In addition to mechanical energy, there are many forms of energy, including thermal, electrical, and light energy. We learn the universal law that although the form changes, the total amount of energy does not change.
Basic Knowledge
Law of conservation of energy(first law of thermodynamics): Energy is converted into various forms, but its total amount does not change.
Main forms of energy:
・Mechanical energy(kinetic energy + potential energy)
・Thermal energy(kinetic energy of molecules)
・Electrical energy
・Light energy・Chemical energy・Nuclear energy
Examples of energy conversion:
・Power plant: chemical/nuclear energy → heat → motion(turbine) → electricity
・Fluorescent lamp: electricity → light + heat
・Photosynthesis: light → chemical energy
・Body movement: chemical energy(ATP) → mechanical energy + heat
Energy conversion efficiency: The proportion converted into the desired form of energy. It is always 100% or less. The remainder is mainly converted into heat.
Law of conservation of energy（Total energy does not change. The first law of thermodynamics）
Energy conversion（The forms of mechanical, thermal, electrical, light, and other energy change into one another）
Conversion efficiency（Useful energy output / total energy input × 100%）
Thermal energy（The total kinetic energy of the particles that make up matter）
Chemical energy（Energy stored in chemical bonds. Found in fuels and food）
ATP（A molecule that transports energy in living organisms. The direct energy source for muscle movement）
Further Study (Background and Significance)
The law of conservation of energy is one of the most fundamental laws in physics. It was discovered independently by several scientists in the mid-nineteenth century and has never been violated in experiments to this day.
The impossibility of a “perpetual-motion machine(a machine that moves forever without external energy)” follows from this law of conservation of energy. Historically, many inventors attempted to create perpetual-motion machines, but all failed.
Conversion efficiency can never exceed 100%(because that would mean energy was created). LED lighting is more efficient than incandescent bulbs(the conversion rate to light is higher), so less energy is lost as heat. Electric vehicles are also more efficient than internal-combustion engines for the same reason.
Energy conversion around us: Charging a smartphone converts electrical energy → chemical energy(a battery). Charging efficiency is usually 90〜95%, and the remainder becomes heat(the reason the phone becomes warm while charging).
- Law of conservation of energy: total amount does not change
- Only the form changes(mechanical → thermal → electrical, etc.)
- Conversion efficiency is always 100% or less
- The remaining energy mainly becomes heat
- Perpetual-motion machines are impossible(the conservation law is the basis)
- Conservation of mechanical energy is a special case of the conservation law
- Mechanical E lost through friction = heat produced
Points to Note (Common Confusions)
① Distinguish between the “law of conservation of mechanical energy(without friction)” and the “law of conservation of energy(always valid).” The former is conditional, while the latter is an absolute law. ② “Energy was lost” means “energy was converted into heat or another form.” It did not disappear. ③ Conversion efficiency is “the proportion of energy that can be used effectively,” and lost energy usually dissipates into the environment as heat. ④ Energy conversion in living organisms(ATP) also follows the laws of physics.
Practice
- An incandescent bulb(conversion efficiency 5%) and an LED(conversion efficiency 20%) were each supplied with 100 J of electrical energy. Find the light energy produced by each.
- When “mechanical energy was lost due to friction,” explain where that energy went.
- Using the law of conservation of energy, explain why a “perpetual-motion machine” cannot be realized.