Energy and efficiency

Energy and efficiency

1. Conservation of energy

   • Energy cannot be created or destroyed, only transferred
     • When energy is transferred, only part of it may be usefully transferred --> the rest is wasted
       • Wasted energy is transferred to its surroundings, which become warmer --> wasted energy becomes more spread out --> less usefel
       • Sankey diagram
         • Shows useful and wasted forms of energy
         • Shows how efficient an appliance is

2. Efficiency = useful energy out/total energy in x 100%

3. Usefulness of electrical appliances

   • Electrical appliances transfer energy at the flick of a switch
     • You can calculate how much energy is transferred
       • E = P x t
         • Amount of energy transferred from the mains electricity (kWh and J)
     • You can calculate how much the appliance costs to run
       • Total cost of bill = no. of kWh used x cost per kWh (rate)
   • Power is the rate at which the object transfers energy
   • Cost effectiveness
     • Getting the best value for money
     • Payback time is the time it takes for an appliance/installation to pay for itself in terms of energy savings

4. Efficiency = useful power out/total power in x 100%

5. Input energy = useful energy + wasted energy

6. Forms of energy

   • Stored energies
     • Gravitational potential
       • A falling object transfers g.p. to kinetic energy
     • Elastic potential
     • Chemical

7. Improving effiency

   • Reduce:
     • Friction (e.g. lubricating moving parts)
     • Electrical resistance (e.g. less resistant wires)
     • Noise due to vibrations (e.g. cut out noise)
     • Air resistance (e.g. streamline the shapes of moving objects)