Energy

Energy Stores

• Kinetic
• Gravitational potential
• Chemical
• Elastic (strain)
• Nuclear
• Electrostatic
• Internal (thermal)

Energy Pathways

Energy is transferred between these stores through four pathways:

• By forces (mechanical work done)
• By electrical currents (electrical work done)
• By heating
• By electromagnetic, sound, or other waves

When describing any energy change, the correct phrasing is that energy is transferred from one named store to another named store via a named pathway.

Key Definition The principle of conservation of energy states that energy cannot be created or destroyed — it can only be transferred from one store to another.

In any process, the total energy before equals the total energy after. Simple flow diagrams illustrate this by showing input and output energies that sum to the same total.

Extended Extended students must also apply the equations for kinetic energy and change in gravitational potential energy.

Eₖ = 1/2mv²

Δ Eₚ = mgΔ h

Kinetic energy depends on the square of speed, so doubling speed quadruples kinetic energy. Gravitational potential energy is directly proportional to both mass and height.

In multi-stage problems, conservation of energy links these equations — for example, a falling object's gravitational potential energy lost equals its kinetic energy gained (assuming no air resistance).

Sankey Diagrams

• Provide a scaled visual representation of energy transfers
• Arrow widths are proportional to energy values
• The widths of all output arrows must add up to the width of the input arrow (conservation of energy)

These diagrams are powerful tools for identifying useful and wasted energy in real-world systems.