Topic 8: Forces Doing Work

• Work - the scientific way of saying that energy has been transferred from one store to another

  There are multiple ways:

  • Mechanical - when a force acts on a body (child kicking a ball)
  • Electrical - charges moving due to a potential difference (battery to a lamp)
  • Heating - temperature differences caused by electric/chemical reactions (radiator)
  • Radiation - energy transferred as a wave (light/sound)
• Diagrams can be used to show how energy is transferred from one store to another:

  • Sankey Diagram - shows how all energy in a system is transferred into different stores
  • Transfer Diagram - boxes show energy stores and arrows show energy transfers

  
• Open Systems - energy can be lost and gained as it interacts with the outside world so the overall charge varies
• Closed Systems - energy and matter cannot enter meaning that the overall charge remains 0 even if energy transfers occur
• Work done - energy transferred by a force when it makes something move
• When work is done, energy has been transferred from one store to another.

  The amount depends on the size of the force acting on the object and the distance through which the force causes the body to move in that direction
• Dissipation of Energy - energy is wasted as it spreads into the surroundings or is converted into wasteful energy
• In a mechanical system, energy is dissipated when two surfaces rub together meaning work is done against friction.

  This heats them so the internal thermal energy store increases which is usually wasteful
• Energy is usually transferred to the internal energy stores of surroundings. The ways in which energy is dissipated depends on the system:

  • Radio/Speakers - electric energy transferred into useful sound energy and wasteful thermal energy is dissipated, causing internal energy store (temperature) of surroundings to rise
  • Tumble Dryer - electric energy transferred into useful thermal energy and wasteful sound energy is dissipated, causing internal energy store (temperature) of surroundings to rise
• Power - the rate at which energy is transferred (measured in watts which is equal to one joule per second)