Topic 8 ~ Forces Doing Work

• Energy


  Energy is never created or destroyed, it is only transferred
• Energy transfer


  When a system changes, energy is transferred from one form to another
• Energy transfer examples


  • Electrical energy changes into kinetic energy in a motor
• Closed system


  A system which experiences no external forces
• Closed system examples


  • A circuit, or a skydiver
• In a closed system, the total energy remains the same at all times
• Gravitational potential energy


  Change in gravitational potential energy (joule, J) = mass (kilogram, kg) × gravitational field strength (newton per kilogram, N/kg) × change in vertical height (metre, m)
• Kinetic energy


  Kinetic energy (joule, J)= ½ ×mass (kilogram, kg) × (speed)2 ((metre/second)2, (m/s)2)
• Work done


  This is when an external force transfers energy to a system
• Work done example


  • A human doing work to push a ball up a hill
• Electric energy transfer


  Chemical potential energy (stored in batteries) forms electric
• Heating energy transfer


  Electric transfers to thermal energy, as work is done against the heating coils causing it to heat up
• Work done


  Work done (joule, J) = force (newton, N) × distance moved in the direction of the force (metre, m)
• Waste


  Energy is rarely 100% transferred into the desired form, some energy is dissipated so it is stored in less useful ways
• Mechanical processes


  • Mechanical processes are wasteful when they get hot (usually due to friction), energy is dissipated as heat, causing a rise in temperature, so the energy is lost to the surroundings
• Power


  The rate at which energy is transferred
• Power


  Power (watt, W) = energy transferred or work done (joule, J) ÷ time taken (second, s), where 1 Watt is equal to 1 Joule per second
• Efficiency


  Measure of how much energy is not lost as waste