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)

	Erstellt von georgie.proctor vor mehr als 12 Jahre

Notiz am GCSE core physics revision, erstellt von georgie.proctor am 08/06/2014.