[II] Physics - P2

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  • Created on: 02-03-14 10:52

Electromagnetic radiation

Light is a form of ELECTROMAGNETIC RADIATION. Sources of light include things that glow such as the Sun, lightbulbs and fires. These sources emit (produce) radiation that travels outwards in all directions. When the light meets an object in its path, the way it behaves depends on the material the object is made from. 

Objects have different colours because they absorb reflect different colours, or frequencies, of the light that lands on them. Light-coloured objeccts reflect most of the light that falls on them, whereas dull, black objects absorb most or all of the light.

We can only see objects that emit or reflect light. Light that reaches our eyes from objects is absorbed by special cells in the eye. Our eyes are examples of detectors because they respond to the light. For something to detect light, a change must happen when light is absorbed. Many objects absorb light but are not detectors. 

A lit candle is a source of light, but mirrors, stars and the Moon are all reflectors.

Almost all light passes through glass and air. Light is transmitted well through transparent materials. Only some light passes through clouded glass and coloured filters. Light is partly absorbed and partly transmitted by translucent materials. Shiny objects reflect most of the light.

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The electromagnetic spectrum

Light belongs to the electromagnetic spectrum, a family of electromagnetic waves that travel st a speed of 300,000km/s through space, which is a vacuum. 

The behaviour of electromagnetic radiation depends on the frequency of the waves. Waves with higher frequencies carry more energy. Electromagnetic waves are groups in ranges of frequency. The types of electromagnetic radiation are:

radio waves > microwaves > infrared > visible light > ultraviolet > xrays > gamma rays

Radio/TV waves have the lowest frequency and lowest energy and Gamma rays have the highest frequency and highest energy. Radio waves do not harm us because they have a low frequency and carry little energy. Ultra-violet radiation from the Sun causes sunburn and skin cancer as it has a much higher frequency and carries much more energy.

In the 20th century, scientists developed a new model for light. They found evidence that not only did it behave as a wave, but it also in some ways behaved as a stream of particles. These "particles" of light are packets of energy called photons. Photons have no mass. Light - and all other types of electromagnetic radiation - can be thought of as a stream of photons. The energy carried by each photon depends on the frequency of the radiation. 

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Radiation intensity

The Sun is our closest star, and its radiation can cause skin cancer, heat stroke and even kill us. Massive stars in our galaxy emit much greater quantities of damaging electromagnetic radiation which eventually reaches the Earth. This does not harm us, because the intensity of the radiation is so small after it has travelled many light-years to reach the Earth.

Solar cells produce electricity. They work by absorbing electromagnetic radiation. The solar cell's surface absorbs some of the energy carried by sunlight and transfers it to electrical energy. The amount of energy absorbed by the solar cell depends on the strength (or intensity) of the radiation arriving at its surface.

The intensity of a beam of radiation is a measure of the energy transferred each second. Electromagnetic radiation transfers energy by photons. The energy arriving per second at a surface from the beam of radiation depends on: the number of photons arriving per second and the energy transferred by the individual photons. 

In winter, sunlight is less intense than in summer. There are fewer photons arriving per second on the surface of a solar cell, so the solar cell absorbs less energy. The energy carrid by individual photons depends on the type of radiation. Photons of ultraviolet radiation have a higher frequency and more energy than photons of infrared radiation. 

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Distance and radiation intensity

The intensity of electromagnetic radiation gets less as you move further away from the source. The intensity of the Sun's radiation decreases further away from it as it is spread over a larger area. This is why planets more distant from the Sun than the Earth are much cooler than our planet. The Earth receives more of the Sun's electromagnetic radiation because the intensity of the radiation is closer to the Sun. The complete definition of the intensity of a beam of electromagnetic radiation is the energy it transfers every second per square metre of the surface:

intensity = energy transferred per second per m²

If you move a torch closer to a surface, its light spreads over a smaller area and becomes brighter. The total energy from the torch each second is the same, but the intensity of the light on the surface area increases. Move the torch away again and the intensity of light on the surface decreases. This happens because as the distance doubles, the area the energy spreads over increases four-fold and so the intensity decreases four-fold. When you shine a torch into a pool of water, the beam of light does not reach very far. The intensity falls to almost 0 in a short distance. The light energy is absorbed by molecules in the water as the light travels through it. Some materials absorb electromagnetic radiation more than others. Light does not travel as far in water as it does in air. The type of electromagnetic radiation also affects how well it is absorbed. 

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Everything is made up from the basic building blocks of matter: atoms, molecules and ions. These basic building blocks are formed from even smaller particles, which include electrons. The building blocks are tiny - if you lined up one million atoms, the line would be less than a mm long.

ATOMS are the smallest particles of an element. Pure elements like copper contain just one type of atom.

MOLECULES are the smallest part of a substance, and are formed from more than one atom joined together. Compounds are formed from two or more types of atoms.

IONS are charged particles, formed when atoms or molecules are broken into smaller pieces. When an ion forms, either electrons are knocked out of an atom or a molecule, or electrons join onto an atom or molecule. Either way, charged particles are formed because electrons have a negative charge. This is called ionisation.

All ionshave a charge. Positively charged ions have lost electrons, and negatively charged ions have gained electrons.

When an atom of sodium loses an electron, it becomes a sodium ion.

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How electromagnetic radiation causes ionisation

When an atom absorbs a gamma ray photon, the photon may have enough energy to knock an electron out of the atom. A molecule that absorbs a gamma ray photon may break into bits, each bit being an ion. Gamma rays are a type of IONISING RADIATION. Other types of ionising radiation include x-rays, and high frequency ultra-violet radiation.

The photons of gamma rays have the highest frequency and so the greatest energy compared to the rest of the electromagnetic spectrum. Ionisation can also happen when a molecule absorbs x-rays and high-frequency ultraviolet radiation, as long as each photon has enough energy to knock an electron out of the molecule. 

Ionisation does not happen with visible light, microwaves, infrared or radio waves because the photon energy of each of these types of radiation is too small to knock electrons out of any molecules or atoms. 

If molecules in our cells are ionised, the processes that occur within the cells change. Ultraviolet rays from sunlight can damage skin cells and excess exposure can eventually lead to skin cancer. Too much exposure to X-rays can cause cancers to develop. Gamma rays can severely damages cells, causing cancers and even cell death. The damage to cells is greater when we are exposed for longer periods of time, or to a higher intensity of radiation. 

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Damage to cells from ionisation

The ions produced by ionising radiation take part in chemical reactions. Because they are charged particles, these reactions are different from reactions involving atoms and molecules. In our body cells, processes can go wrong. Damage to the DNA in our cells results in mutations and some of these are starting points for cancerous growths. DNA molecules in our cells are complex. Ionisation alters how they instruct our bodies to function.

GAMMA RAYS have the most energy of all electromagnetic radiation as they have the highest frequency. They are capable of ionising (knocking electrons out of) molecules of the material they pass through.

RADIOACTIVE materials emit gamma rays as well as other ionising radiation. Gemma rays penetrade easily into the human body, causing ionisation in cells. This changes the molecules in the cells, which can change the reactions taking place inside them. Over time, living cells may die or become cancerous because molecules in the cells are damaged.

XRAYS are slightly less energetic than gamma rays, but can still damage cells by ionising the molecules inside them. Bones absorb X-rays. This is why we can use X-ray shadow pictures to see if a bone has been damaged.

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Protection from ionising radiation

X-rays are used in many different ways. In hospitals, x-rays provide useful information about a patient's bones and tissues. In airports, x-rays are used to inspect objects in a passengers luggage without opening the case. The x-rays pass through the soft case and clothes, but are absorbed differently by metal objects and plastic objects. Detectors produce an image of what is inside the bags. 

People who work with x-rays need protecting from small doses of radiation over months or years. To reduce the damage to their cells, they leave the room while an x-ray is taken, or stand behind a special barrier that absorbs x-rays. Very energetic ionising radiation is best absorbed by heavy, dense materials such as thick lead and concrete.

Barriers built from lead or concrete protect people from exposure to x-rays and gamma rays. Radioactive materials are stored in lead containers. When an x-ray is taken, the rest of the patient's body is shielded with a lead apron to reduce damage to healthy cells nearby.

Radiographers prepare and analyse x-ray images, as well as treating patients with radiation as needed. They tend to wear a badge with a scale of "safe radiation amounts" to ensure that they do not cause damage to themselves.

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Assessing risks

People are more willing to accept risks if:

- They chose the risk rather than having it imposed upon them

- The effects are short-term rather than long lasting.

Radiation workers increase their risk of exposure to x-rays and gamma rays. They can choose whether to accept this risk. The risk can be controlled using dosimeters. These monitor a person's exposure to x-rays and gamma rays over time. Problems can be spotted before too much damage is done.

People who have mobile phone masts built near their homes have exposure to microwaves imposed on them. They tend to be less willing to accept any level of risk, and are more likely to overstate possible risk.

We are surrounded by ionising radiation all the time. Most of our exposure comes from radioactive radon gas seeping naturally from rocks and building materials in our surroundings, from the food we eat and from space (cosmic rays). The risk from radiation to health is very low, but people tend to overstate it because it is unfamiliar to us, it is invisible and hard to detect without special equipment and its effects can be long-lasting (eg: cancer).

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Radiation and heating

When a substance absorbs radiation it warms up because energy from the absorbed radiation is transferred to thermal (heat) energy. If living cells absorb radiation, this heating effect can damage the cells. More energy is absorbed if the radiation is very intense or the radiation is absorbed for a longer time.

Energy from microwave radiation is absorbed by food in a microwave oven. Water molecules in the food strongly absorb microwaves. The energy from the microwaves is transferred to thermal energy (the water molecules move and vibrate faster). This heat cooks the food. Settings on the microwave over alter how long the food is cooked for, and how intense the beams of microwaves are. Larger portions of food need a longer cooking time, or a higher setting.

Microwaves reflect off metal sheets and mesh. The oven is surrounded by a metal case, and the door has a metal screen. This ensures that microwaves stay inside the oven and do not affect people nearby.

The greatest risk from food cooked in microwave ovens is when food is not cooked or heated properly. Some parts of the food cook more quickly than others which is why we have a rotating plate for the food to sit on.

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Protection from microwaves

The risks to living cells from microwaves are very low because microwaves have a low frequency and carry small amounts of energy. Some people wonder if a long exposure to low levels of radiation can still be damaging to health. 

Mobile phones use low levels of microwave radiation. However, people use mobile phones for many years and can use them for several hours a day.

Mobile phone masts communicate with neighbouring masts and phones using low intensity microwave radiation. Although the mast constantly emits microwave radiation, the radiation intensity is extremely small at the base of the mast.

Studies carried out to assess if mobile phone use increases cancer have used different groups/samples of people. These included small groups (matched in as many ways as possible, but with different patterns of phone usage) and very large groups across many countries that have been chosen at random. 

No clear evidence of increased or decreased risk has been found. As more studies are carried out, scientists can be more confident that this conclusion is correct. These studies are important for Governments and public bodies to assess how much risk is acceptable.

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Ozone is a form of oxygen found in the outer layers of our atmosphere. Its molecules contain three oxygen atoms. Ozone is very effective at absorbing ultraviolet radiation from the Sun. This means that less ultraviolet radiation reaches Earth. Most ozone is found at about 32km above the Earth's surface.

Ultraviolet radiation causes sunburn and can lead to skin cancer and eye damage. The ozone layer protects living organisms, especially animals, from the effects of too much ultraviolet radiation. 

Ozone in the upper atmosphere protects us from ultraviolet radiation. Ozone at ground level is harmful, causing difficulties breathing and sore throats.

Fridges used to be cooled using chemicals called CFCs. These were safe and stable chemicals. No one realised CFC molecules damaged the ozone layer in the atmosphere. This meant that more ultraviolet from the Sun reached certain places on Earth, increasing the risk of sunburn and skin cancer. 

Now CFCs are banned.

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The risks from ultraviolet radiation

Everything we do carries a certain risk of accident or harm. Skin cancer is the most common type of cancer diagnosed in the UK. The risk of skin cancer increases with exposure to ultraviolet radiation, so outdoor workers are more likely to suffer from skin cancer. Nothing is risk free, even something as natural as enjoying a sunny day. Staying out of the sun is no solution - too little sunlight increases our risk of rickets, a preventable disease of the bones.

Sometimes new technologies cause new risks. Over 80 years ago, specialist chemicals containing fluorine, chlorine and bromine were invented with many uses including refrigeration and fire-fighting. About 40 years ago, researchers realised that these chemicals were damaging the ozone layer. Particularly above the Antartic, there was less ozone in the atmosphere so more ultraviolet radiation reached Earth. We can assess the risk of skin cancer caused by depletion of the ozone layer. Studies of thousands of people living in affected areas over several years suggest that if the ozone layer thins by 1%, the risk of skin cancer increases by about 4%.

Ozone is produced when oxygen molecules absorb high energy ultraviolet radiation and react together. Ozone molecules may split into an oxygen molecule and a free oxygen atom is they absorb low energy ultraviolet radiation. In both these reactions, infrared radiation is emitted. Ozone also reacts with nitrogen, hydrogen, chlorine and bromine compounds.

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The greenhouse effect

The absorption of ultraviolet radiation in the ozone layer reduces the amount of ultraviolet radiation reaching the Earth's surface. It causes chemical changes in this part of the atmosphere.

The Sun emits different frequencies of electromagnetic radiation, which travel through space to reach Earth. Our Earth is surrounded by a layer of gases called the atmosphere. When the radiation from the Sun reaches the atmosphere, some frequencies pass through. The Earth's surface absorbs most of this radiation, and warms up. 

The Earth's surface itself also emits electromagnetic radiation (infrared) as it warms up. When infrared radiation from the Earth's surface reaches the atmosphere, the atmosphere ABSORBSS and RE-RADIATES most of the radiation. Some of the re-radiated radiation reaches the Earth. The atmosphere then REFLECTS some radiation back to Earth and a small amount passes through the atmosphere to space. 

The infrared radiation is absorbed by the Earth's surface, which both become hotter. The warming of the Earth's atmosphere is called the GREENHOUSE EFFECT. The Earth is about 30oC hotter than expected for its distance to the Sun. The greenhouse effect is one reason why Earth is the right temperature to sustain life.

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Temperature and global warming

All objects, large and small, emit some electromagnetic radiation at all temperatures. This radiation has a range of frequencies in varying amounts. The frequency emitted in the highest intensity is the PRINCIPAL FREQUENCY. The principal frequency of the emitted radiation increases as the temperature rises. This can be seen as a colour change for objects that are hot enough to emit visible light. A metal rod glows red when it is first heated, becoming yellow and then white hot as it gets hotter. Hotter objects radiate higher frequencies of radiation than cooler objects.

The principal frequency of electromagnetic radiation from the Earth's surface is lower than the principal frequency emitted by the Sun. This is because the Earth is cooler than the Sun.

The level of carbon dioxide in the atmosphere has been rising for 50+ years. The global average temperature has also risen over this period. Scientists could see that the two effects were related. It took longer for many scientists to accept that rising carbon dioxide levels could cause the greenhouse effect.

A factor does not always cause the other even if the two are correlated. Once a plausible mechanism was suggested explaining how rising carbon dioxide levels could increase global temperatures, more scientists accepted that this was a possibility.

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Carbon cycling

The atmosphere contains several greenhouse gases. Water vapour is a greenhouse gas, responsible for over 2/3 of the greenhouse effect. The activities of humans have very little effect on the amount of water vapour in the atmopshere. Methane also adds to the greenhouse effect. Each molecule of methane contributes about 20 times more to the greenhouse effect than a molecule of carbon dioxide, although the amount of methane present is much less than carbon dioxide. Different amounts of these gases are present in the atmosphere at different times. 

Carbon is found in all living things. Carbon in your body comes from the food you eat and the air you breathe. The carbon from carbon dioxide in the atmosphere is constantly being recycled through PHOTOSYNTHESIS and RESPIRATION. 

Animals eat cabohydrates for energy > Animals and plants release carbon dioxide during respiration > Green plants absorb carbon dioxide from the air by photosynthesis > Plants grow and store carbon as a carbohydrate [REPEAT]

Carbon dioxide does not have its own cycle, but carbon does. The carbon changes from carbon dioxide in the atmosphere to more complex compounds in living things. 

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Carbon dioxide in the atmosphere

For thousands of years, the levels of carbon dioxide in the Earth's atmosphere stayed approximately constant. Carbon absorbed and stored by plants matched the amount released during rotting (decomposition) and respiration. Trees hundreds of years old in ancient forests stored large amounts of carbon. Over millions of years, fossil fuels formed from dead plants and animals that did not decompose, or decomposed very slowly.

About 200 years ago, a big social change started to spread across the world. During the industrial revolution, machines powered by fossil fuels started manufacturing goods on a large scale in factories. Carbon locked away in the fossil fuels for millions of years was released to the atmosphere. 

Forests were cut down or burned to clear the land and provide fuel. This left fewer trees to absorb carbon dioxide. Carbon dioxide was also released as the wood was burned or left to rot.

In the past 200 years, the amount of carbon dioxide in the atmosphere has been rising steadily. This century, the amount of carbon dioxide released from deforestation and buring fossil fuels has increases greatly. 

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If there is a correlation between two things, then increasing one will increase (or decrease) the other. If there is a correlation between atmospheric carbon dioxide and burning fossil fuels, then:

- burning more fossil fuels will increase the amount of atmospheric carbon dioxide

- burning less fossil fuels will decrease the amount of atmospheric carbon dioxide

There may be a correlation is one factor affects the chance of something happening, even if it does not cause it.

For example, it is likely that a larger population will burn more fossil fuels. Sometimes, two things increase at the same time but are not correlated. The levels of carbon dioxide and worldwide bicycle sales rose at the same time. These factors are not correlated, but both are affected by the increase in population. 

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Global warming

Global warming is the increase in average temperature worldwide caused by the greenhouse effect. The greenhouse effect occurs naturally. But scientists are concerned that recent increases in greenhouse gas emissions will cause global warming to continue and at a possibly increased rate.

If global warming continues, the climate will change, with some places becoming hotter, drier or damper. It may become impossible to grow certain crops in these regions. The ice at the poles may start melting because of global warming. Sea levels may rise if water from melted ice on land (glaciers) flows into the sea. It may also rise as the water in the ocean expands as it warms. Rising sea levels will leave low lying islands vulnerable to flooding.

Global warming may increase the amount of severe weather in places. It may cause more severe or frequent heat waves, storms, hurricanes, floods, rain or snow.

We are not sure what the scale of global warming will be. If we do nothing now, we may be able to adapt successfully, but large changes will be very difficult to adapt to. Governments making decisions about global warming must consider how likely it is that global warming is taking place because of human activity. They must also consider how serious its effects may be.

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Causes of extreme weather

Most governments now believe we have enough evidence that global warming is taking place and we should take steps to reduce it. We do not know for certain how serious the risks and benefits of global waring will be. It is important to consider the effects of global warming on different individuals and groups, and the steps taken to reduce its impact.

More severe weather can happen when the atmosphere is warmer. A larger temperature difference between the atmosphere and the oceans may increase convection - the transfer of energy through the atmosphere by circulating masses of air at different temperatures. Warm air holds more water vapour than cooler air, so the amount of water vapour in the atmosphere increases. In the right conditions it is released as heavy rain or snow. This hypothesis is still being debated and the processes taking place are not yet fully accepted/understood.

Climate scientists rely on computer models to make forecasts. The computer model is tested using past data to confirm which assumptions should be included in the model. These assumptions include changes caused by human activities, such as increased burning of fossil fuels. Once the test model runs, scientists check if its predictions match what actually happened. The accuracy of these models allows scientists to be confident that human activities are one of the principal causes of climate change. There are many factors that affect the world's climate, and natural changes take place over thousands of years.

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Using waves for communication

Conversations, music, pictures and other information can be carried from place to place using electromagnetic waves. The main types of radiation used for communication are radio waves, microwaves, infrared radiation and visible light.

Radio waves transmit terrestrial TV and radio programmes from a transmitter to a receiver. The waves are not strongly absorbed by air so they can travel far. Radio waves can spread around hills and buildings, and reflect off layers in the atmosphere.

Microwaves have a shorter wavelength than radio waves, and are not absorbed much in air. Narrow beams of microwaves travel many km through the atmosphere and communicate with satellites. We use microwaves for satellite TV broadcasts, satnav and mobile phone connections.

Wii controllers and TV remotes use infrared radiation, which only travels short distances in air and cannot pass through walls.

Very narrow glass fibres called optical fibres carry infrared and light signals long distances. Infrared radiation and visible light are not absorbed much in glass, repeatedly reflecting off the sides of the glass fibre. Optical fibres are used in telephone, internet and TV cables. More than one signal can pass through an optical fibre at the same time.

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How waves carry the information

You tune your radio to a certain frequency to listen to your favourite radio station. This frequency is the frequency of the carrier wave. 

A CARRIER WAVE is a radio wave which carries information from the broadcasting station to your radio. 

Information containing the programme's sounds of images is added to the carrier wave. The carrier wave is MODULATED (changed).

This creates a signal that is transmitted to your radio set.

Your radio can separate out the modulation from the carrier, so you can hear the programme.

In analogue broadcasting, the signal added to the carrier wave can vary continuously. This is an analogue signal. The frequency of its carrier wave and its amplitude can have any value. 

You don't hear radio waves. Your radio detects the signal and converts it into sound waves.

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Digital signals

A signal is used to carry information from a transmitter to a receiver. A digital signal is a type of signal that can take one of a small number of fixed (discrete) values, usually two. The values change in discrete steps, and cannot take any values in-between.

Many TV and radio programmes are transmitted using digital signals. Sounds and pictures can be converted into digital signals that are streams of just two values, 0 and 1. You can use a lamp to create a digital signal. The lamp is either on or off. Turning the lamp on represents a signal value of 1, and turning the lamp off represents a signal value of 0.

This is different from analogue signals, which can vary continuously. A lamp with a dimmer switch can be set to any value of brightness to give an analogue signal.

When information is to be broadcast, the heigh of the analogue signal is measured regularly. Each measurements is coded as a strong of numbers, either 0 or 1. An example is 001011010.

The codes for successive measurements are joined together in one long string. The string of numbers can be transmitted using an electromagnetic carrier wave. The carrier wave is turned on or off, creating short burts of waves called PULSES. Once the digital signal is received, the series of pulses is decoded inside the radio, TV or other digital device. 

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Advantages of digital signals

There are several advantages of sending information using digital signals:

- Different types or information can be sent at the same time (such as a picture inside a written document)

- Digital signals can be sent more accurately than analogue signals

- Digital information can be stored and processed by computers

- Unwanted information mixed in with the original signal, called noise, can be removed more easily and the original signal recovered. Noise can be caused by interference with signals from other equipment or the pick-up of a random signal.

Digital signals are not affected by noise as badly as analogue signals. During transmission, all signals may be amplified (made stronger). They are all decoded by the devices that receive them. Analogue signals can take any value so when they are amplified, or decoded, it is not possible to tell which part of the signal is real and which part is noise. Digital signals can only have certain values, so the decoders and amplifiers assume signals close to a certain value have that value. The decoder ignores parts of the signal that are not close to the expected values.

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Storing images and sound

The strings of 0s and 1s that make up digital information are called binary digits. To store text like that on this page, letters are converted to binary digits. Each letter is represented by a string of eight digits, which is called a byte. The letter A is represented by 01000001. All digital information is converted into a string of binary values, measured in bytes: each eight digits make 1 byte of information.

To store an image digitally, the area of the image is divided into lots of tiny boxes. The colour and brightness of each box are represented by a long series of binary digits. To produce an image, the values used to store the images are assigned to tiny areas on the screen called pixels. The quality of a picture improves as more pixels are used. Each pixel then improves as more pixels are used. Each pixel then represents a smaller area of the original picture. The number of pixels that form an image is callled the resolution. High resolution images are of a good quality and have more pixels whereas low resolution images have fewer pixels and a lower quality.

When creating a digital sound from an analogue sound signal, information about the sound signal is collected at regular time intervals. To transmit or store a higher quality sound image, the signal must be sampled more often. More information about the sound is then stored and a higher quality transmission or recording is achieved.

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Advantages of digital information

In general, the more information that is stored (the larger number of bytes), the better the quality when the stored data is converted back to sound.

Computers store and process digital information. They can store and process any information that can be changed into digital code, such as sound, video, text and images. 

Digital information is versatile: it can be processed in many different ways. A digital file stored on a digital camera, for example, can be transferred to a memory stick or a computer, it can be emailed, uploaded onto the internet or sent by Bluetooth to mobile phones, and still stay in its original form.

Bluetooth transmits signals wirelessly over short distances between telephones, computers and other equipment.

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