Our lives are supported by transformations of energy. Let us understand the types of energy and the law of conservation of energy.
Basic Knowledge
Main types of energy:
・Mechanical energy: kinetic energy (1/2 mv²) + potential energy (mgh)
・Thermal energy: thermal motion of the particles that make up matter
・Electrical energy: movement of electric charge (electric current)
・Chemical energy: energy stored in chemical bonds
・Nuclear energy: binding energy of atomic nuclei
・Light energy: energy of electromagnetic waves
Law of conservation of energy: The total amount of energy does not change even when energy is transformed or transferred.
Decrease in energy quality: During transformation, some energy changes into thermal energy (irreversible). The amount of usable energy decreases (energy dissipation).
・Thermal efficiency: η = useful energy extracted / energy supplied × 100%
Law of conservation of energy（the total amount of energy remains unchanged. Its form may change, but the total amount stays the same）
Thermal efficiency (η)（useful energy / supplied energy. Less than 100%）
Energy transformation（changing energy from one form to another, such as chemical→thermal→kinetic→electrical）
Dissipation（the spreading of energy into the surroundings as heat, making it unavailable for use）
Primary energy（unconverted energy resources existing in nature）
Secondary energy（usable energy such as electricity and gasoline, made by transforming primary energy）
Further Exploration (Background and Significance)
The first law of thermodynamics: energy is conserved. The second law of thermodynamics: entropy (disorder) can spontaneously change only in the direction of increasing. Therefore, energy is always transformed in the direction of “lower quality.” Thermal efficiency is about 40〜50% for thermal power generation and about 20〜30% for gasoline engines. Electric vehicles have high overall efficiency because they have fewer conversion losses.
The energy in food is also chemical energy. In nutrition, calculations use 1 kcal = 4.2 kJ. Consuming 2000 kcal during intense exercise means using about 8.4 × 10⁶ J of energy. Converted entirely into electrical energy, this is about 2.3 kWh.
The Sun's energy comes from nuclear fusion (hydrogen→helium). The solar energy reaching Earth is about 1360 W per 1m² (the solar constant). Some of this energy was accumulated through photosynthesis in plants→as fossil fuels over hundreds of millions of years, and modern people are burning it rapidly.
- Law of conservation of energy: total amount unchanged
- Some energy is dissipated as heat during transformation
- Thermal efficiency = useful energy/supplied energy
- Thermal efficiency is always less than 100%
- Primary energy: fossil fuels・nuclear fuels・renewable sources
- Secondary energy: electricity・hydrogen, and others
- Energy quality decreases with each transformation
Notes (Common Confusions)
① Although the law of conservation of energy holds, usable energy decreases after transformation because energy is dissipated. ② No heat engine has a thermal efficiency of 100% (the second law of thermodynamics). ③ “Saving energy” does not physically mean saving energy; it means “reducing energy dissipation.” ④ Distinguish primary energy from secondary energy. Electricity is secondary energy.
Practice
- A heat engine receives 100 J of heat and performs 40 J of work. Find its thermal efficiency.
- Why is energy conservation necessary even though the law of conservation of energy holds?
- Is electrical energy primary energy or secondary energy?