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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