Energy Stores and transfers

• When there is a change to a system, energy is transferred
• If an apple sits on a table, and that table is suddenly removed, the apple will fall
• As the apple falls, energy is transferred
• Defining a system in physics

  It is a way of narrowing the parameters to focus only on what is relevant to the situation being observed
• Energy Stores


  • Objects store energy in different energy stores
• Energy Transfer Pathways

  • Mechanical
  • Electrical
  • Heating
  • Radiation
• Energy transfers in scenarios

  1. A battery powering a torch: Energy transferred from the chemical store of the battery to the thermal store of the bulb via an electrical pathway

  2. A falling object: Energy transferred from the gravitational store to the kinetic store of the object via a mechanical pathway
• When a system is in equilibrium, nothing changes, and so nothing happens
• In physics, a system is defined as an object or group of objects
• Exam Tip: Don't worry too much about the parameters of the system. They are there to help
• Exam Tip: Don't worry too much about the parameters of the system. They are there to help you keep your answers concise so you don't end up wasting time in your exam
• Principle of conservation of energy: Energy cannot be created or destroyed, it can only be transferred from one store to another
• Transfer pathway

  Mechanical
• Ezciency


  • The ratio of the useful energy output from a system to its total energy output
  • If a system has high efficiency, this means most of the energy transferred is useful
• Energy transfer from gravitational store to kinetic store


  Mechanical transfer pathway
• If you follow any process back far enough, you would get many energy transfers taking place
• Total energy transferred into a system must be equal to the total energy transferred out of the system
• Energy can be dissipated to the surroundings by heating and radiation
• Ezciency is a measure of the amount of wasted energy in an energy transfer
• Total energy in a closed system remains constant
• Dissipated energy transfers are often not useful and can be described as wasted energy
• Force is weight


  Acts over a distance (the height of the fall)
• Calculating Useful Energy Output

  Useful energy output = total energy output - wasted energy
• Sankey diagrams can be used to represent energy transfers
• Calculating Ezciency

  efficiency = useful energy output / total energy output × 100%
• Ezciency calculation example: Blades of a fan turned by an electric motor
• Ezciency calculation example: Electric motor lifting a weight
• Total energy in


  Equals total energy out
• If a system has low ezciency, most of the energy transferred is wasted
• Ezciency


  The measure of the amount of wasted energy in an energy transfer
• Sankey diagrams

  • Characterised by splitting arrows showing proportions of energy transfers
  • Different parts of the arrow represent different energy transfers
  • Width of each arrow is proportional to the amount of energy transferred
• Ezciency


  The ratio of the useful energy output from a system to its total energy output
• Ezciency is represented as a percentage
• Materials containing small pockets of trapped air are especially good at insulating because air is a gas and hence a poor conductor
• Wasted energy calculation: 500 - 120 = 380 J
• Conduction is the main method of energy transfer by heating in solids
• Non-metals are poor thermal conductors while liquids and gases are extremely poor
• Objects will continue to lose heat until they reach thermal equilibrium with their surroundings
• Conservation of energy

  Energy cannot be created or destroyed, it can only be transferred from one store to another
• Calculate wasted energy

  Wasted energy = total energy in - useful energy out