IB DP Physics: HL复习笔记8.1.2 Sankey Diagrams

• Diagrams are used to represent energy transfers
  • These are sometimes called Sankey diagrams

• The arrow in a Sankey diagram represents the transfer of energy:
  • The end of the arrow pointing to the right represents the energy that ends up in the desired store (the useful energy output)
  • The end(s) that point(s) down represents the wasted energy

Total energy in, wasted energy and useful energy out shown on a Sankey diagram

• The width of each arrow is proportional to the amount of energy going to each store
• As a result of the conversation of energy:

Total energy in = Useful energy out + Wasted energy

• A Sankey diagram for a modern efficient light bulb will look very different from that for an old filament light bulb
• A more efficient light bulb has less wasted energy
  • This is shown by the smaller arrow downwards representing the heat energy

Sankey diagram for modern vs. old filament light bulb

Step 1: State the conservation of energy

• • Energy cannot be created or destroyed, it can only be moved from one store to another
  • This means that:

Total energy in = Useful energy out + Wasted energy

Step 2: Rearrange the equation for the wasted energy

Wasted energy = Total energy in – Useful energy out

Step 3: Substitute the values from the diagram

500 – 120 = 380 J

Step 1: State the conservation of energy

• • Energy cannot be created or destroyed, it can only be moved from one store to another
  • This also applies to power
  • This means that:

Total power in = Useful power out + Friction losses + Traction losses + Wasted power other losses

Step 2: Rearrange the equation for the wasted power

Wasted power other losses = Total power in – (Useful power out + Friction losses + Traction losses)

Step 3: Substitute the values from the diagram

120 – (33 + 36 + 36) = 15 W