, while nuclear and coal power stations take many hours.
4. Effect on the environment covers carbon dioxide emissions, other pollution, waste and damage to land or habitats.
5. Cost is split into the cost of building the system and the cost of running it, since wind and solar are cheap to run because their energy source is free.
6. Location matters because hydroelectricity needs high valleys, tidal power needs a suitable estuary or coastline, and wind farms need exposed sites.
7. No single resource wins on every factor, which is why the UK uses a mixture of resources to keep the supply secure.

8.2.2 Patterns and trends in energy use

Rising energy demand

1. Energy demand across the world has risen steadily for more than a century and is still rising.
2. One cause is population growth, since every extra person needs energy for food, warmth, light and travel.
3. A second cause is economic development, because as countries industrialise, more people buy cars, fridges, air conditioners and phones.
4. Energy use per person is far higher in wealthy countries such as the UK than in many developing countries, but demand is now growing fastest in developing countries.
5. Fossil fuels still supply most of the world's energy, because they are reliable, the technology already exists and many countries have their own reserves.
6. The UK bucks the global trend, as its total electricity demand has fallen since the mid-2000s.
7. That fall is explained by more energy-efficient appliances, LED lighting, better home insulation and the decline of heavy industry such as steelmaking.

Daily and seasonal patterns

1. Electricity demand follows a pattern that repeats every day, and a second pattern that repeats every year.
2. Demand is lowest in the early hours of the morning, when most people are asleep and most businesses are closed.
3. It rises quickly from about 6 am as people wake up, switch on kettles and lights, and travel to work and school.
4. Peak demand normally comes in the early evening, when people arrive home, cook meals and turn on lights, televisions and heating.
5. Demand is higher in winter than in summer, because evenings are darker and more homes use electric heating.
6. Electricity cannot easily be stored in large amounts, so power stations must match the supply to the demand second by second.
7. Nuclear power stations provide a steady base load, because they run best at a constant output and take hours to change it.
8. Gas-fired power stations and hydroelectric stations cover the peaks, because they can raise their output within minutes.
9. Pumped storage stations such as Dinorwig in Snowdonia pump water uphill when demand is low and release it through turbines within seconds when demand peaks.
10. Solar output peaks around midday in summer, which does not match the winter evening peak, so other resources must fill the gap.

Changes in the UK over time

1. In 1990 about two-thirds of the UK's electricity was generated from coal.
2. During the 1990s, many new gas-fired power stations were built, because North Sea gas was cheap and these stations were quicker to build and released less carbon dioxide than coal.
3. Coal use fell sharply during the 2010s as the government added a cost to carbon emissions and tightened air pollution rules, and the last coal power station closed in 2024.
4. The share of electricity from renewable resources grew from only a few per cent in 2000 to around half by the mid-2020s, with wind now the largest single contributor.
5. Renewables grew because the cost of wind turbines and solar panels fell dramatically and because the UK made a legal commitment to reach net zero carbon emissions by 2050.
6. Nuclear power has supplied a smaller share in recent years as older reactors reached the end of their working lives, although a new station, Hinkley Point C, is being built in Somerset.
7. In transport, oil still supplies most of the energy used, but sales of electric cars are rising each year.
8. In heating, most UK homes still burn natural gas, but electric heat pumps are slowly being installed in their place.

Reasons behind the trends

1. Cost: a resource is used more when it becomes cheaper than its rivals, which is why gas replaced coal and wind is now replacing gas.
2. Availability: countries use resources they have locally, so Norway generates almost all its electricity from hydroelectricity because it has many steep, wet valleys.
3. Environmental concern: worries about climate change and air pollution have pushed countries away from coal and towards low-carbon resources.
4. Government policy: taxes on carbon, subsidies for renewables and targets for cutting emissions all shift the mix of resources.
5. Technology: larger offshore turbines, cheaper solar cells and large batteries have made renewable resources more practical.
6. Security of supply: when gas prices rose steeply in 2022 after Russia invaded Ukraine, many countries sped up plans to depend less on imported gas.
7. Reliability: however cheap wind and solar become, a country still needs some resources that can generate on a calm, dark evening.

8.2.3 The national grid

What the national grid is

1. The national grid links the power stations and wind farms of Great Britain to every home, school, hospital and factory that uses electricity.
2. Power stations are often built far from the towns they supply, for example on the coast for cooling water or out at sea where the wind is strongest.
3. The grid carries electrical power across these distances, so electricity generated in the north of Scotland can be used in London a fraction of a second later.
4. Because every generator feeds into one shared network, a breakdown at one power station does not cut off the homes nearby, since other stations can make up the difference.
5. The grid is made of overhead cables carried on steel pylons, underground cables in towns and cities, and substations where the potential difference is changed.
6. Potential difference is measured in volts, $V$, and is often called voltage, especially when describing the grid.

High voltage from power stations

1. The generators in a typical large power station produce a potential difference of about $25kV$, where $1kV=1000V$.
2. Before the power leaves the site, a substation beside the power station raises the potential difference to a very high voltage.
3. The main cross-country cables in Great Britain run at $275kV$ or $400kV$, which is more than $1000$ times the potential difference of a plug socket.
4. This part of the grid is called the transmission network, and it is the part you see as lines of tall pylons striding across the countryside.
5. The cables hang high above the ground and are held away from the steel pylons by long insulators, because such large potential differences are extremely dangerous.

Lower voltage in each locality

1. When the power reaches a region, substations reduce the potential difference in stages to $132kV$, then $33kV$ and then $11kV$.
2. These lower-voltage cables form the distribution network, which carries power along smaller poles or underground into each locality.
3. A small substation in each street finally reduces the potential difference to about $230V$ for domestic use in homes.
4. Large users such as factories and railways may take their supply at $11kV$ or $33kV$ and reduce it on site.
5. Homes use a low potential difference because it is far safer for people to handle and allows appliances to be built with ordinary plastic insulation.

8.2.4 Step-up and step-down transformers

What a transformer does

1. A transformer is the device the national grid uses to change the size of the potential difference as power is transferred from power stations to consumers.
2. It is built from two separate coils of insulated copper wire wound around a shared iron core.
3. The input coil is connected to the supply, and the output coil delivers power at a different potential difference to the next part of the grid.
4. The two coils are not joined by any wire, so the transfer of energy from one coil to the other happens through the iron core.
5. A transformer has no moving parts, and large grid transformers waste only a very small fraction of the energy passing through them.
6. Transformers work only with an alternating supply, which is one of the main reasons the national grid uses alternating current.
7. Each transformer is either step-up or step-down, depending on whether its output potential difference is larger or smaller than its input.

Step-up transformers

1. A step-up transformer gives out a higher potential difference than the one it receives.
2. In the grid, step-up transformers sit in the substation beside each power station.
3. A typical one raises the $25kV$ produced by the generators to $400kV$ for the transmission cables.
4. Wind farms and solar farms also use step-up transformers to connect their output to the high-voltage network.
5. There is normally only one step-up stage between a power station and the transmission network, because the potential difference is raised in one large jump.
6. A step-up transformer does not create extra energy, because the energy it transfers out each second cannot be greater than the energy supplied to it.

Step-down transformers

1. A step-down transformer gives out a lower potential difference than the one it receives.
2. Step-down transformers are used where the transmission network meets each region and again in substations throughout towns and cities.
3. The potential difference is lowered in several stages, typically from $400kV$ to $132kV$, then to $33kV$, then to $11kV$ and finally to about $230V$.
4. The last step-down transformer is often housed in a fenced green cabinet or small brick building at the end of a street.
5. Step-down transformers are also found in everyday devices, such as the chargers and adaptors that lower the $230V$ mains supply to the few volts needed by a phone or laptop.

8.2.5 Efficiency of the national grid

Energy dissipated in the cables

1. Every cable in the national grid has resistance, even though the cables are thick and made of good conductors such as aluminium.
2. When a current passes through a resistance, the cable heats up, so energy is transferred to the thermal energy store of the cable.
3. This energy then spreads into the air around the cable, so it is dissipated to the surroundings and never reaches a consumer.
4. The rate at which energy is dissipated in a cable is given by $P=I^{2}R$, where $P$ is power in watts ($W$), $I$ is current in amperes ($A$) and $R$ is resistance in ohms ($\Omega$).
5. The current appears squared in this equation, so doubling the current makes the heating four times greater.
6. Halving the current cuts the heating to a quarter, and cutting the current to a tenth reduces the heating to one hundredth.
7. The current in the cables is therefore the single biggest factor that decides how much energy the grid wastes.

Why high voltage improves efficiency

1. The power transferred along a cable is linked to the potential difference and the current by $P=VI$.
2. For a fixed amount of power, a larger potential difference means a smaller current, because $V$ and $I$ multiply together to give the same $P$.
3. Raising the potential difference by a factor of $10$ therefore lowers the current by a factor of $10$.
4. A current ten times smaller makes the heating in the cables $10^2=100$ times smaller.
5. Less energy is dissipated to the surroundings, so a greater proportion of the input energy reaches homes and businesses.
6. A greater useful share of the input energy is what a higher efficiency means, since $efficiency=\frac{useful\; output\; energy\; transfer}{input\; energy\; transfer}$.
7. This is why the national grid transmits power at hundreds of thousands of volts, and only a few per cent of the energy is dissipated in the transmission cables.

Why the voltage is lowered again

1. A potential difference of $400kV$ is lethal, and it can even make a spark jump through the air to a nearby object.
2. Step-down transformers therefore reduce the potential difference before the power reaches each locality, so it is safe to bring into streets and homes.
3. The cables after a step-down transformer carry a larger current, but they are short, so the extra energy dissipated in them stays small.
4. Transformers waste a small amount of energy themselves, but far less than would be wasted by sending a large current over hundreds of kilometres.
5. Using high voltage for long distances and low voltage for the final short distances gives the most efficient and safe system overall.

8.2.6 Transformers and power transmission at high voltage

Coils and turns

1. A transformer has two coils of insulated wire wrapped around a shared iron core.
2. The primary coil is connected to the alternating supply, so it is the coil that power enters.
3. The secondary coil is connected to the output circuit, so it is the coil that power leaves.
4. Each coil is made of many loops of wire, and each complete loop is called one turn.
5. The number of turns on the primary coil is written $N_p$ and the number on the secondary coil is written $N_s$.
6. The potential difference across the primary coil is written $V_p$ and the one across the secondary coil is written $V_s$.

The turns ratio equation

1. The ratio of the two potential differences is equal to the ratio of the numbers of turns on the two coils.
2. This is written as $\frac{V_p}{V_s}=\frac{N_p}{N_s}$, where both potential differences are in volts and the turns are plain numbers with no unit.
3. If the secondary coil has more turns than the primary coil ($N_s>N_p$), the secondary potential difference is larger, so the transformer is step-up.
4. If the secondary coil has fewer turns than the primary coil ($N_s<N_p$), the secondary potential difference is smaller, so the transformer is step-down.
5. A secondary coil with $16$ times as many turns as the primary therefore gives a secondary potential difference $16$ times larger.
6. Both potential differences must be in the same unit before you substitute, so either convert both to volts or keep both in kilovolts.
7. Rearranging for the secondary potential difference gives $V_s=V_p\times \frac{N_s}{N_p}$.

Power in a transformer

1. A transformer that wastes no energy is described as $100\%$ efficient.
2. In such a transformer, the power supplied to the primary coil equals the power delivered by the secondary coil, because energy is conserved.
3. Power is potential difference multiplied by current, so this gives $V_p{I}_p=V_s{I}_s$.
4. In this equation $I_p$ is the current in the primary coil and $I_s$ is the current in the secondary coil, both in amperes.
5. The product of potential difference and current stays the same, so if a transformer raises the potential difference by a factor, it lowers the current by the same factor.
6. A step-up transformer with $16$ times as many secondary turns therefore gives $16$ times the potential difference and one sixteenth of the current.
7. A step-down transformer does the reverse, lowering the potential difference and raising the current.
8. Real grid transformers are slightly less than $100\%$ efficient, so their output power is a little smaller than their input power.

Advantages of high-voltage transmission

1. The power dissipated as heat in a transmission cable is $P=I^{2}R$, so it depends on the square of the current.
2. A step-up transformer that raises the potential difference by a factor $n$ lowers the current by a factor $n$, which reduces the heating in the cables by a factor $n^2$.
3. In the grid transformer above, the current falls by a factor of $16$, so the heating in the cables falls by a factor of $16^2=256$.
4. Less energy is dissipated, so more of the energy generated reaches consumers and the transmission is more efficient.
5. Less fuel has to be burned to deliver the same energy to homes, which lowers costs and carbon dioxide emissions.
6. A smaller current can be carried safely by thinner, lighter cables, which allows lighter pylons spaced further apart.
7. The drawbacks are that transformers, tall pylons and long insulators are expensive, and a very high potential difference is dangerous, but the energy saved over the life of the grid outweighs these costs.

8.2.7 The UK domestic supply

The UK mains supply

1. The electricity that reaches the sockets and lights of a UK home through the national grid is called the mains supply or domestic supply.
2. Three facts describe it, and you need to recall all three together.
3. The supply is alternating current, written a.c., which means the current repeatedly reverses its direction.
4. Its frequency is $50Hz$.
5. Its potential difference is about $230V$.
6. Every home in the UK receives a supply with these values, so an appliance bought anywhere in the country works in any socket.
7. Other countries use different values, for example the USA supplies about $120V$ at $60Hz$, which is why some appliances bought abroad will not work properly in the UK.

Frequency of the supply

1. Frequency is measured in hertz ($Hz$), where $1Hz$ means one complete cycle each second.
2. A frequency of $50Hz$ means the mains supply completes $50$ full cycles every second.
3. In each cycle the current flows one way and then the other way, so it reverses direction $100$ times every second.
4. Each cycle lasts $\frac{1}{50}s=0.02s$, which is the period of the mains supply.
5. The reversals are far too fast for you to notice, so a lamp on the mains appears to glow steadily.
6. The company that runs the grid keeps the frequency very close to $50Hz$ by matching the output of power stations to the demand at every moment.
7. If demand suddenly rises faster than generators can respond, the frequency dips very slightly below $50Hz$, which signals that more power stations must raise their output.

Potential difference of the supply

1. The potential difference of the mains supply is about $230V$, which is roughly $150$ times larger than the $1.5V$ of an AA cell.
2. The word about matters, because the actual value at a socket varies a little during the day as demand changes.
3. UK rules allow the supply to range from about $216V$ to $253V$, so $230V$ is the nominal value around which the supply is kept.
4. Before 1995 the UK quoted its mains supply as $240V$, so older books and appliances sometimes give that figure.
5. A potential difference this large lets mains appliances transfer energy quickly, which is why kettles, ovens and heaters run from the mains rather than from batteries.
6. The same potential difference is also large enough to drive a dangerous current through a person, which is why mains wiring is always insulated.

8.2.8 Direct and alternating potential difference

Direct potential difference

1. Cells, batteries and solar cells all produce a direct potential difference.
2. One terminal of a cell is always positive and the other is always negative, so the potential difference always acts in the same direction.
3. A direct potential difference drives a direct current (d.c.), in which charge flows around the circuit in one direction only.
4. The size of a direct potential difference can change, for example as a battery runs flat, but its direction never reverses.
5. Direction, not size, is therefore what makes a supply direct.
6. Phones, laptops, torches and other portable devices all run on d.c. from their batteries.

Alternating potential difference

1. The generators in power stations produce an alternating potential difference, and this is what the mains supply delivers to homes.
2. The two terminals of an alternating supply repeatedly swap over, so each one is positive for a moment and then negative.
3. An alternating potential difference drives an alternating current (a.c.), in which the charge flows one way, then the opposite way, again and again.
4. During each cycle, the potential difference rises from zero to a maximum in one direction, falls back to zero, rises to a maximum in the opposite direction and returns to zero.
5. An alternating potential difference is therefore changing in both size and direction all the time.
6. The UK mains supply completes $50$ of these cycles every second.
7. The national grid uses a.c. because transformers need an alternating supply to change the potential difference.
8. Devices that need d.c. but plug into the mains, such as phone chargers, contain circuits that convert the alternating supply into a direct one.

Showing the difference on a graph

1. The clearest way to see the difference between the two supplies is a graph of potential difference against time, called a trace.
2. A trace can be recorded with a voltage sensor connected to a data logger, which takes hundreds of readings each second and plots them on a screen.
3. The vertical axis shows potential difference in volts, the horizontal axis shows time, and a horizontal zero line marks $0V$.
4. Points above the zero line show the potential difference acting in one direction, and points below it show the potential difference acting in the opposite direction.
5. A steady d.c. supply gives a flat horizontal line that stays on one side of the zero line for the whole trace.
6. An a.c. supply gives a repeating wave that crosses the zero line again and again, showing that its direction keeps reversing.
7. The height of the wave above the zero line is the peak potential difference, the largest value the supply reaches.
8. The time taken for one complete wave is the period of the supply, and the number of complete waves each second is its frequency.

8.2.9 Live, neutral and earth wires

Three wires in a mains cable

1. Most mains appliances connect to a socket through a plug and a flexible three-core cable.
2. Each core is a bundle of fine copper wires surrounded by its own layer of coloured plastic insulation, and an outer plastic sheath holds the three cores together.
3. The live wire has brown insulation.
4. The neutral wire has blue insulation.
5. The earth wire has green and yellow stripes.
6. The colours let an electrician identify each wire instantly, so the wires are never connected to the wrong pins.
7. Each of the three wires does a different job in the circuit.

The live wire

1. The live wire brings the alternating potential difference from the mains supply into the appliance.
2. Its potential alternates continuously, but it stays at about $230V$ relative to the earth.
3. The switch that turns an appliance on and off is connected in the live wire.
4. The live wire is the most dangerous wire in the circuit, because touching it can let a current flow through your body.

The neutral wire

1. The neutral wire completes the circuit by carrying the current from the appliance back to the supply.
2. In normal use the current flows into the appliance through the live wire and out through the neutral wire, so the two wires carry the same current.
3. The neutral wire is joined to the earth at the local substation, which keeps it at or very close to $0V$.

The earth wire

1. The earth wire is a safety wire that joins the metal case of an appliance, through the earth pin of the plug, to a connection in the ground outside the building.
2. It holds the metal case at $0V$, the same potential as the ground you are standing on.
3. In normal use the earth wire carries no current.
4. If a fault makes the live wire touch the metal case, a large current flows through the low-resistance earth wire into the ground instead of the case staying at $230V$.
5. The case therefore cannot reach a dangerous potential, so a person who touches it does not receive a shock.
6. Appliances with metal cases, such as washing machines, fridges and electric ovens, must have an earth wire.

Potential differences between the wires

1. The potential difference between two wires is the difference between their potentials.
2. The potential difference between the live and neutral wires is about $230V$, and this is what drives the current through an appliance.
3. The potential difference between the live and earth wires is also about $230V$, because the earth wire is at $0V$ like the neutral wire.
4. The potential difference between the neutral and earth wires is about $0V$, because both are held at the potential of the ground.
5. A current flows only along a conducting path that joins two points with a potential difference between them.
6. A path between live and neutral is the normal working circuit, while a path between live and earth is a fault or a shock.
7. No current would flow along a path between neutral and earth, because there is no potential difference to drive it.

8.2.10 Dangers of mains electricity

Why touching a live wire is dangerous

1. The human body conducts electricity, mainly because it contains a lot of salty water.
2. When you stand on the floor or the ground, your body is at about $0V$, the same potential as the earth.
3. If you touch the live wire, there is a potential difference of about $230V$ across your body.
4. That potential difference drives a current through you, from the point of contact down to the ground.
5. The current makes your muscles contract uncontrollably, which can stop you letting go of the wire.
6. It also heats the tissue it passes through, causing deep burns.
7. A current of only a few tens of milliamperes passing through the chest can upset the rhythm of the heart and kill.
8. Wet skin has a much lower resistance than dry skin, so the same $230V$ drives a much larger current, which is why water and mains electricity are such a dangerous mix.

Danger when a switch is open

1. Opening a switch makes a gap in the circuit, so no current flows through the appliance and it turns off.
2. The gap does not disconnect the wiring between the switch and the supply, so that wiring stays connected to the live supply.
3. The live pin of the plug, the live contact in the socket and the live wire up to the switch therefore all stay at about $230V$ relative to earth.
4. A wire does not need to be carrying a current to be dangerous, because the danger comes from its potential difference relative to earth.
5. If you touch a live part while the switch is open, your body completes a new circuit to earth, and a current flows through you.
6. If a switch is wrongly fitted in the neutral wire, opening it turns the appliance off but leaves every part inside it connected to the live wire.
7. This is why the switch is always placed in the live wire, so that opening it cuts the appliance off from the live supply.
8. Even so, the safe way to work on an appliance or a light fitting is to unplug it or switch off the supply at the consumer unit, rather than relying on the wall switch.

Connecting the live wire to earth

1. Any conducting connection between the live wire and earth gives a current a path to flow along, driven by the $230V$ potential difference.
2. If the connection is a person, the current flows through their body and causes an electric shock.
3. If the connection is a metal object or a bare wire, its resistance is very low, so a very large current flows.
4. A very large current heats the wiring rapidly, which can melt insulation, cause sparks and start a fire.
5. Connections like this can happen when a drill bit cuts into a cable hidden in a wall, when a metal tool is pushed into a socket, when water gets inside an appliance or when worn insulation lets a live wire touch a metal pipe.
6. No connection of any kind should ever be made between the live wire and earth, except through the protective earth wire of an appliance during a fault.

Protection by insulation

1. Every core in a mains cable is coated in plastic, an electrical insulator that stops charge passing from the live wire into anything it touches.
2. The outer sheath of the cable and the casing of the plug add a second layer of insulation around the wires.
3. Many appliances, such as hairdryers, vacuum cleaners and phone chargers, are double insulated, so their outer casing is made entirely of plastic with no exposed metal.
4. A double insulated appliance has only two cores in its cable, live and neutral, because there is no metal case that needs an earth wire.
5. Double insulated appliances carry a symbol of one square drawn inside another.
6. Insulation protects you only while it is undamaged, so frayed cables, cracked plugs and scorched sockets must be replaced, and appliances should never be handled with wet hands.
7. Bathrooms use pull-cord light switches and have no ordinary sockets, so that wet hands cannot come close to a live part.