エネルギー、仕事、 power

Energy never disappears

• At the top of a roller-coaster hill the car is slow but high; at the bottom it is fast but low.
• The energy did not vanish — it just moved from one store to another.
• This is the big idea of this lesson: energy is stored, transferred, and always conserved.

Energy stores and transfers

• Energy can be stored as: kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic, and internal (thermal).
• It is transferred between stores by: forces (mechanical work), electric currents, heating, and waves (such as light and sound).
• "Using energy" really means moving it from one store to another.

Wind turbines transfer energy from moving air to electricity

Kinetic and potential energy

• Kinetic energy 动能 is the energy of a moving object:

• Lifting a mass by a height $\Delta h$ changes its gravitational potential energy 重力势能:

• A falling object turns gravitational PE into kinetic energy.

Conservation of energy 能量守恒

• The principle of conservation of energy: energy is never made or destroyed — it only moves between stores.
• Some energy is always wasted — usually spread out as heat to the surroundings.
• A Sankey diagram 桑基图 shows the input energy splitting into useful energy and wasted energy.

On a roller-coaster, stored energy at the top becomes kinetic energy lower down

Work

• Work done 做功 equals the energy transferred. When a force moves an object along its direction:

• The unit of work and energy is the joule 焦耳 (J).
• Lifting a heavier load, or moving it further, does more work.

A swinging pendulum trades gravitational potential energy for kinetic energy and back

Power 功率 and efficiency 效率

• Power is the work done (energy transferred) each second:

• The unit is the watt 瓦特 (W); $1\ \text{W} = 1\ \text{J/s}$.
• Efficiency is how much of the input energy becomes useful:

• It is always less than $100\%$ because some energy is always wasted.

A Sankey diagram: the band widths are proportional to the energy, so the input splits into useful output and wasted heat

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