OCR 21st Century Science GCSE Level Physics Higher

1. PHYSICS – P2 NOTES
OCR 21st Century Science Exam Higher

2. 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
RADI
O
WAV
ES
MICR
O
WAV
ES
INFR
A
RED
VISIB
LE
LIGH
T
ULTR
A
VIOL
ET
X-
RAYS
GAM
MA
RAYS

3. 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

4. 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

5. EM RADIATION – INTENSITY
Total 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

6. 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

7. 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

8. THE CARBON CYCLE
The carbon cycle shows how carbon is recycled in the
atmosphere:
Processes that remove carbon dioxide from the air:
• photosynthesis by plants
• dissolving in the oceans
Processes that return carbon dioxide from the air:
respiration by plants, animals and microbes
Combustion, ie: burning wood and fossil fuels such as coal,
oil and gas
thermal decomposition of limestone, for example, in the
manufacture of iron, steel and cement.

9. 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

10. GLOBAL WARMING
Although 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.

11. 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

12. ANALOGUE VS DIGITAL
Analogue Signals Digital Signals
The amplitude or frequency of an analogue
signal varies continuously
An analogue signal can take any value in a
particular range
Lots of noise
Lower quality
Can 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/false
The information is carried by switching
the EM carrier wave on or off which
creates pulses
A digital receiver will then decode these
pulses to get a copy of the original
signal
Less noise and better quality
Are easy to process using computers