# Physic P1B

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• Created by: Amy GOuld
• Created on: 05-06-10 19:24

## Electromagnetic Waves

• Electromagnetic radiation travels as EM waves. Like others, they transfer energy from one place to another without moving any matter.
• Al waves have WAVELENGTH, FREQUENCY, AMPLITUDE and SPEED.
WAVELENGTH: Distance from one peak to the next.
FREQUENCY : How many complete waves there are per secong, measure in Hz.
AMPLITUDE : The height of the wave.
SPEED : How fast it goes.
EM waves with higher frequencies = lower wavelengths
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Wave Speed = Frequency x Wavelength

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## EM Waves

• EM radiation can be absorbed, reflected or transmitted.
• The different wavelengths interact diefferently with matter, and different things can happen when a wave meets matter, air or glass etc.
It may be transmitted- just pass through the substance, as light passes through glass.
It could be reflected - bounce back, like light reflected from the mirror.
It could be absorbed - the energy of the wave is transferred to the matter.
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• When EM is absorbed => The substance gets hotter, OR it sets up an alternating current with the same frequency as the EM wave.
• Some EM radiation can be harmful - most passes throught soft tissue (radiowaves) but some types are absorbed and cause heating (microwaves).
• Some radiations cause cancerous in living cells (UV).
• The effect radiation depends on what type it is, and the size of dose you get.
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• Mainly used for communications.
• EM radiation with wavelengths longer than 10cm.
• TV and FM Radio - very short wavelengths, 10cm-10m.
• Long and short wave radio signals can both be received from long distances.
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MICROWAVES
• Communication to and from satellites, but microwaves are needed to be able to pass easily throught the watery atmosphere.
• Satellite TV - a transmitter is transmitted into space, its picked up by a satellite receiver dish orbetting above the Earth. The satellite transmits the signal back to Earth in a different direction where it is received by a satellite dish on the ground.
• Used by remote-sensing satellites - through the clouds, monitor oil spills, track iceburg movement, see how the rainforest has been chopped down etc.
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## Optical Fibres, visible light and infrared

• Optical Fibres can carry data over long distances, as pulses of light or infrared radiation.
• They work by :
Bouncing waves off the sides of the thin inner core of glass or plastic.
The wave enters one end of the fibre, is reflected repeatedly until it emerges at the other end.
• They work due to TOTAL INTERNAL REFLECTION.
It can only happen when a wave travels through dense substances such as glass or water, towards a less dense substance (AIR) .
• It depends on whether the angle of incidence is BIGGER than the CRITICAL ANGLE.
• IF LESS: Most of the light passes out but a little is internally reflected.
• IF EQUAL: Emerging ray comes out along the surface more internal reflection
• IF GREATER : No light comes out, all is internally reflected.
• Different materials have different critical angles, glass is 42`.
• Optical fibres can be bent, not sharply as the angle of incidence may be below the critical angle.
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• Higher frequency EM radiation is more dangerous = Due to energy of EM waves is directly proportional to the frequency. HIGHER frequency = More energy ( energy does damage ) .
• Visible light is only harmful if it is too bright.
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• Infared can cause burns, but they are easily avoidable.
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• Microwaves may not be harmful - some wavelengths are absorbed by water, and heated, but mobile phones use microwaves, and some think that they are harmful.
• UV rays can cause cancer, from spending too much time in sunlight, as the rays have UV in them, this damages human cells.
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• X-Rays are used in hospitals but are dangerous; Radiographers take x-ray phtotgraphs to diagnose damage - They pass easily through flesh, and can cause mutations leading to cancer. Lead aprons are used for patients to protect other body parts.
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## Analogue and Digital Signals

• Information, sounds and pictures, are converted into electrical signals before transmittion. Then, it is send down wires or carried on EM waves.
• The amplitude + frequency of analogue signals vary continuously.
• Dimmer switches, thermometer and old watches are analogue devices.
• Digital signals are coded pulses; they can only two values ( ON / OFF ) .
• BOTH signals weaken as they travel, so amplifying them is needed. interference or noise from electrical disturbances are also picked up.
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• DIGITAL signals have better quality :
Noise is less of a problem .
Noisy analogue signals are difficult to know what the original looked like.
Digital signals are higher quality, as the information is receieved the same as the original.
They are easier to process.
Several signals can be transmitted as one using 1 cable or EM wave, so more information can be sent.
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• The nucleus contains protons and neutrons, this takes up most of the mass of an atom, but virtually no space is taken up.
• The electrons are negatively charged.
• Many elements have different ISOTOPES - They are atoms with the same number of protons, but different number of neutrons.
E. G - two common isotopes of carbon. Carbon-14 has two more than normal Carbon (carbon-12). Usually each element has only one or two stable isotopes.
• Others tend to be radioactive, the nucleus is unstable, it decays + emits radiation.
The nuclei of unstable isotopes breaks down randomly, each nucleus decays in its own time. Physical conditions do not change its speed.
When decayed, alpha, beta or gamma is let out.
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• Nuclear Radiation causes ionisation, by electrons being knocked of ( IONS ) .
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## Alpa, Beta, Gamma

• Alpha : Relatively big and heavy, and slow-moving.
They do not penetrate far into materials.
Due to the size, they knock electrons off before slowing down, creating ions.
They are electrically charged ( + )
Alpha particles are charged by electric and magnetic fields.
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• Beta : An electron emitted from the nucleus of an atom when a neutron turns into a proton or neutron.
Every particle emitted, the number of protons in the nucleus increases by 1.
They move quite fast and are small.
They penetrate before colliding and are moderately ionising.
They are electrically charged ( - )
Beta particles are deflected by electric and magnetic fields.
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• Gamma : Opposite to alpha, no mass, just energy.
They penetrate a long way into materials withour being stopped.
They are weakly ionising as they pass through rather than colliding.
Gamma rays have no charge, not deflected by electric or magnetic fields.
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## Blocking Alpha, Beta, Gamma

• Alpha : Blocked by paper, skin and a few centimetres of air.
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• Beta : Stopped by thin metal.
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• Gamma : Blocked by thick lead, or thick concrete.
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## Half - Life

• Nuclear waste stays reactive for a long time, this is its half - life.
• When unstable nucleus' decay, they emit radiation, meaning there is one less radioactive nucleus that isn't there to decay.
• As more unstable nuclei decay, the radioactivity as a whole decreases, the less radiation it emits.
• The speed in which activity decreases varies, some can take just hours but some, years.
• Half - life measure how quickly the activity decreases.
HALF LIFE IS THE TIME TAKEN FOR HALF THE NUCLEI NOW PRESENT TO DECAY.
• Half life can be measured by sampling the count rate ( decay per. min ) using a Geiger counter.
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## Uses and Risks of Radiation

• Long half - lifes are needed for devices that have to last.
Sterilising machines in hospitals use gamma to kill bacteria. They are powerful, with a long half life, so lasts for a long time.
Smoke detectors use sources of alpha to ionise the air between electrodes, smoke absorbs the radiation, the current stops = alarm sounds.
• Medical tracers use Beta of Gamma as they penetrate the skin, allowing the damage internally to be detected externally.
• Gamma is used in industrial tracerss, such as pipes, it will penetrate and some is absorbed, so cracks are detected by seeing high radioactivity at this point.
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• Radiation harms living cells, when penetrated ( ALPHA ) .
Beta + Gamma are less dangerous inside, more dangerous outside.
If radiation enters the body = it collides with molecules in cells, causing ionisation and damages / destroys molecules.
• Radiation is used to treat cancer however, using radiotherapy to kill the cells.
• In the laboratory, exposure to radiation shoul always be minimised, no skin contact, source at arm's length, point it away, keep the soure in its box.
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## The Origin Of the Universe

• When looking at light from distant galaxies, the frequencies are lower than they are to be => they are shifter towards the red part of the spectrum.
This is the red shift. The galaxies are moving away from us.
• The Doppler Effect :
The sound waves from stationary objects are equally spaced. but when they move.
The wavelengths seem longer at the front end, due to movement.
The frequency waves seem lower as the object moves further away.
• More disant galaxies have greater red-shifts.
• The more distant galaxies are moving away faster than the closer galaxies.
• This gives evidence to suggest the Universe is expanding.
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## Looking into Space

• There are different objects in space, they emit of reflect different frequencies of EM radiation.
The atmoshpere can get in the way, as a lot of light is absorbed from space before reaching Earth.
There is pollution ( light ) and this makes it difficult to pick out objects that are dim/darker in the sky.
• The problems are resolved by having telescopes in space, ( Hubble, optical telescope 1st ) and they can see objects that may be a billion times fainter than what is seen unaided on Earth.
• Bigger telescopes are needed to see disant or faint objects, so more light is collected per second, making the image brighter.
• The size of the telescope is compared to the wavelength of the radiation.
• Radio telescopes need to be large, as the waves are very long.
• Optical telescopes detect visible light. They are used to look at objects that are in close other galaxies.
• X-ray telescopes are useful in seeing violent events in space, but they only work from space, as the Earth's atmoshpere absorbs x-rays.
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