Conservation of energy and power

• Power


  The rate at which energy is transferred or the rate at which work is done
• Calculating power


  1. Energy transferred in Joules divided by time in seconds

  2. Work done in Joules divided by time in seconds
• Watt


  Unit of power, equal to an energy transfer or work done of 1 Joule per second
• Calculating power of Bunsen burners

  • Left Bunsen burner: 250,000 J of thermal energy transferred in 500 s, power = 500 W
  • Right Bunsen burner: 180,000 J of thermal energy transferred in 500 s, power = 360 W
• Calculating power of car brakes

  • 40,000 J of work done by brakes in 5 s, power = 8,000 W
• Calculating power of motors

  • Motor A: 500 J of electrical energy transferred to gravitational potential energy in 40 s, power = 12.5 W
  • Motor B: 500 J of electrical energy transferred in 60 s, power = 8.3 W
• Work done


  Whenever energy is transferred from one store to another
• Types of work


  • Mechanical work
  • Electrical work
• Mechanical work


  • Using a force to move an object
• Example of mechanical work


  • Dog pulling a sledge
• Energy transfers in mechanical work

  1. Chemical energy in dog

  2. Kinetic energy in sledge

  3. Thermal energy in sledge runners and ground
• Work done (formula)

  Work done (J) = Force (N) x Distance (m)
• Example of calculating work done

  • Force = 50 N, Distance = 3 m, Work done = 150 J
• Example of work done in braking

  • Car traveling at 20 m/s, Brakes apply 2000 N force, Car stops in 20 m, Work done = 40,000 J
• The work done equation is not given in the exam, so it must be learned
• Energy transfers in a bungee jump

  1. Jumper starts with gravitational potential energy

  2. Energy transfers to kinetic energy as jumper falls

  3. Kinetic energy reaches maximum when bungee rope starts to tighten

  4. Kinetic energy transfers to elastic potential energy in the bungee rope

  5. Bungee rope recoils, transferring energy back to kinetic energy

  6. Kinetic energy transfers back to gravitational potential energy as jumper ascends

  7. At top of ascent, all energy is in gravitational potential energy store
• How is Energy dissipated in a bungee jump(2)?

  • Due to friction with air particles as the jumper passes through the air
  • Due to stretching effects in the bungee rope (bungee ropes are not fully elastic)