P1a 3 (AQA Core Physics)

summary notes for AQA Core physics, unit 3 (electrical energy) made using the Nelson thornes revision guide (really useful!)

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  • Created on: 15-01-12 11:48

Unit summary

  • Power is the amount of energy transferred each second
  • Power is measured in watts (W)
  • Energy is measured in joules (J)
  • Electricity is transferred around the country via the National Grid
  • Energy transferred = power x time
  • A kilowatt hour (Kwh)  is used to measure the amount of electricity used in a home 
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P1a 3.1 Electrical devices

  • Electrical devices are devices which transform electrical energy into useful energy
  • An example of an electrical device is a speaker (electrical energy to sound energy)
  • Electrical energy is transferred due to electrical currents uses for electrical devices  include; heating, lighting, movement and sound/visual 
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P1a 3.2 Electrical power

  • Power is measured in watts (W)  
  • Power is a measure of how much electricity a device used per second (the rate it uses energy)
  • Electricity is measured in joules (J)
  • One watt means that a device uses one joule per second (W = 1 J/s)
  • A watt is a small measurement so sometimes a kilowatt (kW) is used
  • One kilowatt = 1000 watts
  • Power (watts) = (energy transferred in joules) ÷ (time taken in seconds) 
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P1a 3.3 Using electrical energy

  • Electricity companies charge customers for the amount of electricity that they use, they charge per kilowatt hour (kW h)
  • A kilowatt hour is the amount of energy transferred by a one kilowatt device in one hour
  • Energy transferred (kW h) = power of device (kW) x time used (hours)   
  • Cost of electricity = number of kW h x cost per kW h

E.g. a 9 kW shower is used for 12 min at the cost of 7p per unit. What was the total charge?

9 (kW) x 0.2 (hrs) = 1.8 (kW h)

1.8 (kW h) x 7 (p per unit) =  12.6p 

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P1a 3.4 The national grid

  • The national grid is a network of cables and transformers
  • Step-up transformers increase voltage at power stations
  • Increased voltage reduces energy loss and makes it more efficient
  • Step-down transformers reduce voltage at local points for use in homes etc.
  • Voltage is reduced for safety reasons

Power station à step-up transformer (increases voltage to reduce energy loss) à cables  à step-down transformers for safety) à homes, offices etc. 

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summary questions

  • What unit is used to measure power other than a watt?
  • In a 3 kW kettle, how many joules are transformed each second?
  • How much does it cost to run a 1200 W vacuum cleaner for 10 minutes at 10p per kW h?
  • Why is the voltage of the national grid reduced before electricity is delivered to homes? 
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answers to summary questions

  •  Kilowatt (kW
  • 3kW = 3000 W = 3000 J/s
  • 1200 W= 1.2 kW, 10 min =10/60 h, 1.2 kW x 10/60 h = 0.2 kWh, 0.2 x 10 = 2, 2p
  •  Because high voltage is dangerous
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