# P1 - Energy for the home

## P1a: Heating houses

Measuring temperature:

• thermogram uses colour to show temperature
• temperature = a measurement of hotness on an arbitrary scale
• heat = a measurement of internal energy on an absolute scale

Specific Heat Capacity:

• = amount of energy needed to change temperature of substance by 1 degrees C (in J/kg degree Celcius)
• water = 4200 joules per kg degree celcius
• energy transferred = mass x specific heat capacity x temperature change

Specific latent heat:

• energy needed to melt/boil 1kg of material in joules per kilogram (j/kg)
• energy transferred = mass x specific latent heat
• when a substance changes state, energy is needed to break bonds of molecules
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## P1b: Keeping homes warm

Insulation homes:

• double glazing reduces energy loss by conduction:
• gap between two panes of glass is filled with gas or contains a vacuum
• particles of gas are far apart so energy is difficult to transfer
• no particles in a vacuum so impossible to transfer energy
• loft insulation reduces energy loss by conduction and convection:
• warm air in home rises
• energy transferred through ceiling by conduction
• air in loft warmed by top of ceiling and trapped in loft insulation
• cavity wall insulation reduces energy loss by conduction and convection:
• air in foam is good insulator
• air cannot move by convection because it is trapped in foam
• insulation blocks used to build new homes reduce energy transfer by radiation:
• blocks have shiny foam on both sides
• energy from Sun reflected back to keep home cool in summer
• energy from home is reflected back to keep home warm in winter
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## P1b: Keeping homes warm

Energy is transferred by:

• conduction - due to transfer of kinetic eergy between particles
• convection - gas expands when heated and makes it less dense so it rises
• density = mass / volume (in g/cm3)
• radiation - energy transferred through waves not by particles, works in a vacuum

Energy efficiency

Payback time

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## P1c: A spectrum of waves

Properties of waves:

• amplitude of wave = maximum displacement of a particle from rest position
• crest = highest point on a wave above rest position
• trough = lowest point on a wave below rest position
• wavelength = distance between two sucessive points
• frequency = number of complete waves passing a point in one second
• speed of wave = frequency x wavelength

Electromagnetic spectrum:

• radio, microwave, infrared, visible light, ultraviolet, X-ray, gamma ray

Refraction: when the speed of a wave decreases as the wave enters a more dense medium

Diffraction: spreading out of a wave as it passes through a gap

• larger gap = less diffraction, most diffraction occurs when gap is similar size to wavelength
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## P1d: Lights and lasers

Morse code:

• dots and dashes to represent the alphabet
• code is used by signalling lamps as a series of short and long flashes of light
• example of a digital signal

Laser light:

• white light = made up of different colours of different frequencies out of phase
• laser light = one single frequency, in phase and shows low divergence
• used to read from the surface of a CD:
• surface of CD is pitted
• pits represent digital signal
• light shone onto CD surface and difference in reflections provides info for digital signal
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## P1d: Lights and lasers

Refraction:

When light travels from more dense material to less dense material, angle of refraction is larger than the angle of incidence

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When angle of refraction = 90 degrees, angle of incidence = critical angle

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When angle of incidence = bigger than critical angle --> total internal reflection

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Endoscopes: allows doctors to see inside a body without need for surgery

• light passes through one set of optical fibres and illuminates inside of body
• light is reflected and reflected light passes up another set of fibres to an eyepiece / camera
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## P1e: Cooking and communicating using waves

• energy is absorbed by surface of food
• kinetic energy of surface of food particles increases
• rest of food heated by conduction

Cooking with microwaves:

• penetrate up to 1cm into food
• microwave ovens cook food by microwave radiation
• water of fat molecules in outer layer of food vibrate more, kinetic energy of food particles increases, and rest of food cooked by conduction

Properties of microwaves:

• have wavelengths between 1mm and 30cm
• mobile phones use longer wavelengths than microwave ovens so less energy is transferred
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## P1e: Cooking and communicating using waves

• used to communicate over long distances
• transmitter and reciever must be in line of sight
• aerials are normally situated on top of high buildings

Microwave communication:

• satellites use microwave communication
• signal from Earth recieved, amplified and re-transmitted back to Earth
• satellites are in line of sight because there are no obstructions in space

Signal strength for mobile phones can change over a short distance:

• microwaves do not show much diffraction
• adverse weather and large areas of water can scatter signals
• curvature of Earth limits line of sight so transmitters have to be on tall buildings or close together
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## P1f: Data transmission

How electrical devices are controlled:

• pressing button on device completes circuit which sends a coded signal to LED at front
• signal includes start command, instruction command, device code and
• LED transmits series of pulses that recieved by device and decoded to change channel

Analogue to digital switchover:

• gives improved signal quality for picture and sound and greater choice of programmes
• allows user to interact with programme
• provides information servies such as programme guides and subtitles

• interferences also occur on digital signals but does not affect signal as digital signals have only two values, 0 and 1
• multiplexing allows a large number of digital to be transmitted at the same time
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## P1g: Wireless signals

Radio waves reflected and refracted by Earth's atmosphere:

• amount of refraction depends on frequency
• less refraction at higher frequencies

• Digital Audio Broadcasting provides greater choice of radio stations but not of good quality
• DAB eliminates interference between other radio stations

Radio waves are reflected from ionosphere and undergo total internal reflection:

• water reflects radio waves, land mass doesn't
• reflection by ionosphere allows radio waves to be recieved from aerial not in line of sight

Microwaves pass through the ionosphere:

• signals recieved by satellites, amplified and retransmistted back to Earth
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## P1h: Stable Earth

Earthquake waves:

L waves travel round the surface very slowly.

P waves are longitudinal pressure waves:

• travel through the Earth at between 5km/s and 8km/s
• can pass through solids and liquids and are refracted by the core
• path taken by P wave means that scientists can work out size of Earth's core

S waves aretransverse waves:

• travel through the Earth at between 3km/s and 5.5 km/s
• can only pass through solids and don't travel through liquids
• not detected on the opposite side of Earth to an earthquake which tells scientists that Earth's core is a liquid
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## P1h: Stable Earth

Tans are caused by the action of UV light on the skin:

• cells in the skin produce melanin which then produces a tan
• people with darker skin do not tan as easily because UV radiation is filtered out
• use a sunscreen with high SPF to reduce risk:
• max length of time to spend in the Sun = published normal burn time x SPF

Ozone depletion:

• ozone is found in the stratosphere
• helps to filter out UV radiation
• CFCs from aerosols and fridges destroy ozone and reduce the thickness of the ozone layer which increases the potential danger to humans
• the ozone layer is thinnest above the South Pole because ozone depleting chemicals work best in cold conditions
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