energy stores/systems

• System

  A single object (e.g. the air in a piston) or a group of objects (e.g. two colliding vehicles) that you're interested in
• When a system changes, energy is transferred
• Energy can be transferred from the system, between different objects in the system or between different types of energy stores
• Closed system


  A system where neither matter nor energy can enter
• The net change in the total energy of a closed system is also zero
• Energy transfer by heating

  1. Energy is transferred to the water's thermal energy store (causing the temperature of the water to rise)

  2. Energy is transferred electrically to the thermal energy store of the kettle's heating element, which transfers energy by heating to the water's thermal energy store
• Work done


  Energy transferred
• Energy transfer by doing work


  1. Work done against resistance in a circuit

  2. Work done by a force moving an object

  3. Work done by the gravitational force

  4. Work done by the friction between a car's brakes and its wheels

  5. Work done by the normal contact force between a car and a stationary object
• In a collision between a car and a stationary object, the normal contact force between the car and the object does work
• It causes energy to be transferred from the car's kinetic energy store to other energy stores, e.g. the elastic potential and thermal energy stores of the object and the surroundings
• Some energy might also be transferred away by sound waves
• Energy stores pop up everywhere in physics