Energy exists in various forms, which can be converted into one another. Every device around us is an energy-conversion device.
Basic Knowledge
Main forms of energy:
① Kinetic energy: Possessed by moving objects
② Potential energy: Possessed because of height or deformation(gravitational・elastic)
③ Thermal energy: Raises an object's temperature; motion of molecules
④ Light energy: Transmitted as electromagnetic waves
⑤ Electrical energy: Effects of electric current・voltage
⑥ Chemical energy: Possessed by fuels・batteries・food
⑦ Nuclear energy: Obtained from reactions of atomic nuclei
⑧ Sound energy: Transmitted as sound waves
Examples of energy-conversion devices around us:
・Light bulb: Electricity → light + heat
・Motor: Electricity → motion
・Speaker: Electricity → sound
・Solar cell: Light → electricity
・Battery: Chemical → electricity
・Steam turbine: Heat → motion → electricity
Energy（Ability to do work. Unit is J(joule)）
Energy conversion（Change from one form of energy into another）
Thermal energy（Caused by the motion of molecules・atoms）
Light energy（Energy carried by electromagnetic waves）
Chemical energy（Energy stored in substances. Fuels・batteries）
Nuclear energy（Energy extracted from reactions of atomic nuclei）
Further Study (Background・Significance)
The unit of energy is standardized as the joule(J).
Examples:
・Lifting one apple(about 100g) by 1m: About 1J
・Energy needed to raise 1L of water by 1℃ in 1 second: About 4200J
・One day's smartphone charge(about 5Wh): 18000J
・Energy for a car to travel 1km: About 3〜5MJ(several million J)
Historically, mechanical energy(kinetic energy) was first systematized, and then expanded to thermal・electrical・chemical・nuclear energy. In the mid-19th century, Mayer・Joule・Helmholtz and others established the law of conservation of energy: the total amount of energy remains unchanged even when its form changes.
The human body is also an energy-conversion device, converting the chemical energy in food into motion・body temperature・electrical signals, and so on. It uses about 6000kJ(1500kcal) per day for basal metabolism and consumes 2000〜3000kcal/day including activity.
- Energy exists in multiple forms
- Kinetic・potential・thermal・light・electrical・chemical・nuclear・sound, and others
- All can be measured in joules(J)
- Devices are energy-conversion devices(light bulbs・batteries・motors...)
- The law of conservation of energy was established in the mid-19th century
- The human body also converts the chemical energy in food to support activity
- 1kcal = about 4200J
Points to Note (Common Confusions)
① Energy changes form; it does not disappear. ② Work (J) and energy (J) have the same unit. Work can be described as the amount of energy transferred. ③ Everyday kcal and physical J are both units of energy(1kcal ≒ 4200J). ④ Fuel is chemical energy, not thermal energy itself. It becomes heat only through combustion.
Practice
- State the unit of energy.
- What form of energy does a light bulb convert, and into what form?
- What is the name of the device that converts light energy into electrical energy?