GCSE OCR Gateway Science - P1 E-F

Analogue and Digital signals

Signals are used to transmit date, analogue and digital.

Analogue signals have a continuous variable value.

Digital signals are either on (1) or off (0). They allow more information to be transmitted because many signals can be interleaved on the same data line without interfering with each other. This is called multiplexing.

All signals weaken as they travel and need to be amplified. When an analogue signal is amplified any unwanted signals (noise) is amplified and this causes distortion.

When a digital signal is amplified any noise is removed beforehand and is not amplified.23

Total Internal Reflection

When a ray of light travels through a transparent material it is refracted when it reaches a boundary.

The ray only passes out of the material if the angle of incidence is less than the critical angle.

If the angle of incidence is the same as the critical angle the light travels along the surface. If the angle of incidence is greater than the critical angle total internal reflection takes place.

CRITICAL ANGLE = APPROX. 42 DEGREES

This reflection is useful in optical fibres which are used to transmit infrared or visible light as data very rapidly.

Uses of Wireless Technology

• radio
• mobile phones
• connecting laptop computers to the Internet.

Transmitting Signals

Satellites can be used for global commnication. A signal is sent from a ground station transmitter dish to a satellite reveiver dish. A return signal is then sent by the satellite transmittter to a ground receiver dish, which may be in a different country, continent, etc.

The ionosphere is an electrically charged layer in the Earth's upper atmosphere. Longer wavelength radio waves are reflected by the ionosphere. This enables radio and television programmes to be transmitted between different places.

The refraction and diffraction of radiation can affect communications:

• refraction at the interfaces of different layers of the Earth's atmosphere results in the waves changing direction
• diffraction (changes to the direction and intensity of waves)
• 

Interference from similar signals limits the distance between transmitters. Positioning transmitters in high places can help to overcome the nuisance of obstacles blocking signals.

Advantages of Digital Radio

• Uses digital coding. This results in better reception with less noise or interference.
• Each station has the same channel all over the country, so there is no need to re tune.
• Many different stations can broadcast in each area.

Transverse Waves consist of crests and troughs.

• The amplittude of a transverse wave is the height of a crest.
• The wavelength is the distance from one crest to the next.
• The unit of wavelength is metres.
• The frequency is the number of waves per second. The unit of frequency is hertz.

wave speed (m/s) = frequency (Hz) * wavelength (m)

Morse code which uses dots and dashes to represent letters. It was first used to send messages through wire by pulsing the current in them on or off. It can also be used with a signal lamp or torch that is switched on and off to produce pulses of light.

Light Signals -

• instantaneous
• simple equipment - lamp or torch
• not a secure method
• if using optical fibres... you need fibres

Electricity Signals -

• special equipment - transmitter, receiver, wires
• signal easily lost so needs to amplified regularly

Radio Waves or Microwaves -

• can travel large distances and through space
• no wires are needed
• radio waves can refract around obstacles, microwaves cannot

Lasers produce an intense beam of light in which all light waves are :

• the same frequency
• in phase with each other.

This means that all the peaks and troughs match up: they go up together and down together. When waves are in phase they have a lot of energy.

Waves out of phase will have less overall energy.

Compact Discs

Digital information can be stored as a sequence of tiny pits in a metal layer inside a CD.

A CD player spins the disc and laser light is reflected from the pits. The reflected pulses of light are then turned into electrical signals on their way to the amplifier.

Earthquakes produce shock waves which can travel inside the Earth and cause damage. These waves are called seismic waves and can be detected by seismometers. there are two types of seismic wave.

• P-waves, primary waves, are longitudinal and travel through both solids and liquids
• S-waves, secondary waves are transverse waves and travel through solids but not through liquids. They travel slower than P-waves.

The properties of seismic waves provide evidence for the structure of the Earth. After an earthquake occurs, the waves are detected all over the world.

Comparing the time difference between P-waves and S-waves arriving at seismic measuring stations located at different points on the Earth's surface, indicates that the Earth is made up of :

• a thin crust
• a mantle which is semi-fluid and extends almost halfway to the centre
• a corewhich is over half of the Earth's diameter with a liquid outer part and a solid inner part.

Ultraviolet Radiation

The Earth receives ultraviolet radiation from the Sun and prolonged exposure can cause sunburn or skin cancer. Darker skins are less risk of damage from ultraviolet radiation because they absorb more of the radiation in the top layers, so less reaches the live skin cells below the surface.

Sun cream reduces the damage caused by ultraviolet radiation. The higher the factor, the lower the risk of burning.

Safe time = Recommended exposure time * Sun block factor

The Ozone Layer

- absorbs much of the ultraviolet radiation from the sun, stopping it reaching the Earth. Recently, the ozone layers is becoming thinner and more people are suffering from skin cancer. Many people blame the use of CFCs in factories and aerosol cans.

Climate Change

Three contributing factors to global warming are :

• increaed enrgy use both in homes and industry
• increased carbon dioxide emissions from fossil fuels
• deforestation, which is the cutting down of large numbers of trees

Weather patterns are also affected by human activity, as well as by natural phenomena :

• dust from factories reflects radiation back into cities, causing warming
• dust from volcanoes reflects radiation from the Sun back into space, causing cooling