P1B
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- Created by: Dr Afzal
- Created on: 24-01-10 12:12
Electromagnetic Waves
Wave speed=
Frequency
x
Wavelength
Electromagnetic Waves
EM Radiation can be:
Transmitted- pass through the substance
Reflected- bounce back
Absorbed- enerfy of wave is transfered through matter
Electromagnetic waves
Radio Waves:
Long radio wavelengths: 1-10km:
Bend around the curved surface of the Earth
Can also get around hill, into tunnels etc
Used for TV and FM radio
Short radio wavelenths: 10cm-10m:
Be recieved lond distances by transmitter
Reflected from ionsphere
EM Radiation
Microwaves:
Communication to and from satellites but you need to use microwave signals which pass easily through earth's watery atmosphere
For satellite TV, the signal from a transmitter is transmitted into space
EM Radiation
Optical fibres:
Can carry data over long distances as pulses of light or infrared radiation
Work by bouncing waves of the sides of a thin core of glass or plastic
Optical fibres work because of total internal reflection
Higher frequency usually more dangerous
Higher frequency waves have more energy- enerygy of wave does damage
visble light isn't harmful unless really bright
Infrared can cause burns or heatstroke
Microwaves
Some wavelengths of microwaves are absorbed by water molecules and heat them up
If water molecules reach skin- skin could start to cook
Ultraviolet
Ultarviolet radiation damages DNA in your cells- Causes skin cancer
Dark skin- protects against UV rays because absorbs more vulnerable tissues deeper in the body
If you have pale skin and your in the sun you need to take extra care
The gas inside fluorescent tubes emits Ultraviolet radiation
X-Rays
X-rays easily pass through flesh but not through denser material like bones or metal
X-rays can cause mutations which can lead to cancer
Lead aprons minimise exposure to X-rays
Lead shields are given to the parient on parts of body where they don't want x-rays
Pregnant women shouldn't have X rays
Analogue and Digital Signals
Information such as sounds and pictures is converted into electrical signals
Amplitude and frequency of an anlogue signal vary continously
Old fashion watches, thermometers- Analoge devices
Digital Signals only take 2 values (on or off)
Signals pick up interference or noise from electrical disturbances or other signals
Digital signals are better quality
Noise is less problem with digital signal
If you amplify a noisy anologue signal, you'll amplify the noise as well
Digital signal, the signal is recieved same as the original
Digital signals easier to process with computers
With Digital technology you can transmit several signals at once using just one cable or EM wave
Radioactivity
Nucleus contains protons and neutrons (make up most mass of atom)
Electrons are negatively charged and really really small- paths take up a lot of space
The word "ATOM" was being bandied around over 2400 years ago by a greek philosopher, Democritus
Radioactivity- Isotopes
Isotopes are atoms with the same number but different number of protons
Usually each element has only one or two stable isotopes
The other isotopes tend to be radioactive- the nucleus is unstable, so it decays (breaks down) and emits radiation
AND RADIOACTIVE DECAY BREAKS DOWN SPONTANEOUSLY IT'S UNAFFECTED BY ANYTHING ELSE IT'S A RANDOM PROCESS
Radioactive decay is a Random process
Radioactivity- Nuclear Radiation and Alpha particl
Nuclear Radiation causes ionisation, by bashing into atoms and knocking electrons off them, atoms are turned into ions
Alpha particles are big, heavy and slow moving
Because of their size they bash into a lot of atoms and electrons off them, before they slow down
Alpha particles are deflected by electric and magnetic fields
Alpha particles are electrically charged with a positive charge
Beta Particles
A beta particle is an electron which has been emitted from the nucleus of an atom when a neutron turns into a proton and an electron
Move quite fast and are quite small
Negatively charged- deflected by electric and magnetic fields
Gamma Rays
Gamma rays have no mass- just energy
Penetrate a long way into materials without being stopped
This means they are weakly ionsing- tend to pass through rather than collide with atoms
Gamma rays have no charge
Alpha, Beta, Gamma
Alpha particles blocked by pape, skin or few mm of air
Beta particles stoppes by thin metal
Gamma rays are blocked by thick lead or very thick concrete
Half Life
HALF TIME IS THE TIME TAKEN FOR HALF OF THE NUCLEI NOW PRESENT TO DECAY
A short half life means the activity falls quickly, because lots of nuclei decays quickly
A long half life, activity more slowly because most of nuclei don't decay for long time
You can work out the hallf life of a sample by monitoring its count rate (the number of atoms which decay per minute)
Uses of Radiation
Gamma radiation- Sterilise machines and kill bacteria on medical instruments and find underground leaks of pipes
Smoke detectors use a weak source of alpha radiation to ionise the air between two electrodes
A source of with a fairly long half-life is used so detector will work for many years
Beta and gamma radiation will penetrate skin and other body tissues
So it's injected or just swallowed
Risks of Radiation
Beta and Gamma sources, however are less dangerous inside the body because they pass thorugh the body without causing damage
If radiation enters body it will collide with molecules in cells- collision causes ionisation
Higher doses tend to kill cells completely
Radiotherapy is used to kill cancer (gives high dose of gamma rays)
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