- Created by: MPR
- Created on: 20-10-10 19:44
radiation, conduction, convection
-transfer of heat energey by infra red waves -no particles are involved, works in a vacuum
-Dark, matt=good absorbers and emiters
-Light, shiny= poor absorbers and emits
-Larger surface area (thin, flat) radiates more heat energy
-the hotter an object, the more heat it emits
-An object that is warmer than surroundings, loses heat energy and cools down.
-An object that is cooler than surroundings, gains heat energy and warms up.
Conduction: (solids only)
-if one end of a solid is heated the partiles gain kinetic energy, vibrate more. The energy is passed on to neighbouring particles.
-METALS: coduction occurs faster bc metals have free electrons. These gain kinetic energy and can freely move, as well as having vibrating particles.
Convection: Fluids (does not work in any solids bc particles are not free to move)
-Fluid is heated, partiles move away from each other, becomes less dense and rises. Cool -liquid: particles move close together, becomes dense, sinks.
max/min heat loss & vacuum flask
- Maximising heat loss:
painted dull black
maximised air flow
Reducing heat loss In a House:
- BY CONDUCTION: using insulators eg trapping a layer of air - double glazing/cavity wall insulation
- BY CONVECTION: preventing convection currents e.g trapping air in small pockets- fibre glass
- BY RADIATION: using poor emitters e.g. light, shiny surfaces - aluminium foil behind radiators
The vacuum flask:
- there is vacuum between two wall of the container- stops conduction&convection
- Liquid is in a glass container. Glass - insulator- stops conduction
- Silverly glass surfaces - poor emitter-stops radiation from the outer wall
Energy forms and efficiency of a device
-There are 9 forms of energy
-CONSERVATION OF ENERGY -Not possible to create or destory energy. It only transforms/transfers
A device transfers energy from one place to another/transfers energy.
-Input energy= useful + wasted energy
-Both useful and wasted energy is transformed to the surroundings, making them warm up
-As the energy spreads out, it becomes more diffucult to use it.
-Wasted energy is ofetn heat if it has moving parts (due to friction). Sound is also ofted wasted energy
-Devices have vents so that the wasted (heat) enery will warm surroundings/not overheat it.
Efficieny - useful energy transformed (J) / total input energy (J)
mains: power, energy transferred, cost of energy,
Electrical devices -transform electric energy to whatever form
-POWER - rate at which a device transforms energy. the faster the rate, the more powerful the device
-power (W) = energy transferred (J) / time (s)
-one watt= one joule per second
-one kW=1000 W
Think mains electricity -
-energy transferred (kWh) = power (kW) x time (h)
-total cost= energy transferred (kWh) x cost per kWh
-electicity meter- records no of kWh used.
Current reading - previous reading = energy used between readings
step up transformer- increase voltage, decrease current =reduce energy loss,more efficient
step down- decrease voltage, increase current = safer for consumers
-power stations: burning fuel/heats water to produce steam, steam drives turbines, drives generators, produce electricity.
(coal- co2&so2)(oil-co2&so2)(gas -co2, starts up quickly)
-nuclear power stations- uranium/plutonium used. The nucleus undergoes nuclear fission which releases heat energy. This energy turns water into steam etc.
-more energy from each kg of nuclear fission than fossil fules
-no polluting gases but nuclear waste, risk of big accidents
-Wind and water- drive turbine directly
(turbine- dilute, sound/visual pollution) (hydroelectric- pumped storage system, starts up quickly, must be wet and hilly, flooding) (waves- hazardous to boats) (tides -reliable, affects ecology)
-Earth and Sun- (solar- coverts electromagentic radiation into electrical energy/ heats water flowing through/ light to electric, dilute)
(geothermal- radioactive processes heat rocks. hot water comes to the surface-heats buildings). (naturally in some places or deep holes are drilled, cold water is pumped down. steam comes up) Steam drives turbines etc, only in few places,