Energy

• Stores of Energy

  • Kinetic store
  • Gravitational potential store
  • Chemical store
  • Elastic potential store
  • Nuclear store
  • Internal store
• Kinetic store

  The energy stored by an object's movement
• Gravitational potential store

  The energy stored in objects raised above the Earth's surface. This energy exists because of the Earth's gravitational field.
• Chemical store

  The energy stored by the chemical bonds between atoms
• Elastic potential store

  The energy stored when an object is being stretched, compressed or squashed
• Nuclear store

  The energy stored in the nucleus of an atom. The energy in the nuclear store can be released by radioactive decay.
• Internal/Thermal store

  The sum of the kinetic energy stored in the particles of an object and the potential energy stored in chemical bonds between particles in the object
• The temperature of a body (object) is a measure of the energy in the kinetic stores of its particles
• Conservation of Energy

  Energy cannot be created or destroyed, it can only be transferred from one store of energy into another
• Ways energy can be transferred

  • Mechanical work
  • Electrical work
  • Heating
  • Waves
• Mechanical work

  Energy transferred from one object to another via a force
• Mechanical work

  • When an object falls from a building, the weight does mechanical work to transfer energy from the gravitational potential store to the kinetic store
  • When you push a wheelchair, energy is transferred to the kinetic store of the wheelchair
• Electrical work

  An electrical current transfers energy from one object to another, such as a battery powering a torch
• Heating

  Heating an object transfers energy to its internal store (the sum of the energy in the kinetic and chemical stores of its particles)
• Waves

  Light and other types of waves can transfer energy from one object to another
• Energy conversion: A moving object hitting an obstacle

  1. The object's energy is in the kinetic store to begin with

  2. When the object collides with the obstacle, energy is converted to the kinetic store of the obstacle, the thermal store of the object and the obstacle

  3. Some energy remains in the object's kinetic store as it moves away after the collision
• Energy conversion: An object projected upwards

  1. The object's energy is initially in the kinetic store as it moves upwards

  2. The object's energy is slowly transferred from the kinetic store to the gravitational potential store as it slows down and climbs higher

  3. Once the object reaches its highest point, all of its energy is in the gravitational potential store

  4. As the object falls again, energy transfers from the gravitational potential store to the kinetic store
• Energy conversion: A vehicle slowing down

  1. To begin with, the vehicle's energy is in the kinetic store

  2. The brakes do work slowing the car down, during this process energy is dissipated (lost) through heat and sound
• Energy conversion: Boiling water in an electric kettle

  Energy transfers from the electrical store of the mains power supply to the thermal store of the water
• Mechanical Work Done

  The work done by a force is equal to the energy transferred to the object
• Mechanical Work

  • More work is done if you push with the same force for a longer distance
  • More work is done if you push with a larger force for the same distance
• Energy Losses & Efficiency

  When one store of energy is transferred to another, some energy is wasted (transferred into unwanted stores of energy). This dissipates (spreads out) to anything nearby.
• Car engines

  • The useful energy store is the kinetic store because you want the car to move you from one place to another
  • The heat and sound energy are wasted energy because they are of no use to you
• When wires are used in an electrical circuit, they heat up. This is often not useful energy and so it is wasted.
• In almost every energy transfer, friction creates heat energy and this energy is often wasted.
• Work done

  force x distance (moved in the direction of the force)

  
• Reducing Unwanted Energy Transfer: Lubrication

  • Reduces frictional forces
  • Lubricants can reduce the friction between the surfaces of objects that are rubbing together
  • Lubricants are normally liquids
• A moving object normally has at least one frictional force acting against it


  This causes some energy to be dissipated
• Reducing unwanted energy transfer: Thermal insulation

  • Heat will be lost through the roof, windows, walls and floor
  • To reduce the energy transfer, we might: Fit loft insulation (the roof), Use double glazing (windows), Use cavity wall insulation (walls), Fit carpets (floors)
• Reducing unwanted energy transfer: Walls in homes

  • We can decrease the rate a house cools down by: Having thicker walls, Having walls made from a material with a lower thermal conductivity
• Thermal conductivity

  The more heat that a material conducts, the higher the rate of energy transfer by conduction across the material
• Power

  The rate of energy transfer to a component, or the rate at which work is done
• Power - energy transfer

  Power (watts, W) = energy transferred (joules, J) / time (seconds, t)

  
• Power - work done

  Power (watts, W) = work done (joules, J) / time (seconds, t)