Electrical energy

Electrical energy

1. Power

   • Energy supplied to an electrical device per second is the power supplied to it
     • We say that the more powerful a device, the faster the rate at which it transforms energy
       • Power (in watts, W) = rate of transfer of energy
         • = energy transferred (in joules J) _________________________________ time taken (in seconds, s)
     • Measure the power of a device in watts (W) or in kilowatts (kW)
       • For any device: - the input energy is the energy supplied to it - the output energy is the useful energy per second transferred by it
         • 1 watt is a rate of transfer of energy of 1 joule per second (J/s)
           • 1kW device in an hour is 1 kilowatt-hour (kW h)
         • 1 kilowatt 1000 watts
     • Energy transferred (kW h) = power of device (kW) x time in use (hours, h)
   • a 2kW heater switched on for 4 hours uses 8kW h of electrical energy
     • When paying the electricity bill, we use this equation: total cost = number of kW h used x cost per kW h

2. Electrical devices & energy

   • When we use an electrical device, some energy transformed is useful, and the rest is wasted

3. National Grid

   • we use step-up transformers in power stations and stepdown transformers in sub-stations near homes
     • Step-up transformer increases the votlage
     • Step-down transformer decreases the voltage
     • The National Grid’s voltage is 132000 volts plus, and power stations only produce electricity at a voltage of 25000 volts. This is when we use step-up transformers at power stations to step this voltage up to the National Grid voltage, and then we use step-down transformers at the sub-stations near our homes to step the voltage back down to 230 volts for homes and offices
       • Some people constantly debate over whether they should be built above or below ground. If they are built above ground they are visible and not aesthetically pleasing, but campaigners for underground mains could be argued with, because underground cables are far more expensive, much more difficult to repair and are extremely difficult to bury where they cross canals, rivers and roads
   • Electricity reaches our home through the national grid