P1.1. Energy Transfer by heating

Transcription

1 P1.1 Energy Transfer by heating

2 Infrared radiation Infrared camera detect infrared radiation Every object emits infrared radiation The hotter an object is, the more infrared radiation it emits in a given time Infrared radiation can be detected using a thermometer with a blackened bulb placed just beyond the red on the spectrum The atmosphere acts like a greenhouse... Short wavelength infrared radiation can pass through the glass to warm objects inside Infrared radiation from the warm objects is trapped inside because these objects emit longer wavelength radiation that can t pass through the glass Gases in the atmosphere such as water vapour, carbon dioxide, methane trap infrared radiation from the Earth making it warmer than if we had no atmosphere

3 Surfaces and radiation A light, shiny surface emits less radiation these are used to keep accident survivors warm - than dark, matt surfaces Infrared radiation Reflection and absorption Infrared radiation Scattering and absorption Light, shiny surfaces absorb less radiation than dark, matt Light, shiny surfaces reflect more radiation than dark, matt

4 States of matter Kinetic theory Solids, liquids and gases are made of particles Solid fixed shape, particles vibrate Liquid particles in contact, move, flow Gas move fast, low density Melting Freezing Boiling Condensing

5 Conduction Metals conduct better than non-metals Metals contain free electrons which move about at random and hold the positive metal ions together When a metal rod is heated the free electrons gain energy and move faster The electrons diffuse and collide with other free electrons and ions in the cooler parts The kinetic energy is transferred

6 Convection Convection happens when we heat fluids (gas or liquid) due to circulation currents within the fluid The fluids rise when they are heated (as heating makes them less dense) They then fall when they cool down (more dense) Convection currents transfer energy from the hotter to the cooler parts Using convection -Hot water at home Hot water from the boiler rises and flows into the top of the tank -Sea breezes Ground heats faster than the sea, the hot air rises and is replaced by cool air from the sea

7 Evaporation and condensation Evaporation water molecules escape from the surface of the water Rate of evaporation is increased by increasing surface area, temperature or creating a draught Condensation water molecules from the air hit a cool surface (mirror) and condense Rate of condensation increases by increasing surface area or reducing surface temperature Cooling by evaporation Before an injection skin is wiped with a liquid that easily evaporates making your skin too cold to feel pain Weak attractive forces between molecules - The faster molecules with more kinetic energy escape the liquid the average kinetic energy has decreased so is cooler

8 Energy transfer by design The bigger the temperature difference between an object and its surroundings, the faster the rate energy is transferred The rate an object transfers energy depends on design including: The materials the object is in contact with The objects shape The objects surface area Mass and material Vacuum flask

9 Specific heat capacity... The energy transferred to 1kg of the substance to raise the temperature by 1 o C

10 Heating and insulating buildings Reducing the rate of energy transfers in the home reduces the cost of heating bills Loft insulation Cavity wall insulation Aluminium foil between radiator and wall reflects radiation away from the wall Double-glazed windows insulates U-values The energy per second that passes through one square metre of material when the temperature difference is 1C The lower the U-value, the more effective the material is as an insulator Payback time If you paid 1000 for solar panels and they save you 100 a year the payback time would be..? 10 years

11 Using Energy P1.2

12 Forms of energy We describe energy stored or transferred in different ways as forms of energy Chemical energy stored in fuel, released when chemical reactions take place Kinetic energy energy of a moving object Gravitational potential energy the energy of an object due to its position Elastic potential energy stored in a springy object when we stretch or squash it Electrical energy energy transferred by an electric current Energy can be transferred from one form to another. E.g. Torch Chemical energy in the battery Electrical energy Light energy Energy heating surroundings

13 Conservation of energy Energy cannot be created or destroyed The total energy after a transfer is the same as the energy before the transfer Bungee jumping: Gravitational potential energy kinetic energy Elastic strain energy Graviational potential energy kinetic energy Some of the energy is transferred to the surroundings as heat by the rope stretching Lots of energy transfers happen at the rollercoaster It gains gravitational potential energy as it climbs As it descends: gravitational potential energy kinetic energy + sound + heating

14 Useful energy A machine transfers energy for a purpose Friction between the moving parts causes them to warm up so some energy is wasted Useful energy every transferred to where it is wanted in the form wanted Wasted energy energy that is not usefully transferred Spreading out Wasted energy is dissipated (spreads out) to the surroundings Useful energy eventually transfers to the surroundings too Energy becomes less useful the more it spreads out Friction between the pads and disc slows the wheel

15 Energy and efficiency Weight is measured in Newtons (N) Energy is measured in Joules (J) Sankey diagrams Shows how we can represent any energy transfer where energy is wasted Input energy = useful energy delivered + energy wasted Efficiency = useful energy transferred by the device Total energy supplied to the device X 100

16 Energy and efficiency Efficiency limits No machine can be more than 100% efficient because we can never get more energy from a machine than we put into it Improving efficiency Why machines waste energy Friction between moving parts causes heating The resistance of a wire causes the wire to get hot when a current passes through it Air resistance causes energy transfer to the surroundings Sound created by machinery causes energy transfer to the surroundings How to reduce the problem Lubricate the moving parts to reduce friction In circuits, use wires with as little electrical resistance as possible Streamline the shapes of moving objects to reduce air resistance Cut out noise (e.g. Tighten loose parts to reduce vibration)

17 Electrical Energy P1 3.1

18 Electrical appliances Transfer electrical energy into useful energy (with some wasted energy) Appliance Useful energy Energy wasted Light bulb Light from the glowing filament Energy transfer from the filament heating the surroundings Hairdryer Kinetic energy of the air being driven by the fan Energy heating air flowing past the heater Sound of fan motor Energy heating the hairdryer itself

19 Electrical power The more powerful an appliance, the faster the rate at which it transfers energy The power of an appliance is measured in watts (W) or kilowatts (kw) Power (W) = energy transferred to the appliance (J) Time taken for the energy to be transferred (s) Example: A motor transfers 10,000J of energy in 25s. What is its power? Efficiency = useful power out total power in (x100%) Answer: P = E = 10,000 = 400W t 25 Example: the useful power out of an electric motor is 20W and the total power is 80W, the percentage efficiency is 20 / 80 (x100) = 25%

20 Using electrical energy For any appliance the energy supplied to it depends on: How long it is used for The power supplied to it To work out the energy transferred by a mains appliance in a certain time: The energy supplied to a 1kW appliance in 1 hour is 1 kwh (kilowatt-hour) - The electrical meter measures how much electrical energy the home uses and records the kwh used

21 Cost effectiveness matters To compare the cost effectiveness Capital costs buying and installing Running costs fuel and maintenance Environmental costs removal, disposal, tax Payback time 1. Loft insulation costs 200 and saves 100 per year on her fuel - The payback time is 2 years 2. Double-glazed windows cost 200 and save 10 per year what is the payback time?

22 P1.4 Generating electricity

23 Fuel for electricity Biofuels: methane gas from cows, manure, sewage works can be used in power stations. Biofuels are renewable and carbon neutral because the carbon dioxide taken in can balance the amount released when it burns Almost all of the electricity you use is generated in power stations

24 Fuel for electricity nuclear power The fuel in a nuclear power station is uranium The nucleus of a uranium atom is unstable and can split in two, releasing energy this is nuclear fission There are lots of uranium atoms in the core so it becomes very hot The energy of the core is transferred by a coolant, used to turn water to steam Nuclear power stations release much more energy than fossil fuel power stations (10,000x) but has radioactive waste that needs to be stored for many years. Uranium doesnt produce greenhouse gases

25 Energy from wind and water Wind A wind turbine is an electricity generator at the top of a narrow tower, the wind drives the turbines blades around and power increases with wind speed Wave A wave generator uses the waves to make a floating generator move up and down, turning the generator. A cable links this with the shore and grid system. They need to withstand storms and lots of cables might be needed Tidal Traps water from each high tide behind a barrage which is then released into the sea through turbines which drive the generators in the barrage Hydroelectric Rainwater collected in a reservoir flows downhill, the floating water drives turbines that turn the generators at the foot of the hill

26 Power from the sun and the Earth Solar radiation Transfers energy to generate electricity using solar cells We can also use the suns energy to heat water directly in solar heating panels They are useful in remote places or where only small amounts of energy They are expensive to buy but cost nothing to run Lots of them and sunshine are needed Geothermal energy Comes from the energy released by radioactive substances deep within the Earth The energy released heats the surrounding rocks and energy is transferred by heating towards the Earth s surface water is pumped down to produce steam which drives the turbines

27 Energy and the environment Fossil fuel problems Greenhouse gases (carbon dioxide) are released which may cause global warming Sulphur dioxide which causes acid rain we can remove sulphur from a fuel to reduce this Non-renewable they will run out Carbon capture and storage (CCS) Nuclear vs renewable Advantages: no greenhouse gases, more energy per gram of fuel Disadvantages: used fuel rods contain radioactive waste, an explosion could release radioactive material Renewable Advantages: never run out, no greenhouse gases or acid rain, no radioactive waste, can be used in remote areas Disadvantages: wind turbines make a whirring noise, unattractive, tidal barrages affect estuaries and habitats, hydroelectric may flood habitats to create dams, solar cells need to cover large areas

28 The national grid Electricity reaches your home through the national grid This is a network of cables that distributes electricity from power stations Power stations produce electricity at a voltage of 25000V Step-up transformers at power stations Step-down transformers at local substations Underground or overhead? Some people object to electricity pylons saying they ruin the countryside or affect their health electric currents produce electric and magnetic fields Underground cables would be more expensive, difficult to repair and difficult to bury at roads, canals and rivers Overhead cables are high above the ground but underground may affect people more as they wont be buried very deep

29 Big energy issues Supply and demand The demand for electricity varies during each day and is higher in winter than summer our generators need to match these changes Power stations cant just instantly start up Renewable energy resources are unreliable Hydroelectric Wind, waves Tidal Solar Upland reservoir could run dry Wind and waves too weak on calm days Height of tide varies on a monthly and yearly cycle No solar energy at night and variable at daytime The variable demand for electricity is met by: Using nuclear, coal and oil fired power stations to provide a constant supply Using gas and pumped storage to meet daily variations using renewable resources when demand is high Using renewable resources when demand is low to store

30 P1.5 The nature of waves

31 The nature of waves Transverse waves The vibrations of a transverse wave are perpendicular to the direction in which the waves transfer energy Longitudinal waves The vibrations of a longitudinal wave are parallel to the direction in which the waves are travelling

32 Measuring waves Wavelength: distance from one wave crest to the next Frequency: the number of complete waves produced by a source in one second, Hz Amplitude: the height of a wave from its undisturbed position Wave speed = frequency x wavelength m/s Hz m

33 Wave properties: reflection Incidence ray from ray box Virtual image is one from which the light rays appear to come but don t actually come from that image like in a mirror. Real image is the image formed where the light rays are focussed Reflected ray angle of incidence = angle of reflection

34 Wave properties: refraction Sound waves and light waves change speed when they pass between the boundary of two substances with different densities (e.g. air and glass). This causes them to change direction and this effect is called refraction Refraction doesn't happen if they cross the boundary at an angle of 90 - in that case they carry straight on.

35 Wave properties: diffraction The spreading of waves when they pass through a gap or past an obstacle The narrower gap, the more the waves spread out The wider gap, the less the waves spread out Plane barrier reflects the waves If the gap is smaller than the wavelength the waves are reflected If the gap is bigger than the wavelength the waves are diffracted The larger the gap the less the waves diffract

36 P1.6 Electromagnetic waves

37 The electromagnetic spectrum

38 Light, infrared, microwaves and radio waves Light and colour Light from ordinary lamps and the sun is called white light It has all colours of the visible spectrum in it You can use a glass prism to split a beam of white light Microwaves Shorter wavelength than radio waves used : Communications e.g. satellite TV because they can pass through the atmosphere, to beam signals as they spread out less than radio waves, Mobile phones Infrared radiation All objects emit infrared radiation The hotter the object, the more infrared radiation Optical fibres use infrared radiation instead of light Also used in remotes, scanners and cameras Radio Waves Carry radio, TV and mobile phone signals, bluetooth

39 Communications Optical fibres transmit signals carried by light or infrared radiation. They have smaller wavelengths than micro- or radio waves so carry more information and are more secure because signals stay in the fibre Radio wavelengths Microwaves and radio waves of different wavelengths are used for different purposes: Shorter wavelengths: carry more information, diffract less, have a shorter range Microwaves: satellite and TV as they diffract less and can travel between space and the ground Radio waves of wavelength less than 1m: TV broadcasting as they carry more information than longer wavelengths Radio waves of wavelength from 1-100m: local radio stations, emergency services as their range is limited Radio waves of wavelength greater than 100m: national and international radio stations Are mobile phones dangerous? Radiation is quite weak but it is close to your brain Children have thinner skulls than adults so could affect them more. Overall... We re not sure

40 The expanding universe The Doppler effect: Change in observed wavelength of waves due to the motion and source of the waves Edwin Hubble Discovered that: The light from distance galaxies was red-shifted, The further away, the bigger the red shift Concluded that: Distant galaxies are moving away from us, Greater the distance the greater the speed its moving away This must mean that the whole universe is expanding

41 The Big Bang The Big Bang theory states that: The universe is expanding after exploding suddenly in a Big Bang from a very small initial point Space, time and matter were created in the Big Bang Evidence for the Big Bang: Cosmic microwave background radiation (CMBR) It was created as high-energy gamma radiation after the big bang and has been travelling through space since then As the universe has expanded it has stretched out to longer and longer wavelengths and is now microwave radiation It has been mapped out using microwave detectors An alternative theory... The Steady State theory suggests that the galaxies are being pushed apart caused by matter entering the universe through white holes Future of the universe: If the density of the universe is less than a certain amount then it will expand forever.. If not, it will stop expanding and reverse ending in a crunch