P1 Revision (once again...)

P1 Revision (once again...)

1. Wireless signals

   • Radio waves can be reflected off a hill or other objects (like light)
   • TV aerial- receives two signals: one from a transmitter and one reflected from an obstacle
   • Reflected signal- travels further, if it's analogue it produces a picture shifted to the right (ghosting)
   • Refraction of waves
     • Amount of refraction depends on the frequency of the wave. (Long wl and low fq shows most refraction)
   • DAB and FM
     • DAB- more radio stations, choose it by name, coverage can be poor, interference can be removed
     • FM- better audio quality
   • Radio waves can be reflected from the ionosphere and water and they undergo total internal reflection
     • Sky wave propagation- the propagation of radio signals from one place to another because of reflection from the ionosphere
     • As a radio wave travels up in the ionosphere, it passes from a denser to a less dense medium. It continuously bands away from it's path until it is totally reflected back to the Earth. Coupled with reflection from the oceans, radio signals can be received well beyond line of sight
     • Microwaves are not reflected back to Earth. They pass through the ionosphere where they are received by satellites orbiting the Earth. The satellites amplify the signal and then re-transmit it back to Earth
     • Radio waves are diffracted when they meet an obstruction. The diffraction is only significant if the size of the obstacle is similar to the wavelength
   • Diffraction at edges of dish. Wavelengths must be small compared to the dish diameter to reduce this diffraction. Radios now receive digital signals and this reduces the amount of interference

2. Stable Earth

   • An earthquake occurs when rocks deep below the surface of the Earth move suddenly at a fault and it happens below the Earth's surface at the focus
     • L wave- surface wave. Travels out from the epicentre relatively slowly
       • Epicentre- point on Earth's surface above the focus
     • Seismic waves
       • P wave- primary pressure wave. Longitudinal wave and is similar to a sound wave. Travels through Earth at a speed between 5 and 8 km/s
         • Pass through solids and liquids
         • Refracted by the core. Paths taken help to calculate the size and density of Earth's layers
       • S wave- secondary shear wave. Transverse wave. Travels through Earth at a speed between 3 and 5.5 km/s. Slightly slower than P wave
         • Only pass through solids
         • Can't be detected from the opposite side of the Earth, liquid in the way
   • Skin cancer
     • UVA has a lower frequency and energy than UVB rays. However, UVA rays are more penetrable. Dark skin absorbs more UV rays so that less UV can penetrate living tissues
     • Suntans occur because exposure to UV produces a black pigment called Melanin
     • Cancer is caused because radiation has damaged DNA in the skin cells. The cells start to divide out of control. The result is a tumour and this can prove fatal if not successfully treated
     • SPF 45- means you can stay out in the sun 45 times longer than if you didn't have any on
     • safe length of time to stay in the Sun with sunscreen on= SPF number x time given in sun index
   • Ozone
     • The problem- UV is absorbed by ozone layer, ozone gas is found in a layer in the upper atmosphere, CFCs damage ozone
     • Strayosphere- absorbs UV radiation
     • Made up of 3 oxygen atoms
     • Catalytic cycle- chlorine destroys oxygen molecules
     • CFCS don't break down easily and are not water soluble
     • CFC- Chlorofluorocarbons are man made and contain halogens

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