GCSE core physics revision

Página 1

ENERGY TRANSFER BY HEATING

INFRARED RADIATION

every object emits infrared radiation

the hotter an object is, the more infrared radiation it emits in a given time

IR is beyond the red part of the visible spectrum

use a thermometer with a blackened bulb to detect it

radiowaves, microwaves, IR and visible light are parts of the electromagnetic spectrum ( so are UV and X rays)

electromagnetic waves are electric and magnetic waves that travel through space

sun emits all types of electromagnetic radiation- Earth's atmosphere blocks most of it that would harm us- doesnt block IR

earth's atmosphere acts like a greenhouse- short wavelength IR passes through to the Earth- objects on the earth emit longer wavelengths that can't pass through the atmosphere

Water vapour, methane and CO2 trap IR from the Earth, making it warmer- global warming- therefore need to reduce production of greenhouse gases

SURFACES AND RADIATION

dark matt surfaces are better at emitting radiation than light, shiny surfaces

a light, shiny surface absorbs less radiation that a dark, matt surface, a matt surface has lots of cavities and the radiation reflected from the matt surface hits the surface again (so scattering and absorption takes place), in a shiny surface just reflected and only absorbed from one point

light shiny surfaces absorb less radiation than dark, matt surfaces

light, shiny surfaces reflect more radiation than dark, matt surfaces

ABSORPTION TEST- coin stuck with wax on different surfaces, heater between, one which drops off first absorbs most

STATES OF MATTER

solid, no flow, fixed volume and shape, density much higher than gas

liquid, flows and fits container shape, fixed volume and density much higher than a gas

gas flows and fills container, volume can be changed and low density

can change states of matter by heating or cooling- solid to gas = sublimation

particles in a solid- fixed positions, vibrate about fixed positions, solid keeps its shape

particles in a liquid are in contact with each other, move about at random

particles in a gas move about at random (have the most energy)

CONDUCTION

metals conduct energy better than non-metals

copper is a better conductor than steel

wood conducts better than glass

metals contain lots of free electrons, these electrons move about at random inside the metal and hold the positive ions together- when a metal rod is heated at one end the free electrons at the hot end gain kinetic energy and move faster, these electrons diffuse and collide with other free electrons in the cooler parts, energy transferred

non-metals, no free electrons, can only transfer energy because atoms vibrate and shake each other (less effective)

CONVECTION

convection currents occur in fluids, hot fluid rises and cold fluid is drawn down to replace the hot fluid

circulation currents caused because fluids rise where they are heated (become less dense), fall where they cool down (more dense)- transfer energy from hotter parts to cooler parts

fluids expand when heated, particles move about more, density decreases ( same mass of fluid occupies a bigger volume)

EVAPORATION AND CONDENSATION

evaporation causes a cooling effect- as weak attractive forces exist between the molecules in the liquid, the faster molecules, which have more kinetic energy break away from the attraction of the other molecules and escape from the liquid, after they leave the liquid is cooler because the average kinetic energy of the remaining molecules as decreased

factors affecting the rate of evaporation- surface area of the liquid, increasing the temperature of the liquid and creating a draught of air across the liquid's surface

factors affecting the rate of condensation- increasing surface area and reducing the surface temp

ENERGY TRANSFER BY DESIGN

Vacuum flas- double-walled glass container, vacuum between two walls prevents conduction and convection, glass is a poor conductor so less conduction, silvery glass surfaces to reduce radiation from outer wall, spring made of plastic (insulator), plastic cap stops cooling by evaporation (and conduction due to plastic)

however, still eventually cools

factors affecting rate of energy transfer- bigger the temperature difference, faster the rate, also the materials the object is in contact with, the object's shape and the object's surface area, also the object's mass and material it is made from affect how quickly its temperature changes when it loses or gains energy

SPECIFIC HEAT CAPACITY

when a substance is heated its temperature rise depends on the amount of energy supplied to it, the mass of the substance, what the substance is

specific heat capacity of a substance is the energy needed or energy transferred to 1kg of the substance to raise its temperature by 1 degree- unit = J/kg/degree

E=m*c*weird0

storage heaters use electricity at night (off peak to heat special breaks or concrete blocks, energy transfer from the bricks keep the room warm, bricks have a high specific heat capacity so store lots of energy, warm up slowly and cool down slowly- cost effective as off peak charged less

HEATING AND INSULATING BUILDINGS

reduce energy transfer at home to reduce home heating bills- loft insulation (fibreglass reduces conduction and radiation), cavity wall insulation (insulation pumped in traps air in small pockets and reduces convection currents), aluminium foil (reflects radiation away) and double glazed windows (vacuum in between, reduces conduction and convection)

U-values- the energy per second that passes through one square metre of material when the temperature difference across it is 1 degree. - the lower the U value, the more effective the material is as an insulator

solar heating panels- don't use fuel to heat water but they are expensive to buy and install- payback time (time taken to recover the up-front costs from the savings on fuel bills)

USING ENERGY

FORMS OF ENERGY

energy stored or transferred in different ways- chemical energy (energy stored in fuel including food), kinetic energy, gravitational potential energy, elastic potential energy and electrical energy

when an object falls and gains speed, its gpe decreases and its ke increases

CONSERVATION OF ENERGY

energy cannot be created or destroyed

USEFUL ENERGY

useful energy is transferred to where it is wanted, wasted energy is that is not usefully transferred

wasted energy is dissipated to the surroundings, useful energy eventually transfers to the surroundings too, energy becomes less useful the more it spreads out as it is more difficult to use for further energy transfers

ENERGY AND EFFICIENCY

weight is measured in newtons, energy in joules

energy transfers represented in sankey diagrams

improving efficiency- friction between moving parts- lubricate, air resistance- streamlining, noise due to vibrations

ELECTRICAL APPLIANCES

transfer electrical energy into useful energy, but some is wasted

an electrical appliance is designed for a particular purpose and should waste as little energy as possible

ELECTRICAL POWER

energy we supply per second is the power supplied

the more powerful an appliance, the faster the rate at which it transfers energy

P = E/ t

efficiency = useful power out/total power in *100%

USING ELECTRICAL ENERGY

the energy supplied to an appliance depends on how long its used for and the power supplied to it

energy supplied to a 1kW appliance in 1 hour is 1 kilowatt-hour

use the kilowatt-hour as the unit of energy supplied by mains electricity

electricity meter

total cost = no. of kWh used * cost per kWh

COST EFFECTIVENESS MATTERS

need to consider the capital costs such as buying and installing equipment, the running costs including fuel and maintenance, environmental costs eg tax costs, and other costs such as interest on loans

payback time

low energy bulbs use much less electrical energy than filament bulbs

GENERATING ELECTRICITY

FUEL FOR ELECTRICITY

all electricity you use is generated in power stations

burning fuel heats water in a boiler, produces steam, steam drives a turbine which drives a generator

coal, oil and gas are fossil fuels obtained from long-dead biological material

in some gas- fired power station, we burn natural gas directly in a gas turbine engine, gas-fired turbine can be switched on very quickly

biofuels- methane gas from animal manure and cows and from sewage works, decaying rubbish and other sources- also ethanol, biofuels are renewable and carbon-neutral

nuclear power- energy released through nuclear fission- energy of the core transferred by a fluid (the coolant) that is pumped through the core- transferred to heat exchanger, turbines,

nuclear releases more energy per kg of fuel, but produces radioactive waste, no greenhouse gases

ENERGY FROM WIND AND WATER

renewable- dont produce a constant supply of energy, wave power can spoil areas of coastline and can affect the habitats of marine life and birds as tidal flow patterns might change

hydroelectric- can be pumped up when demand low then released when demand is higher

a tidal power station traps each high tide and uses it to turn generators

solar cells- useful when need small amounts of electricity, expensive to buy, cost nothing to run

geothermal- comes from energy released by radioactive substances, water pumped into hot rocks underground producing steam to drive turbines that generate electricity

ENERGY AND THE ENVIRONMENT

greenhouse gases, global warming, acid rain (sulfur can be removed before fuel is burnt), carbon capture and storage could be done

nuclear power- ads- no greenhouse gases, much more energy per kg of fuel, disads- radioactive waste, reactors are potentially dangerous

renewable power- ads- will never run out, no greenhouse gases or acid rain, no radioactive waste products - disads- noise and sound pollution, tidal barrages affect marine habitats, hydroelectric habitats flooded- unreliable source of energy (not constant), can affect plant and animal life

THE NATIONAL GRID

a network of cables and transformers that distributes electricity from power stations to homes and other buildings

step up transformers used at power stations- raised from 25,000V to 132,000 or more V (the grid voltage)

step down transformers at local substations step the grid voltage down to 230V for safe use in buildings

step up- increase the voltage and decrease the current ( so less energy loss due tot he heating effect of the current)

underground or overhead electricity pylons- underground more expensive, more difficult to repair, difficult to bury

BIG ENERGY ISSUES

start up time depends on the type of power station, renewable energy resources are unreliable

nuclear coal and oil provide a constant amount of energy (base load demand)

gas and renewable help meet daily variations in demand

renewable energy sources when deman is low to store energy in pumped storage schemes

carbon capture and storage is a new technology and likely to be expensive

cost of building and running a nuclear power station is very high. So is the cost of decommissioning it.

WAVES

THE NATURE OF WAVES

use waves to transfer information and energy

mechanical waves = sound, water, seismic- vibrations that travel through a medium (substance)

electromagnetic waves = light waves, radio and microwave- can travel through a vacuum, no medium needed 300 000 km/s

transverse wave- moves up and down, oscillation moves repeatedly between two positions (peaks)- electromagnetic waves- the vibrations of a transverse wave are perpendicular to the direction in which the waves transfer energy

longitudinal wave- slinky- areas of compression and rarefaction- sound- the vibrations of a longitudinal wave are parallel to the direction in which the waves are travelling

mechanical waves can be transverse or longitudinal

MEASURING WAVES

crest is the top, trough is the bottom, the amplitude is the height of the wave crest from the middle (bigger the amplitude, the more energy the waves carry), wavelength is the distance from one wave crest to the next

the no of wavecrests passing a fixed point every second is the frequency- unit is hertz

straight waves are called plane waves- waves all move at the same speed and keep the same distance apart

speed is the distance travelled by a wave crest every second

wavespeed = frequency * wavelength

WAVE PROPERTIES: REFLECTION

plane mirror

when plane waves reflect from a flat reflector, the reflected waces are at the same angle to the reflector as the incident waves

perpendicular line to the mirror is the normal

angle of incidence is the angle between the incident ray and the normal

angle of reflection is the angle between the reflected ray and the normal

reflected by a plane mirror: i=r

energy transfer by heating

using energy

generating electricity

waves