power and efficiency

• potential difference - how much energy is in one unit of charge
• power - energy transferred in a given time
• current - rate of flow of charge
• specific heat capacity - the amount of energy required to rasie 1 kg of a substance by 1 degree
• specific latent heat - energy required to change the state of 1 kg of a substance
• specific latent heat of fusion - solids and liquids
• specific latent head of vapourisation - liquids and gases
• energy = Voltage x charge
• energy = power / time
• energy = current x volts x time
• electrical energy rating

  • in Watts or KWatts
  • tells you the rate at which energy transfers between stores.
  • electrical work done: wd (J) = power (W) x time (seconds)
  • domestic appliances: wd (KWh) = power (kW) x time (Hours)
  • efficiency = useful energy output / total energy input x 100
• • electrical work done: wd (J) = power (W) x time (seconds)
  • domestic appliances: wd (KWh) = power (kW) x time (Hours)
  • efficiency = useful energy output / total energy input x 100
• energy dissipation is the energy lost from a device to its surroundings.

  • 'wasted energy' is transferred to the surroundings
• lubrication - dissipation due to friction
• insulation - dissipation due to heating
• conduction: the direct transfer of thermal energy from one substance to another
• convection: the transfer of heat energy in a fluid (gas or liquid)
• radiation: transfer of heat energy through empty space
• convection only happens in fluids because their particles are free to move
• efficiency: the rate of useful work done to energy supplied.
• how can efficiency of a system be increased?

  • reducing waste output [by lubrication, thermal insulation]
  • recycling waste output
• the rate of energy transfer through a material ,with high thermal conductivity, is higher than for a material with low thermal conductivity
• double - glazed windows have less thermal conductivity than single - glazed windows, meaning less energy transfers through them
• 3 methods of reducing heat loss in a building

  • double glazing
  • loft and wall insulation
  • thicker walls
• 3 methods of reducing heat loss in a building

  • double glazing
  • loft and wall insulation
  • thicker walls
• useful and waste energy for a kettle


  useful: heat energy to the water

  waste: sound energy and infrared into the surroundings
• useful and waste energy for a hairdryer


  useful: heating the air, turning the motor to blow the air

  waste: sound energy, heating the motor, releasing infrared into the surroundings
• thermal equilibrium: when the temperature inside the house is the same as outside the house
• insulation


  • thicker walls help to retain heat in the house for longer
  • turning off the heat in the house, the energy transferred to the outside and will be quicker for thin-walled buildings. thinner walls cool the building faster
• thermal conductivity: energy transferred per second per metre2 of wall area, 1 m of wall thickness and 1 degree of temperature difference between the sides of the walls.
• thermal conductivity is a measure of how quickly energy transfers through a wall. low conductivity walls help retain heat for longer
• how to increase efficiency


  • use insulation to reduce the temperature of the surroundings
  • make devices from materials that reduce unwanted energy transfer
  • use technology to produce devices that are more efficient at their job
  • more efficient devices that operate at a lower power so they use up fuels more slowly
• useful energy = input energy - waste energy