P1a 3 (AQA Core Physics)

• 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 

• 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 

• 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) 

• 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 

• 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. 

• 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? 

•  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