PHYSICS – P2 NOTESOCR 21st Century Science Exam Higher

EM RADIATION• Light is a type of EM Radiation• Radiation is just a transfer of energy• All types of EM Radiation transfer energy• There are seven types of EM Radiation making up a spectrum:

Increasing Frequency

RADIO

WAVES

MICRO

WAVES

INFRA

RED

VISIBLELIGHT

ULTRA

VIOLET

X-RAYS

GAMMARAYS

EM RADIATION – PHOTONS • The energy transferred by EM radiation is delivered by photons• A photon is a ‘packet’ of energy• Each photon carries a small amount of energy, but not all photons have

the same amount of energy• The amount of energy carried by a photon depends on the frequency of

the radiation• The higher the frequency the more energy carried by a photon• Many objects emit EM radiation, these are called sources

EM RADIATION• All types of EM radiation can travel through space.• The speed of light in a vacuum is about 300 000 km/s • When radiation is emitted from a source, it spreads out until it reaches

some matter three things can happen:• The radiation can be transmitted – just keep going• It could be reflected – ‘bounce back’• The radiation could be absorbed – like a sunbather and UV• Objects that absorb radiation are called detectors

EM RADIATION – INTENSITYTotal energy = number of photons x energy of each photons

• The intensity of radiation means how much energy arrives at each square metre of surface per second

• The units of intensity are W/m2• The intensity of a beam of radiation decreases with distance from the

source because:• The beam spreads out• The beam gets partially absorbed as it travels• All substances are made of atoms and molecules

EM RADIATION – IONISATION• When a photon hits an atom or molecule, it sometimes has enough

energy to remove an electron – this is ionisation• It takes a lot of energy to remove an electron from an atom or molecule,

only types of radiation with high enough photon energy can cause ionisation

• Ultraviolet, x-rays and gamma rays are ionising radiation• Radioactive materials emit ionising gamma radiation all the time• If your cells are exposed to ionising radiation, the damage to DNA

molecules can cause mutations• Very high doses of radiation can kill your cells altogether

EM RADIATION - MICROWAVES• Non-ionising radiation doesn’t have enough energy to change atoms. When its absorbed by

a substance it transfers energy to the atoms or molecules of the substance – and heats them up

• Microwaves make particles vibrate, heating them up. • Some microwaves are strongly absorbed by water molecules and so can heat things

containing water• Microwave ovens produce microwave radiation.• Microwave radiation can heat up the water in body cells• Microwave ovens have metal cases and screens over the glass doors which reflect and

absorb the microwaves, stopping them getting out• Microwaves are used to send signals between mobile phones and mobile phone masts• There are concerns that heating of tissues could increase the risk of medical conditions

such as cancer

THE CARBON CYCLEThe carbon cycle shows how carbon is recycled in the atmosphere:Processes that remove carbon dioxide from the air:• photosynthesis by plants• dissolving in the oceansProcesses that return carbon dioxide from the air:respiration by plants, animals and microbesCombustion, ie: burning wood and fossil fuels such as coal, oil and gasthermal decomposition of limestone, for example, in the manufacture of iron, steel and cement.

THE ATMOSPHERE AND OZONE LAYER

• The ozone layer protects us from too much UV radiation• The greenhouse effect helps regulate Earth’s temperature • The earth absorbs EM radiation from the sun:

• Ozone is a form of oxygen; an ozone molecule is just three oxygen atoms joined together

• The ozone layer protects us from too much UV radiation

GLOBAL WARMINGAlthough the changes have been gradual, most - but not all - scientists agree that the climate is getting gradually warmer. This is called global warming.Most - but not all - scientists lay the blame for this on human activities increasing the amount of carbon dioxide in the atmosphere.Global warming could cause:• climate change• extreme weather conditions in some areas.Climate change may make it impossible to grow certain food crops in some regions. Melting polar ice, and the thermal expansion of sea water, could cause rising sea levels and the flooding of low-lying land. Extreme weather events become more likely due to increased convection accompanied by more water vapour being present in the hotter atmosphere.

SIGNALS (ANALOGUE AND DIGITAL)

• EM radiation has been used to send information for years – eg: using light to send signals in Morse code

• Different frequencies are used for different things:• Infrared – TV remote controls and ‘night vision’ cameras• Microwaves – Mobile phones and satellite communication• Radio – TV & radio transmissions and radar• Radio waves and microwaves are good at transmitting information over long

distances • Information is being transmitted everywhere all the time• Whatever the information type is, it is converted into electrical signals before

it’s transmitted• It’s then sent over long distances • Its then sent out as either analogue or digital signals

ANALOGUE VS DIGITAL

Analogue Signals Digital SignalsThe amplitude or frequency of an analogue signal varies continuouslyAn analogue signal can take any value in a particular rangeLots of noiseLower qualityCan transmit one signal at a time

Digital signals can only take on of a small number of discrete values (usually two – on/off, 0/1 or true/falseThe information is carried by switching the EM carrier wave on or off which creates pulsesA digital receiver will then decode these pulses to get a copy of the original signalLess noise and better qualityAre easy to process using computers