[III] Physics - P3

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  • Created on: 16-03-14 10:41

Primary and secondary energy sources

Primary energy sources are those that are used in the form in which they are found. They may be processed, or refined, but their energy has not been transferred from other energy stores. Examples of primary energy sources include:

- Fossil fuels (coal, oil and gas)

- Nuclear fuels (uranium and plutonium)

- Biofuels (plant and animal material used as fuels)

- Wind

- Waves

- Radiation from the Sun

Secondary energy sources are produced from primary energy sources. Electricity is a secondary energy source because it is generated from primary energy sources. Energy stored in fossil fuels, for example, is transferred to electrical energy.

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Our changing use of energy

During the 1800s, our use of energy sources began to increase. Factories started making things in large quantities. 

By the 1880s it was possible to generate and distribute electricity on a large scale. Our use of energy sources then increased dramatically, especially in the 20th century. A bigger percentage of an increasing world population used electricity in their daily lives. To cope with these growing energy needs, more and more fossil fuels were extracted and burned.

In 1850, coal provided about 10% of our energy needs, with 90% coming from wood. Coal provided about three-quarters of our energy needs in 1910. In 1970, coal only provided 20% of our energy with nearly 50% coming from oil. Nuclear energy started providing significant amounts of energy from 1970 onwards.

Fossil fuels take millions of years to form, and we are now using them much quicker than they can be replaced. Stores of fossil fuels are becoming harder to find and more expesive to extract. At some point, it will not be economic to use fossil fuels in power stations because they will become too expensive. Reserves of fossil fuels may become too small to be used for generating large amounts of power.

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The problems with fossil fuel use

Burning fossil fuels produces carbon dioxide, which is a GREENHOUSE GAS. As more greenhouse gases are produced worldwide, global temperatures have gradually risen. It is believed that this global warming causes climate change and erratic weather patterns bringing, for example, droughts in some places and floods in others.

There are other environmental effects caused by using fossil fuels:

- Damage to local areas from coal mining

- Production of large quantities of flue ash, which contains small amounts of toxic elements such as heavy metals and traces of radioactive elements.

- Oil spills when drilling for oil and transporting it

- Production of sulfur dioxide, which contributes to acid rain. 

Recognising the damage we are causing and designing better methods of extracting and transporting fuels can reduce the impact of these activities. Safety features, more efficient extraction techniques and proper treatments of waste will all help. Whatever energy source is used to generate electricity on a large scale, there are environmental effects.

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Measuring energy transferred

When you listen to music on a radio, electrical energy is being transferred. The amount of energy transferred each second is the radio's power. 

If the radio is on for a period of time, the energy transferred is its power multiplied by the time: 

energy transferred = power x time

The units used in this equation depend on the equipment and how long it is switched on for. Power is measured in watts (W) if energy is measured in joules (J) and time is measured in seconds (s). 

For example if the power of an analogue radio was 2 watts, each second it would transfer 2 joules of energy. An 8W digital radio would be more powerful, transferring 8 joules every second. Power is measured in kilowatts (kW) if energy is measured in kilojoules (kJ) and time is measured in seconds (s). A kilojoule is 1000 joules. 

The amount of energy transferred is measured in kilowatt-hours (kWh) if the power is measured in kilowatts and time is measured in hours. For example, if the power of a fridge is 50W, or 0.05W, every day it uses 0.05 x 24 hours = 1.2 kilowatt-hours.

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Calculating power

When a kettle is turned on, an electric current flows through the circuit transferring energy from the power supply to the kettle. The power of an electrical device like a kettle is calculated using:

power (watts, W) = voltage (voltage, V) x current (amps, A)

Voltage is supplied by the mains supply or a battery. A higher voltage means more energy is transferred. The voltage of the mains supply in the UK is 230V; the voltage of many batteries is much lower at 1.5V.

Current is the flow of electricity through a circuit. It is measured in amps (A). A large current means more electricity flows in the circuit, so the current can carry more energy. 

For example, the power of a kettle may be 2.3kW, the mains voltage is 230V, and the current in the kettle element 10A. 

2300W = 230V X 10A

Power measures how quickly energy is transferred by a device. This is the rate at which it transfers energy. The rate of energy transfer is quicker at higher powers. The power of a device is usually given as a rating.

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Transferring energy

An electric current transfers energy from one part of a circuit to another. Whether electricity is generated in a power station or a battery, energy is transferred as the current flows around the circuit. A component (or device) is a part of the circuit that transfers electrical energy into a different useful form. For example:

- a bulb transfers electrical energy to the environment in the form of light and also heat.

- a motor in a food mixer transfers electrical energy into the kinetic energy of the blades and food mix, and also heat and sound to the environment.

Whenever an energy transfer takes place, the component heats up by a certain amount. The wires and cables of a circuit also heat up slightly as current flows through them. The energy transfer is not always wanted because it transfers some energy to warm the surroundings - this is not useful. 

Other unwanted energy transfers result in noise and vibration. After several energy transfers, all the energy from the power supply spreads to the environment.

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Buying electricity

All homes are fitted with an electricity meter,which records the amount of electricity used. It is not practical to measure the large amount of electrical energy used at home in joules, or even kilojoules, so we use a kilowatt-hours instead. One kilowatt-hour is equal to 3.6 million joules - the energy used by a 100 watt bulb in 10 hours. Kilowatt-hours are called units by electicty supply companies. 

Meter readings help us to work out how much our electricity bill will be. The electricity meter shows how many units of electricity have been used. The number of units used since the last reading wis the current reading minus the previous reading. The cost of units used is price per unit x number of units used.

It is useful to work out the cost of using a single piece of equipment: 

the amount of electricity supplied in kilowatt-hours is the power (in kilo-watts) x time in hours.

A 2kW electric fire switched on for 5 hours uses 10kWh (2 x 5 = 10)

A 100W bulb switched on for 10 hours uses 1kWh (0.1 x 10 = 1)

A 2kW kettle switched on for 15 minutes uses 0.5kW (2kW x 0.25 hours)

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Energy diagrams

Information about energy use can be provided in different ways.

A pie chart shows proportions. They can show how the total energy used in the UK is shared between different purposes. 

A bar chart is also used for comparisons, but can show actual amounts. For example, it could compare the powers of different pieces of household electrical equipment.

A graph showing the change in use of energy over a period of time can be very useful for comparing data. It can show how the overall world energy demand has grown, as well as showing the trends for individual energy sources. 

A SANKEY diagram shows how energy supplied to a device is transferred into different forms of energy. It is assumed that energy is conserved so the energy input and the energy output match. For example, a filament lightbulb of 100J electrical energy changes into 5J or light energy and 95J of heat energy. 

The useful energy (eg: light) is shown in a straight arrow and the wasteful energy (eg: heat) are shown sloping down. The size of the arrow correlates to the amount of energy.

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Nearly 200 years ago, Michael Faraday realised that moving a magnet near a circuit caused an electric current to flow in the circuit. The current flowed only when the magnetic field was changing - that is, when the magnet was moving.

When the magnet moves into or out of the coil, a current flows in the coil.

Power stations use this idea to produce mains electricity. The magnet and wire coil are called an electric generator. An ELECTROMAGNET constatntly spins inside the coil. An electromagnet is used because its magnetism is much stronger than a permanent magnet. 

Power stations generate electricity using the same idea as this simple generator - whereby the coil spins and the magnets are stationary. 

A power station generator has two main sections:

A STATIONARY COIL made of hundreds of turns of wire - the stator.


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Electricity production

When the electromagnet spins, it produces a voltage across the coil of wire. If the electromagnet spins faster, a larger voltage is produced. When the generator is connected to an outside circuit, a current flows in the circuit.

Primary fuels such as fossil fuels, nuclear fuels or biofuels, or other primary energy sources such as wind or moving water, provide the energy to turn the turbines, which make the rotor spin. When a larger current is needed, the rotor is made to spin faster, or more generators can be used. More primary fuel is needed every second for this to happen.

Generators in power stations supply their electricity to a national network of cables (the National Grid). For our electricity supply to be safe, reliable and consistent across this network, all power stations have to conform to the same set of standards. As new methods of generating electricity are developed, regulations are needed to ensure that these standards are being met, and that safety standards are acceptable. New developments in power station technology include:

- ways of reducing pollution levels

- more efficient power stations

- new ways of generating electricity

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How power stations work

A TURBINE is basically a set of blades that are linked to a generator by an axle. The blades spin when water, steam, gas or wind are aimed at it. Fossil-fuel power stations and nuclear power stations use jets of steam to spin turbines. The turbine axle is connected to the electromagnet in the generator, which is made to spin inside the coil, generating electricity. 

When fossil fuels are burned, or when fission reactions take place in a nuclear reactor, heat is produced. This heats water in pipes, which produce jets of steam to turn the turbine. In a coal-fired power station, coal is burned in the furnace. Heat from the burning coal heats water in a set of pipes. Steam is prodiced and this is used to spin turbines. 

Coal burnt in furnace ---> Water heated to produce steam ---> Jet of steam turns a turbine ---> Turbine spins the generator ---> Electricity produced by generator

Processes in a coal-fired power station. Oil-fired power stations are very similar - oil is burned instead of coal to produce heat. They are called "thermal" power stations.

Gas-fired power stations have two sets of turbines. When the gas is burned, one set of turbines spins when jets of hot burning gases flow over them. These hot gases are used to change water into steam, which spins a second set of turbines. Gas-fired power stations are much more efficient than coal or oil-fired power stations.

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Nuclear power stations

Turbines are forced to spin in a different way in hydroelectric power stations. Water is trapped behind a dam. When gates are opened, water rushes down pipes and through the turbines at the base of the dam. This generates electricity very quickly. 

The REACTOR contains nuclear fuel rods, which produce heat. Control rods are raised or lowered to control the amount of heat produced. Water circulating through pipes leaving the reactor is put under high pressure which means it can be heated to high temperatures. A second set of pipes filled with water surrounds these heated pipes. Heat is transferred to water inside the second set of pipes, changing it into steam.

Fossil fuels are burnt to release heat. Nuclear fuels do not burn - a nuclear FISSION reaction takes place which releases heat. 

Nuclear power stations use uranium and plutonium to generate electricity. After nuclear fuels have been used, the waste products are radioactive. This means that the waste emits ionising radiation, which may affect molecules and cells, and can pose a serious health risk. Some of the waste can be recycled and reused in power stations, but ost of it needs to be stored safely until the radioactivity dies away naturally. Different types of radioactive waste from nuclear power stations have different strengths, so will be treated and stored differently. 

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Dealing with radioactive waste

93% of the volume of waste does not need special handling because its radioactivity is so low.

Less than 1% of the volume of waste contains 95% of the radioactivity fro nuclear power stations. This is combined into solid glass (by heating it with helted glass and allowing it to cool). Then it is kept cool in large tanks of water and eventually stored underground in sealed stainless steel containers. After about 40 years, the radioactivity typically falls to 1/1000th of its original amount.

If an object is in the path of ionising radiation, it will be IRRADIATED - as soon as the object moves out of the path, it stops being irradiated. The object does noto become radioactive. Thick shielding around a radioactive object stops it irradiating its surrounding. Radioactive waste is stored in specifically built underground storage units because the ground provides shielding.

Something is CONTAMINATED if radioactive gases, liquids, or particles mix with it, making the material radioactive. Contamination is hard to remove when particles of both materials have mixed. Radioactive waste is turned into solid gases or mixed with concrete and contaminated in steel drums so it cannot contaminate its surroundings.

Contamination is a bigger health threat than irradiation because it is hard to limit how far the contamination spreads and what it spreads to, it is harder to clear up contamination than to prevent irradiation and it leaves substances radioactive, increasing exposure to radiation.

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Understanding risks

Ionising radiation cannot be seen and its effects are not immediately obvious, so people tend to overestimate the risk it causes. We underestimate familiar risks such as health risks from acid rain, smog and other pollution from fossil fuels. It was recently estimated that 50,000 people die early each year in the UK as a result of pollution from fossil fuels.

We tend to trust familiar technology and to worry about unfamiliar technology. Coal-fired power stations do not have to limit their radioactive emissions but nuclear power stations do. Coal contains traces of uranium, so the area surrounding coal-fired power stations is several ties more radioactive than the area surrounding nuclear power stations. It is still much lower than radioactivity from natural causes and is not believed to be a health risk.

Statistics provide information that can be used to help compare different risks.

Using statistics helps us to select the risks that cause problems, and which need to be controlled, rather than worrying about risks that are not a problem.

Risks from coal-fired stations may be greater than those from nuclear power stations but they are viewed differently. 

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Renewable energy sources

Renewable energy sources will not run out. About 7% of the UK's electricity comes from renewable energy sources. Some schemes involving spinning turbines directly include: hydroelectric turbines, wind turbines and wave technology. Wave technology is still being developed for widespread commercial use.

Hydroelectric schemes use dams to trap river water in a reservoir. When water is released, it spins the turbines directly. 

Across the world, hydroelectric schemes are providing large amounts of electricity. In the UK, hydroelectricity tops up surges of demand for electricity. When demand for electricity is low, water is pumped back up to the reservoir. 

However, large areas are flooded, and downstream vegetation on the sides of the river die and rot, releasing greenhouse gases. Although hydroelectricity can seem cheap to run, it is costly to build and the dams require maintenance. 

Wind turbines use a set of blades to trap the energy from wind and spin a turbine directly. Wind turbines are tall and are built in windy locations both on land and offshore. The amount of electricit generated depends on how many wind turbines there are and whether it is windy enough.

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Unintended consequences

One problem is that winds are unreliable. Demand for electricity continues even if the weather conditions are wrong, so thermal power stations must be available too. One future solution may be to store surplus energy in giant batteries.

Wave power is useful on remote islands that cannot use other methods to generate electricity. The waves spin turbines directly, using structures moving in the water or on the shoreline. It is not easy to capture the energy from waves, so this method is not yet used to generate electricity widely. The technology is still being developed.

A reliable electricity supply is fundamental to our way of life, but it has had an impact on the environment. The benefits of using renewable energy sources should help to reduce these impacts. However, there are unintended consequences - especially when renewable energy sources are used widely. Careful planning can reduce these effects:

- The impact of noise and visual pollution is reduced by building offshore wind farms

- Careful siting and use of hydroelectric dams reduces the impact of flooding and disruption to normal river-flow patterns.

We need to weigh up the benefits against the costs when evaluating renewable power.

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The National Grid

The National Grid is a network of electric cables that supply electricity throughout the UK. It's job is to carry electricity from power stations to all users. Electricity is a convinient form of energy because it can be transmitted over long distances. When it is used it does not pollute and can be used in many ways.

Energy is wasted at all stages in the generation and transmission of electricity. When power cables carry an electric current they warm up. Over hundreds of miles, a significant amount of energy is transferred to the surrounding air. Transmitting electricity using a very small current reduces this energy loss, but a very high voltage is needed for enough electrical energy to be transmitted. This can be up to 400,000V. The voltage supplied to homes is 230V, which is much lower for safety reasons. Very high voltages are reduced in substations before they reach homes.

Electricity is supplied at 230V in the UK because this is large enough to supply useful amounts of energy. However, this voltage can electrocute people if equipment is faulty or misused. Some countries use lower voltages, but equipment overheating may be more likely because the current is larger if the same amount of energy is supplied. 

Energy from fuels and other sources is transformed into electricity in power stations. The energy wasted at each stage of generation can be shown in a Sankey diagram. 

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Advantages of energy sources

The efficiency of electricity generation (including the efficiency of the National Grid) is the proportion of the energy in the original fuel that is finally delivered to customers as electricity. 

The efficiency of a particular power station does not take into account the energy wasted in the transmission system. 

The advantages of different energy sources are:

- fossil fuels, nuclear power, hydroelectricity and biolfuels generate large amounts of electricity when needed.

- nuclear power and energy from water, waves and the Sun do not produce greenhouse gases.

- energy from water, wind and the Sun are free once the power stations are built.

- renewable energy sources will not run out.

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Disadvantages of energy sources

All energy sources have disadvantages:

- wind, waves and solar power are weather dependent, producing small amounts of energy.

- coal mining and drilling for oil and gas are dangerous occupations, causing deaths every year.

- hydroelectricity and tidal barrages need suitable sites.

Some disadvantages affect the environment:

- fossil fuels and biofuels produce greenhouse gases

- extracting and mining large quantities of fossil fuels damages the landscape, causing pollution and even oil spills.

- nuclear power creates radioactive waste.

- hydroelectricity and tidal barrages flood large areas, disrupting normal river flow.

- large scale wind farms cause visual and noise pollution.

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Choosing an energy source

Our choice of energy source depends on: the IMPACT of the energy source on the environment, how EXPENSIVE it is to build and run power stations, the QUANTITY and TYPE of waste products produced and whether or not it PRODUCES CARBON DIOXIDE.

Many power stations use fossil fuels or nuclear power to produce large amounts of electricity reliably and cost effectively. Small and remote islands often use small-scale renewable energy sources to reduce the environmental impact and waste produced. But renewable energy is expensive and less reliable.

Our choice of electricity generation needs to balance the benefits with the costs and the other disadvantages. If we choose to use new technology to create electricity, we may have more power cuts if the technology is unreliable and much larger electricity bills to cover development costs. These can affect vulnerable people badly.

Different types of power stations produce different amounts of electrical energy. The power output of power stations is stated in megawatts (MW) - one megawatt is one million watts and is the energy in Joules produces every second. Nuclear power stations produce about 1000MW, compared with fossil fuel power stations at 500-2000MW. The power output of wind farms varies with the weather. Wind farms are designed to produce up to 300MW, but most produce 10-50%.

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Reducing energy demand

The setting-up cost of a power station can be huge, so the lifetime of a power station is another important consideration. The lifetime of nuclear power stations was originally planned to be around 30-40 years, but this has been extended as new technology has developed. Coal-fired power stations are also designed to last 40 or more years, and hydroelectric power stations are designed to last for up to 100 years. Wind turbines have an expected life of 20 years, but many individual turbines can be built for the cost of one large power station. 

Changes must be made to reduce the world's energy demands. When people make changes at home, the effect is very small. However, small changes made by everyone add up - switching on lights only when needed and turning off equipment can make a big difference. Changes made by businesses have a bigger effect.

Improved efficiency of production and of vehicle engines reduces the fuel used and emissions released.

The UK produces only 2% of the world's carbon dioxide emissions, but global warming is a worldwide problem. All countries must make some changes to ensure that emissions fall across the world. Many industrialised countries have agreed to limit their carbon dioxide emissions, setting targets together.

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Reducing our emissions

Energy demand is increasing, causing emissions of greenhouse gases to increase. Ideally we would produce our energy from sources that don't produce emissions, but it is unlikely that these can meet all our energy needs. As the world's population increases, so does the demand for energy. To reduce, or even stabilise our emissions, we need to significantly reduce the energy demand per person.

People in different countries diagree about who should reduce emissions and how this should be achieved. Emissions from many developing countries are increasing as they become more industrialised. They often rely on older, dirtier technology because it is cheaper. Individuals use more energy as their lifestyle improves. Reducing emissions from these countries may unfairly limit access to healthcare and education. Running schools and hospitals uses electricity and other energy-hungry resources.

Wealthy industrialised countries increasingly use cleaner technology, but current pollution levels are due to them having used less efficient technology in the past. Such countries often import goods made elsewhere, and they should take responsibility for the carbon emissions these create. There is a real risk that imposing the same carbon dioxide emission targets for everyone will result in industrialised countries consuming as much as before, while developing countries make sacrifices to cover the emissions caused by the production of exported goods. 

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Achieving a secure energy supply

If all countries reduced carbon dioxide limits by the same amount, the impact of the greenhouse effect would be reduced, benefitting most people. However, this would be achieved by developing countries sacrificing their increased standards of living. Any international decision on emissions targets that affects economic and social development in some countries is controversial.

Even if we manage to reduce our energy demands, we will still need vast amounts of electricity to sustain our lifestyles. We must reliably generate enough electricity now and in the future to avoid power cuts and to keep electricity affordable. We need enough power stations, and constant access to sources of energy. Many power stations are reaching the end of their planned lives and will need to be replaced.

Our supplies of fossil fuels are running low, and even though we can import fossil fuels from elsewhere, the cost of this would be controlled by other countries. In any case, to reduce carbon dioxide emissions, we must depend less on fossil fuels. It is likely that a mix of cleaner thermal power stations and nuclear power stations along with power generation using renewable energy, such as hydroelectricity and wind, will provide our future energy needs. Until renewable energy technology develops further (providing cheap, reliable energy), fossil fuel power stations will continue to provide a significant amount of our electricity.

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