Energy Stores and Transfers

• Energy exists in different 'stores.'
• Energy cannot be created or destroyed.
• Energy can be transferred, dissipated, or stored in different forms.
• Energy can remain in the same store for varying durations, from millions of years to fractions of a second.
• Whenever a system changes, the way energy is stored also changes.

Examples of Energy Transfers

• A boat moving through water: Chemical energy converts to kinetic energy.
• Boiling water in an electric kettle: Electricity increases the internal energy of the element, which in turn, raises the internal (thermal) energy and temperature of the water.
• A swinging pirate ship ride: Kinetic energy converts to gravitational potential energy.

Types of Energy Transfer

• The potential difference (voltage) is a measure of energy given to charge carriers in a circuit, measured in volts (V).
• Voltage between two points causes electric current to flow.
• Energy is 'given-out' and the internal energy of the material decreases.
• Infrared radiation from the Sun is emitted into space.
• Doing 'work' means energy has been transferred.
• Examples of doing work: a grazing cow, a firing catapult, and a boiling kettle.

Diagrams to Show Energy Transfer

• Transfer diagrams use boxes for energy stores and arrows for energy transfers.
  • Example: A child at the top of a slide:
    • Gravitational energy stored in the child is transferred as mechanical work to speed up and overcome friction.
    • Energy shifts from gravitational potential energy to kinetic energy and internal energy (raising the temperature of the child and the slide).
• Sankey diagrams use arrows splitting to show energy transfer into different stores.
• Sankey diagrams are useful when the amount of energy in each source is known.
• The width of the arrow is proportional to the amount of energy.