The total amount of energy does not change, but the efficiency of conversion into the desired form of energy differs greatly among devices.
Basic Knowledge
Law of conservation of energy: Even when energy is converted into other forms, its total amount does not change.
This universal law always holds when losses such as heat・friction・sound are included.
Conversion efficiency: The proportion of input energy converted into the desired form.Conversion efficiency (%) = (useful energy / input energy) × 100
Efficiency of typical devices:
・Incandescent light bulb: About 10% becomes light(the remaining 90% is heat)
・LED light bulb: About 40〜50% becomes light
・Gasoline engine in a car: About 25〜30%(the rest is waste heat・friction)
・Motor of an electric vehicle: About 90%
・Solar cell: About 15〜25%
・Thermal power plant: About 40%
Law of conservation of energy（The total amount of energy remains unchanged even when converted）
Conversion efficiency（Useful energy ÷ input energy）
Loss of thermal energy（Loss that inevitably occurs during conversion. Friction・resistance, etc.）
Second law of thermodynamics（Heat cannot be completely converted into work. Entropy）
Energy conservation（Efforts to reduce waste and use energy effectively）
LED（High-efficiency light-emitting diode. Replacement for incandescent bulbs）
Further Study (Background・Significance)
The law of conservation of energy is a generalization of the law of conservation of mechanical energy. Even when friction produces heat, energy is always conserved if that heat is included.
It is important that thermal energy cannot be returned to other forms of energy with 100% efficiency(the second law of thermodynamics). Some energy that has become heat is lost, and the entropy of the universe increases.
An incandescent bulb works by heating a filament until it glows, so most of its energy is lost as heat. An LED directly converts the energy of electrons into light, making it more efficient because less energy becomes heat.
During the 2010s, replacement of incandescent bulbs with LEDs progressed rapidly worldwide, greatly reducing electricity consumption for lighting.
Improving conversion efficiency is a central theme in measures against global warming and is also connected to the SDG goal “Affordable and Clean Energy.” Even for the same work, raising efficiency by 10% can reduce energy consumption and CO2 emissions by 10%.
- Total energy remains unchanged even after conversion(conservation law)
- Conversion efficiency = useful E / input E ×100%
- Incandescent bulb efficiency about 10%, LED 40〜50%
- Gasoline cars 25〜30%, electric-vehicle motors over 90%
- Thermal efficiency of thermal power plants about 40%
- Heat cannot be completely converted into work(second law)
- Energy conservation = improving conversion efficiency + reducing waste
Points to Note (Common Confusions)
① The law of conservation of energy holds at the level of the entire universe. A decrease in a device's output is due to losses; the energy has not disappeared. ② The law of conservation of mechanical energy applies only when friction is ignored, whereas the law of conservation of energy always holds when heat is included. ③ The upper limit of conversion efficiency is less than 100%(the second law of thermodynamics). Perpetual motion machines are impossible. ④ Efficiency depends on the purpose(the purpose of a car engine is “power,” so heat is a loss).
Practice
- Briefly explain the law of conservation of energy.
- Which has higher conversion efficiency, an incandescent bulb or an LED bulb?
- Write the formula for finding conversion efficiency.