Physics P1 Energy

• Energy changes in a system

  Examples:

  • A vehicle slowing down e.g. When you apply the brakes in a lorry
  • Bringing water to a boil on a camping stove
• What happens when a lorry slows down

  1. The moving lorry has a store of kinetic energy

  2. When the brakes are applied the kinetic energy store decreases

  3. The energy is transferred to the internal (thermal) energy store in the brakes and the brakes get hot

  4. You will hear a sound when the brakes are applied, the energy of the sound is also transferred to the internal energy store of the surroundings

  5. When the lorry stops its kinetic energy store is zero
• What happens when water is boiled on a camping stove

  1. As the fuel burns the chemical energy store in the fuel decreases

  2. The internal (thermal) energy store of the water increases

  3. The temperature of the water increases and as bubbles form the kinetic energy store of the water increases
• Energy


  Measured in Joules (J)
• Kilojoule (kJ)


  1000 J (103J)
• Megajoule


  1000 000 J (106J)
• Mechanical work


  The amount of energy transferred by a force
• When a pushback truck is used to move an aircraft, it does work
• Electrical work


  The amount of energy transferred when charge flows in a circuit
• Kinetic energy


  Energy of a moving object
• Elastic energy


  Energy stored in stretched or bent objects that can recover their original shape
• Gravitational potential energy


  Energy gained by an object raised above ground level
• Specific heat capacity


  The ability of a 1kg object to store transferred energy
• When a bowl of water and a stone are left in hot sunshine, the stone feels much hotter than the water. The water has a higher heat capacity as it takes more heat energy to raise its temperature to that of the stone
• Alternative units of power

  • Joules/second
  • Watts
• Energy transfers in a blowtorch

  Chemical energy is transferred into thermal energy usefully and light energy is wasted
• Energy wasted in a blowtorch


  As thermal energy to the environment
• The blowtorch transfers 2 kJ of energy in 4 mins. Power of the blowtorch = 8.33 Watts
• Crane A takes 30 s to lift the load


  Crane B lifts the same load in 9 s
• Crane A power = 600 W, Crane B power = 2000 W, Difference in power = 1400 Watts
• Change in thermal energy of storage heater bricks

  1. Change in thermal energy = Mass x Specific Heat Capacity x Temperature Change

  2. ΔE = 40 x 850 x 22

  3. ΔE = 748 000 J or 748 kJ
• Closed energy system


  Energy can be transferred but not mass, total energy in the system remains constant
• In a closed energy system all the energy can be accounted for even when energy stores change
• Unwanted energy transfers result in energy stores that are not useful
• Efficiency


  The amount of useful energy you get from an energy transfer, compared to the energy put in
• Efficiency = useful output energy transfer / total input energy transfer
• The wind turbine is 24% efficient
• The water pump is 22.2% efficient
• In a closed system, energy can be transferred but there is no net energy loss
• The mobile phone dissipates 19 J of thermal energy
• The thermal energy produced by a bus driving along a road is dissipated by increasing the kinetic energy of the air particles, warming up the surroundings
• Wool has the highest thermal conductivity as it lets thermal energy through at a faster rate so a thicker layer is needed for the same insulation as fiberglass
• The air trapped inside the fiberglass acts as an insulator because air has a very low thermal conductivity and thermal energy can not pass through it easily
• 160 J of electrical energy is transferred to a thermal energy store, the total dissipated energy is 240 J, and the efficiency of the useful energy transfer is 29.4%
• The lift motor is 83% efficient
• The low energy bulb gives off 14,040 J of light energy in one hour
• y


  100 J
• Thermal?


  Sound 80 J
• Lift motor operation


  1. 12 000 W power used

  2. 500 kg mass of lift and passengers

  3. 20 m height raised

  4. 10 s time taken
• The motor for a lift in a tall building uses 12 000 W of power. The lift and its passengers has a mass of 500 kg. The lift motor takes 10 s to raise the lift and its passengers through a height of 20 m.