physics

• Closed system


  A system where no energy can escape to or enter from the surroundings. The total energy in a closed system never changes.
• System


  An object or group of objects
• Energy stores


  • Kinetic
  • Gravitational potential
  • Elastic potential
  • Thermal (or internal)
  • Chemical
  • Nuclear
  • Magnetic
  • Electrostatic
• Calculating the energy in an energy store


  1. Kinetic energy = 0.5 x mass x (speed)^2

  2. Gravitational potential energy = mass x gravitational field strength x height

  3. Elastic potential energy = 0.5 x spring constant x (extension)^2
• Systems


  • Whenever anything changes in a system, energy is transferred between its stores or to the surroundings
• Closed system


  • No energy can escape to or enter from the surroundings. The total energy in a closed system never changes.
• Examples of energy transfers


  • Stretching a rubber band
  • Dropping a block from a height
  • Electric current in a kettle
  • Object slowing down due to friction
• Work done


  Energy transferred when a force moves an object. Calculated using: work done = force x distance moved along the line of action of the force
• Gravitational potential energy


  Depends on height above ground, gravitational field strength, and mass
• Kinetic energy


  Depends on mass and speed
• Elastic potential energy


  Calculated using: 0.5 x spring constant x (extension)^2
• Power


  How much work is done (or how much energy is transferred) per second. Calculated using: power = energy transferred / time or power = work done / time
• Energy cannot be created or destroyed – it can only be transferred usefully, stored, or dissipated (wasted)
• Useful and dissipated energy


  • Energy is never entirely transferred usefully – some energy is always dissipated, meaning it is transferred to less useful stores. All energy eventually ends up transferred to the thermal energy store of the surroundings.
• Reducing energy dissipation


  • Lubrication reduces unwanted energy transfer due to friction
  • Streamlining reduces energy wasted due to air resistance or drag in water
  • Thermal insulation reduces energy wasted due to heat dissipated to the surroundings
• Efficiency


  A measure of how much energy is transferred usefully. Calculated using: efficiency = useful output energy transfer / total input energy transfer or efficiency = useful power output / total power input
• Thermal conductivity


  How quickly energy is transmitted through a material by thermal conduction
• Testing thermal conductivity of rods

  1. Same diameter and length

  2. Same temperature difference between ends

  3. One end covered in wax

  4. Other ends heated equally

  5. Faster wax melts, higher thermal conductivity
• Thermal insulator


  • Material with low thermal conductivity
  • Low rate of energy transfer
• Thickness of walls and roof

  Affects rate of heat loss from building
• Thermal conductivity of walls and roof

  Affects rate of heat loss from building
• Lower thermal conductivity


  Lower rate of heat loss
• Specific heat capacity


  Amount of energy needed to raise the temperature of 1 kg of a material by 1 °C
• Calculating change in thermal energy

  Change in thermal energy (J) = mass (kg) x specific heat capacity (J/kg°C) x temperature change (°C)
• This equation will be given on the equation sheet, but you need to be able to select and apply it to the correct questions
• Thermal conductivity tells you how well a material conducts heat
• Thermal insulators have low thermal conductivity
• Factors determining rate of thermal energy transfer: thermal conductivity of material, temperature difference, thickness of material
• Factors affecting rate of heat loss from building: thickness of walls and roof, thermal conductivity of walls and roof, temperature difference between two sides
• Specific heat capacity is the amount of energy needed to raise the temperature of 1 kg of a material by 1 °C