What is the basic meaning of energy
Lexicon> Letter E> Energy
Definition: a physical quantity related to the ability to do work or accomplish other things
Categories: Basic Concepts, Physical Basics
Formula symbol: E.
Unit: Joule (J), kilowatt hour (kWh)
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
Original creation: 03/06/2010; last change: 25.07.2020
The term “energy” plays a very important role in various fields of physics and technology - for example in thermodynamics and power plant technology. Although initially based on a relatively abstract concept in physics, it can also be used in an intuitive manner to great benefit. Physically educated people recognize energy flows with their mental eye even in countless everyday situations, e.g. E.g. when sunlight falls through a window, a vehicle starts or brakes, or when a jet of hot water disappears in the kitchen drain.
Amounts of energy and units
Energy can be recorded quantitatively. The basic unit of energy in the system of international units (SI system) is the joule (J). Approximately this amount of energy is required to lift an object weighing 1 kg on earth by approx. 10 cm (regardless of how fast you do that). Larger amounts of energy can be specified with units derived therefrom:All forms of energy can be quantified with the same units, since they can be at least partially converted into one another.
- 1 kJ (kilojoules) = 1000 J = 103 J
- 1 MJ (megajoules) = 1000 kJ = 106 J
- 1 GJ (gigajoules) = 1000 MJ = 109 J
- 1 TJ (terajoule) = 1000 GJ = 1012 J
- 1 PJ (petajoules) = 1000 TJ = 1015 J
- 1 EJ (exajoule) = 1000 PJ = 1018 J
For example, an atomic bomb can release hundreds of terajoules or even several PJ. The worldwide primary energy turnover per year is currently a little over 400 EJ.
The following units are also used:
- 1 cal (calorie) = 4.19 J is the amount of heat that is required to heat one gram of liquid water by 1 ° C (more precisely: by 1 Kelvin). The kilocalorie (1 kcal = 1 cal = 4.19 kJ) is a thousand times this amount. Calories (mostly in the form of kilocalories, although the “kilo” is unfortunately often forgotten) are still used in connection with food.
- 1 kWh (kilowatt hour) = 3,600,000 J = 3.6 MJ is the energy that is converted within one hour at an output of 1 kW.
- 1 SKE (hard coal unit) is released when 1 kg of hard coal is burned. It corresponds to 8.141 kWh or 29 MJ.
- 1 RÖE (crude oil unit, English OE = oil equivalent) corresponds to 41.868 kJ. 1 Mtoe (million tons of oil equivalent) corresponds to 41.868 PJ.
- 1 BTU (British Thermal Unit) corresponds to approx. 1055 J. This unit is z. B. in the USA still used for the energy content of fuels and fuels.
- 1 eV (electron volt) is the energy that an elementary electric charge absorbs when it passes through a potential difference of one volt in an accelerating electric field. This unit is mainly used in atomic physics and nuclear physics when it comes to the amount of energy per atom or molecule.
Conservation of energy
In physics, energy is a “conserved quantity”, which essentially means that the total amount of energy in a closed system can neither increase nor decrease - regardless of which physical or chemical processes take place. However, an incomplete system can exchange energy with its surroundings. For example, the earth constantly receives large amounts of radiant energy (mainly infrared and visible light) from the sun and at the same time emits roughly the same amount as thermal radiation into space. Energy can also be converted between different forms of energy (see below), whereby the amounts of the individual forms can change and only the entire amount of energy is retained.
See also the articles on Conservation of Energy, Perpetual Motion, and the Principles of Thermodynamics, the first of which concerns conservation of energy.
Forms of energy
Energy comes in very different forms:There are many forms of energy that can be at least partially converted into one another.
- Mechanical energy is required, for example, to accelerate or lift bodies, i.e. to perform mechanical work. For example, it takes 12.5 J to bring an object with a mass of 1 kg to a speed of 5 m / s; this amount of kinetic energy (kinetic energy) increases with the square of the speed, so it would already be 50 J for 10 m / s. Mechanical energy can be stored, for example in raised masses (e.g. positional energy = potential energy of water in a high-level reservoir of a water storage power plant) or as elastic energy (e.g. tensioned spring or compressed air storage).
- Thermal energy is needed to heat matter. For example, you need 4.19 kJ to heat one liter of water by 1 ° C (more precisely: by 1 Kelvin). Latent heat occurs during phase changes such as B. the melting of ice. Many other forms of energy can easily be completely converted into heat, while conversions in the opposite direction are usually only partially possible. The second law of thermodynamics provides more information about such restrictions.
The Withdrawal of heat (Cooling) is easy if a corresponding cooler medium (such as cold water) is available. However, particularly at very low temperatures requires the provision of cold with a refrigeration machine an effort z. B. of electrical energy.
- light is also a form of energy. In addition to visible light, there are also invisible forms of electromagnetic radiation that propagate in a similar way to visible light: infrared light (thermal radiation) and ultraviolet light. A spatially and temporally very targeted and intensive energy supply is possible in the form of laser light, which is used in laser material processing. See also the article on lighting.
- Electrical power is a very universally usable form of energy. It can be easily converted into mechanical energy and vice versa, and conversion into other forms of energy such as light or chemical energy is also relatively easy to accomplish, albeit with higher energy losses. In addition, electrical energy can be easily transported via electrical lines (e.g. high-voltage lines).
- Chemical energy is energy that can be released during chemical processes. For example, heat and (less efficient) light can be generated in combustion processes, and in fuel cells chemical energy is converted directly and relatively efficiently into electrical energy. The fossil fuels coal, crude oil and natural gas all contain chemical energy. In the future, secondary chemical energy carriers such as hydrogen could become important (especially for storing energy), even if they were initially e.g. B. must be obtained with electrical energy. Animals and humans also live on chemical energy that is ingested in the form of food.
- Nuclear energy (nuclear energy) is energy released when atomic nuclei transform. These processes can in particular be nuclear fission and nuclear fusion, but also simple radioactive decays. This energy is initially released in the form of high-energy radiation, which is then absorbed in the surrounding material and converted into heat.
Limits on energy conversions; Qualities of energy
When different forms of energy are converted into one another, the entire energy is retained; so no energy is lost in this sense. However, often not all energy is converted into the desired form of energy. The proportion of the desired form of energy obtained is referred to as the efficiency.Not every energy conversion that would fulfill the law of conservation of energy is physically possible. The entropy often sets narrow limits here.
Limits to the possibilities of converting energy from certain forms into others result partly from technical imperfections that can be avoided in principle, but partly from fundamental physical laws. The latter applies in particular to the conversion of heat into mechanical energy or electrical energy: heat engines can at most achieve the so-called Carnot efficiency, which depends on the available temperature gradient. This results from the second law of thermodynamics and can also be explained by the fact that the complete conversion of heat into mechanical energy would reduce the overall entropy, which is fundamentally not possible with whatever technical process.Money is only worth exergy, i. H. high quality energy. Anergy is available indefinitely, but can only be used with the help of exergy.
You can assign different “qualities” or values to different forms of energy, the higher the less restricted the conversion into other forms of energy. In this sense, mechanical energy and electrical energy are the most valuable forms of energy; they are pure exergy, in other words, energy without entropy content. Warmth has a lower value, which is higher the more its temperature level deviates from that of the surroundings. The lowest quality is ambient heat (anergy), which is available in practically unlimited quantities, but cannot be used without the additional use of exergy (e.g. to drive a heat pump).
Importance of energy for prosperity and the environment
The industrial revolution was based in large part on the development of techniques with which large amounts of energy were made available for use. Enormous amounts of energy are converted today, especially in industrialized countries, and the importance of energy supply for prosperity is extremely great.
On the other hand, there are a number of negative side effects of the enormous energy turnover, in particular damage to the environment (e.g. due to climate hazards and toxic pollutants) and thus also to the foundations of human life. In addition, there are political and social problems, which are likely to worsen considerably in the future, in particular due to the increasing scarcity of fossil fuels. For these reasons, there is growing awareness of the need for a fundamental energy transition, which not only affects energy generation (more precisely the methods of providing usable energy), but also the efficient and economical use of energy (→Save energy, Sufficiency, Energy efficiency).
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See also: energy sources, energy conservation, energy generation, energy consumption, work, performance, force, joules, primary energy, final energy, energy efficiency, exergy, anergy, entropy, enthalpy, thermodynamics, energy saving, sufficiency, energy tax, external costs, energy transition, energy poverty
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