Physics
AQA GCSE Physics Unit 1
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- Created by: Alice
- Created on: 15-06-11 22:48
Conduction
- Occurs mainly in solids
- Metals are better conductors than plastic or wood
- Vibrating particles pass on kinetic energy to other particles
- Metals have free electrons and at the hot end the electrons move faster and collide with other electrons transferring energy
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Radiation
- Depends on the colour and texture
- Dark matt = absorb heat radiation better than bright glossy surfaces (white or silver)
- Silvered = reflect nearly all heat
e.g. black solar panels or silvered survival blankets
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Convection
- Occurs in liquids and gases only
- More energetic particles move from the hotter region to the cooler region and take their heat energy with them.
- Convection currents are all about changes in density
e.g. immersion heaters in kettles and hot-water tanks
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Useful Heat Transfers
- There's usually more than one type of heat transfer
- Heat conduction through the bottom of a pan to heat up contents is useful
- Heat radiation out the sides of pan is not useful as it's the food you want to heat up
- To work it out: List main types of heat transfers and where, write down if it's useful or not, make sensibe suggestions
- Always mention which form of heat transfer is involved: conduction, convection or radiation.
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Energy Transfer
- Types of energy:
- Electrical: whenever a current flows
- Light: from the Sun, lightbulbs etc.
- Sound: from loudspeakers or anything noisy
- Kinetic: anything that's moving has it
- Nuclear: realeased only from nuclear reactions
- Thermal/heat: flows from hot objects to colder ones
- Gravitational potential: possessed by anything which can fall
- Elastic potential: stretched springs, elastic, rubber bands etc
- Chemical: foods, fuels, batteris etc
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Efficiency of Machines
- Some of the input energy is always lost of wasted, normally as heat
- The less energy wasted the more efficient a device
- Calculating the efficiency:
Efficiency = Useful Energy output / Total energy input
- No device is 100% efficient & wasted energy always dissipated as heat
- Other factors when comparing devices: Initial cost, running cost, ease of use, looks
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Efficiency in the Home
- Cavity wall insulation - Foam squirted into gap between bricks so reduces convection & radiation across gap
- Loft insulation - thick layer of fibreglass wool across loft floor reduces conduction & radiation into roof from ceiling
- Draught-proofing - strips of foam & plastic around doors & windows stops draughts of cold air - reduces convection
- Double glazing - two layers of glass means more radiation reflected back & air gap between glass reduces conduction
- Thermostatic radiator valves - prevent house being overwarmed
- Hot water tank jacket - lagging such as fibreglass wool reduces conduction & radiation
- Thick curtains - big bits of cloth over windows reduces heat loss by conduction & radiation
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Electricity Costs
- kWh = amount of energy
- A killowatt-hour is the amount of electrical energy used by a 1kW appliance left on for 1 hour
- Turning 1 kWh into 3 600 000 joules by:
E= P x T = 1 kW x 1 hour = 1000 W x 3600 s = 3600000J (=3.6 MJ) (Energy (in joules) = Power (watts) x Time (in seconds)
Calculating the cost:
No. of UNITS (kWh) used = POWER (in kW) x TIME (in hours)
COST = No. of UNITS x PRICE per UNIT
**Always turn the power into kW not watts and the time into hours not minutes
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Electricity & National Grid
- National grid takes electrical energy from power stations to where it's needed (homes; industries)
- Transmitting a lot of power needed needs a high voltage or high current
- Problem with a high current is loads of energy lost through heat in cables
- Much cheaper to boost voltage high (400 000) & keep current very low
- To get the voltage to 400000V requires transformers & big pylons with huge insulators
- Transformers have to step up voltage at one end and bring it down to safe levels at other end
- Voltage increased using a step-up transformer and reduced using a step-down transformer
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