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[[File:Lightning over Oradea Romania zoom.jpg|thumb|right|300px|Transformasi energi. [[Kilat]] mengubah 500 [[megajoule]] [[energi potensial listrik]] menjadi [[energi cahaya]], [[energi bunyi]], dan [[energi panas]].]]
Ditinjau dari perspektif [[fisika]], setiap sistem fisik mengandung (secara alternatif, [[penyimpanan|menyimpan]]) sejumlah '''energi'''; berapa tepatnya ditentukan dengan mengambil jumlah dari sejumlah persamaan khusus, masing-masing didesain untuk mengukur energi yang disimpan secara khusus. Secara umum, adanya energi diketahui oleh [[pengamat]] setiap ada pergantian [[sifat]] [[objek]] atau [[sistem]]. Tidak ada cara seragam untuk memperlihatkan energi;
Dalam [[fisika]], '''energi''' adalah [[properti fisika]] dari suatu [[objek fisik|objek]], [[perpindahan energi|dapat berpindah]] melalui [[interaksi fundamental]], yang dapat [[transformasi energi|diubah]] [[bentuk-bentuk energi|bentuknya]] namun [[konservasi energi|tak dapat diciptakan maupun dimusnahkan]]. [[Joule]] adalah satuan [[SI]] untuk energi, diambil dari jumlah yang diberikan pada suatu objek (melalui [[kerja mekanik]]) dengan memindahkannya sejauh 1 [[meter]] dengan gaya 1 [[newton (satuan)|newton]].<ref>Energy units are usually defined in terms of the [[Work (physics)|work]] they can do. However, because work is an indirect measurement of energy, (One example of the difficulties involved: if you use the [[first law of thermodynamics]] to define energy as the work an object can do, you must perform a perfectly [[Reversible process (thermodynamics)|reversible process]], which is impossible in a finite time.) many experts emphasize understanding how energy behaves, specifically the [[conservation of energy]], rather than trying to explain what energy "is". {{cite web|url=http://www.colorado.edu/physics/phys1110/phys1110_fa10/Feynman_energy.pdf|title=The Feynman Lectures on Physics Vol I.|accessdate=3 Apr 2014}}</ref>
[[Kerja (termodinamika)|Kerja]] dan [[panas]] adalah 2 contoh proses atau mekanisme yang dapat memindahkan sejumlah energi. [[Hukum kedua termodinamika]] membatasi jumlah kerja yang didapat melalui proses pemanasan-beberapa diantaranya akan hilang sebagai [[panas terbuang]]. Jumlah maksimum yang dapat digunakan untuk kerja disebut [[energi tersedia]]. Sistem seperti mesin dan benda hidup membutuhkan energi tersedia, tidak hanya sembarang energi. Energi mekanik dan bentuk-bentuk energi lainnya dapat berpindah langsung ke bentuk [[energi panas]] tanpa batasan tertentu.
== Satuan ==
=== SI dan satuan berhubungan ===
Satuan [[SI (satuan ukur)|SI]] untuk energi dan kerja adalah [[joule]] (J), dinamakan untuk menghormati [[James Prescott Joule]] dan percobaannya dalam [[persamaan mekanik panas]]. Dalam istilah yang lebih mendasar [[1 E0 J|1 joule]] sama dengan 1 [[newton]]-[[meter]] dan, dalam istilah [[satuan dasar SI]], 1 J sama dengan 1 [[kilogram|kg]] [[meter|m]]<small><sup>2</sup></small> [[detik|s]]<small><sup>−2</sup></small>.
Ada berbagai macam [[bentuk-bentuk energi]], namun semua tipe energi ini harus memenuhi berbagai kondisi seperti dapat diubah ke bentuk energi lainnya, mematuhi hukum konservasi energi, dan menyebabkan perubahan pada benda bermassa yang dikenai energi tersebut. Bentuk energi yang umum diantaranya [[energi kinetik]] dari benda bergerak, [[energi radiasi]] dari cahaya dan [[radiasi elektromagnetik]], [[energi potensial]] yang tersimpan dalam sebuah benda karena posisinya seperti [[medan gravitasi]], [[medan listrik]] atau [[medan magnet]], dan [[energi panas]] yang terdiri dari energi potensial dan kinetik mikroskopik dari gerakan-gerakan partikel tak beraturan. Beberapa bentuk spesifik dari energi potensial adalah [[energi elastis]] yang disebabkan dari pemanjangan atau deformasi benda padat dan [[energi kimia]] seperti pelepasan panas ketika bahan bakar terbakar. Setiap benda yang memiliki massa ketika diam, memiliki [[massa diam]] atau [[neraca massa-energi|sama]] dengan energi diam, meski tidak dijelaskan dalam fenomena sehari-hari di [[fisika klasik]].
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An energy unit that is used in [[particle physics]] is the [[electronvolt]] (eV). One eV is equivalent to [[1 E-19 J|1.60217653×10<small><sup>−19</sup></small> J]].
Menurut [[neraca massa-energi]], semua bentuk energi membutuhkan massa. Contohnya, menambahkan 25 kilowatt-jam (90 megajoule) energi pada objek akan meningkatkan massanya sebanyak 1 mikrogram; jika ada timbangan yang sebegitu sensitif maka penambahan massa ini bisa terlihat. Matahari mengubah [[energi potensial nuklir]] menjadi bentuk energi lainnya; total massanya akan berubah ketika energi terlepas ke sekelilingnya terutama dalam bentuk [[energi radiasi]].
(Note that [[torque]], which is typically expressed in newton-metres, has the same dimension and this is not a simple coincidence: a torque of 1 newton-metre applied on 1 radian requires exactly 1 newton-metre=joule of energy.)
Meskipun energi dapat berubah bentuk, namun hukum [[kekekalan energi]] menyatakan bahwa total energi pada sebuah sistem hanya berubah jika energi berpindah masuk atau keluar dari sistem. Hal ini berarti tidak mungkin menciptakan atau memusnahkan energi. Total energi dari sebuah sistem dapat dihitung dengan menambahkan semua bentuk energi dalam sistem tersebut. Contoh perpindahan dan transformasi energi adalah pembangkitan listrik, [[reaksi kimia]], atau menaikkan benda.
=== Satuan lain energi ===
Organisme hidup juga membutuhkan [[energi tersedia]] untuk tetap hidup; manusia misalnya, membutuhkan energi dari makanan beserta oksigen untuk memetabolismenya. Peradaban membutuhkan pasokan energi untuk berbagai kegiatan; [[sumber energi]] seperti [[bahan bakar fosil]] merupakan topik penting dalam ekonomi dan politik. [[Iklim]] dan [[ekosistem]] bumi juga dijalankan oleh energi radiasi yang didapat dari matahari (juga [[energi geotermal]] yang didapat dari dalam bumi.
In [[cgs]] units, one [[erg]] is 1 [[gram|g]] [[centimetre|cm]]<small><sup>2</sup></small> [[second|s]]<small><sup>−2</sup></small>, equal to [[1 E-7 J|1.0×10<small><sup>−7</sup></small> J]]. Another obsolete metric unit is the litre-atmosphere (101.325 J).
== Satuan ==
The [[imperial units|imperial]]/[[US customary units|US units]] for both energy and work include the [[foot-pound force]] (1.3558 J), the [[British thermal unit]] (Btu) which has various values in the region of 1055 J, and the [[horsepower]]-hour (2.6845 MJ).
=== SI dan satuan berhubungan ===
Satuan [[SI (satuan ukur)|SI]] untuk energi dan kerja adalah [[joule]] (J), dinamakan untuk menghormati [[James Prescott Joule]] dan percobaannya dalam [[persamaan mekanik panas]]. Dalam istilah yang lebih mendasar [[1 E0 J|1 joule]] sama dengan 1 [[newton]]-[[meter]] dan, dalam istilah [[satuan dasar SI]], 1 J sama dengan 1 [[kilogram|kg]] [[meter|m]]<small><sup>2</sup></small> [[detik|s]]<small><sup>−2</sup></small>.
The energy unit used for everyday [[electricity]], particularly for utility bills, is the [[kilowatt-hour]] (kW h), and one kW h is equivalent to [[1 E6 J|3.6×10<small><sup>6</sup></small> J ]] (3600 kJ or 3.6 MJ; the metric units usually are self-consistent, and this particular one may seem arbitrary; it's not, the metric measurement for time is the second, and there are 3,600 seconds in an hour -- in other words, 1 kW second = 1 kJ, but the kW h is a more convenient unit for everyday use).
The [[calorie]] is mainly used in nutrition and equals the amount of [[heat]] necessary to raise the [[temperature]] of one [[kilogram]] of [[water]] by 1 degree [[Celsius]], at a [[pressure]] of 1 [[atmospheric pressure|atm]]. This amount of heat depends somewhat on the initial temperature of the water, which results in various different units sharing the name of "calorie" but having slightly different energy values. It is equal to [[1 E0 J|4.1868 kJ]].
The calories used for [[food energy]] in nutrition are the large calories based on the kilogram rather than the gram, often identified as ''food calories''. These are sometimes called kilocalories with that calorie being the small calorie based on the gram, and as a result the prefixes are generally avoided for the large calories (i.e., 1 kcal is 4.184 kJ, never 4.184 MJ, even if "calories" are also used for the other, larger unit in the same document or the same nutrition label). Food calories are sometimes noted as ''C''alories (1000 calories) or simply abbreviated Cal with the capital C, but that convention is more often found in chemistry or physics textbooks—which do not use these large calories—than it is in real-world applications by those who do use these calories. (This convention is also, of course, useless when the word calorie appears in a location where it would ordinarily be capitalized, as at the beginning of a sentence or in the first column of a nutrition label as a substitute for the quantity being measured, which is energy, when all the other quantities such as "Iron" and "Sugars" are also capitalized.)
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== Transfer energi ==
Persamaan di atas mengatakan bahwa kerja (<math>W</math>) sama dengan integral dari [[dot product]] [[gaya]] (<math>\mathbf{F}</math>) di sebuah benda dan [[infinitesimal]] posisi benda (<math>\mathbf{s}</math>).
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=== Panas ===
{{Utama|Panas}}
''Heat'' is an amount of energy which is usually linked with a change in temperature or in a change in phase of matter. In chemistry, heat is the amount of energy which is absorbed or released by a given chemical reaction.
The relationship between heat and energy is similar to that between work and energy. Heat flows from areas of high temperature to areas of low temperature. All objects (matter) have a certain amount of internal energy that is related to the random motion of their atoms or molecules. This internal energy is directly proportional to the temperature of the object. When two bodies of different temperature come in to thermal contact, they will exchange internal energy until the temperature is equalised. The amount of energy transferred is the amount of heat exchanged. It is a common misconception to confuse heat with internal energy, but there is a difference: the change of the internal energy is the heat that flows from the surroundings into the system plus the work performed by the surroundings on the system. Heat Energy is transferred in three different ways: [[Heat conduction|conduction]], [[convection]] and/or [[radiation]].
=== Konservasi energi ===
The first law of [[thermodynamics]] says that the total inflow of energy into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the system. This law is used in all branches of physics, but frequently violated by quantum mechanics (see [[off shell]]). [[Noether's theorem]] relates the conservation of energy to the time invariance of physical laws.
An example of the conversion and conservation of energy is a [[pendulum]]. At its highest points the kinetic energy is zero and the potential gravitational energy is at its maximum. At its lowest point the kinetic energy is at its maximum and is equal to the decrease of potential energy. If one unrealistically assumes that there is no [[friction]], the energy will be conserved and the pendulum will continue swinging forever. (In practice, available energy is '''never''' perfectly conserved when a system changes state; otherwise, the creation of [[perpetual motion]] machines would be possible.)
Another example is a [[Chemical_explosive|chemical explosion]] in which potential chemical energy is converted to kinetic energy and heat in a very short time.
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== Jenis energi ==
Persamaan di atas menyatakan bahwa energi kinetik (<math>E_k</math>) sama dengan [[integral]] dari [[dot product]] [[kecepatan]] (<math>\mathbf{v}</math>) sebuah benda dan [[infinitesimal]] [[momentum]] benda (<math>\mathbf{p}</math>).
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For non-[[special relativity|relativistic]] velocities, that is velocities much smaller than the [[speed of light]], we can use the Newtonian approximation
:<math>E_k = \begin{matrix} \frac{1}{2} \end{matrix} mv^2</math>
where
''E''<sub>k</sub> is kinetic energy
''m'' is mass of the body
''v'' is velocity of the body
At near-light velocities, we use the correct relativistic formula:
:<math>E_k = m c^2 (\gamma - 1) = \gamma m c^2 - m c^2 \;\!</math>
:<math>\gamma = \frac{1}{\sqrt{1 - (v/c)^2}} </math>
where
''v'' is the velocity of the body
''m'' is its rest mass
''c'' is the speed of light in a vacuum, which is approximately 300,000 kilometers per second
<math>\gamma m c^2 \,</math> is the ''total energy'' of the body
<math>m c^2 \,</math> is again the rest mass energy.
In the form of a [[Taylor series]], the relativistic formula for can be written as:
:<math>E_k = \frac{1}{2} mv^2 - \frac{3}{8} \frac{mv^4} {c^2} + \cdots </math>
Hence, the second and higher terms in the series correspond with the "inaccuracy" of the Newtonian approximation for kinetic energy in relation to the relativistic formula.
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=== Energi potensial ===
{{utama|Energi potensial}}
Berlawanan dengan [[energi kinetik]], yang adalah energi dari sebuah [[sistem]] dikarenakan gerakannya, atau gerakan internal dari partikelnya, [[energi potensial]] dari sebuah sistem adalah energi yang dihubungkan dengan konfigurasi ruang dari komponen-komponennya dan interaksi mereka satu sama lain. Jumlah partikel yang mengeluarkan gaya satu sama lain secara otomatis membentuk sebuah sistem dengan energi potensial. Gaya-gaya tersebut, contohnya, dapat timbul dari interaksi elektrostatik (lihat [[hukum Coulomb]]), atau [[gravitasi]].
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In an isolated system consisting of two stationary objects that exert a force <math>f(x)</math> on each other and lay on the x-axis, their potential energy is most generally defined as
:<math>E_p = -\int f(x) \, dx</math>
where the force between the objects varies only with distance <math>x</math> and is integrated along the line connecting the two objects.
To further illustrate the relationship between force and potential energy, consider the same system of two objects situated along the x-axis. If the potential energy due to one of the objects at any point <math>x</math> is <math>U(x)</math>, then the force on the that object <math>x</math> is
:<math>f(x) = -\frac{dU(x)}{dx}</math>
This mathematical relationship demonstrates the direct connection between force and potential energy: the force between two objects is in the direction of decreasing potential energy, and the magnitude of the force is proportional to the extent to which potential energy decreases. A large force is associated with a large decrease in potential energy, while a small force is associated with a small decrease in potential energy. Notice how, in this case, the force on an object depends entirely on its potential energy.
These two relationships – the definition of potential energy based on force, and the dependence of force on potential energy – show how the concepts of force and potential energy are intimately linked: if two objects do not exert forces on each other, there is no potential energy between them. If two objects do exert forces on each other, then potential energy naturally arises in the system as part of the system's total energy. Since potential energy arises from forces, any change in the system's spatial configuration will either increase or decrease the system's potential energy as the objects are repositioned.
When a system moves to a lower potential energy state, energy is either released in some form or converted into another form of energy, such as kinetic energy. The potential energy can be "stored" as gravitational energy, elastic energy, chemical energy, rest mass energy or electrical energy, but arises in all cases from the spatial positioning and interaction of objects within a system. Unlike kinetic energy, which exists in any moving body, potential energy exists in any body which is interacting with another object.
For example a mass released above the [[Earth]] initially has potential energy resulting from the [[gravity|gravitational attraction]] of the Earth, which is transferred to kinetic energy as the gravitational force acts on the object and its potential energy is decreased as it falls.
Equation:
:<math>E_p = mgh \;</math>
where ''m'' is the mass, ''h'' is the height and ''g'' is the value of [[acceleration]] due to gravity at the Earth's surface (see [[gee]]).
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=== Energi internal ===
{{utama|Energi internal}}
''Energi internal'' adalah [[energi kinetik]] dihubungkan dengan gerakan [[molekul|molekul-molekul]], dan [[energi potensial]] yang dihubungkan dengan [[getaran]] [[rotasi]] dan energi [[listrik]] dari [[atom|atom-atom]] di dalam molekul. Energi internal seperti energi adalah sebuah [[fungsi keadaan]] yang dapat dihitung dalam sebuah sistem.
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== Sejarah ==
In the past, energy was discussed in terms of easily observable effects it has on the [[property|properties]] of objects or changes in state of various systems. Basically, if something changed, some sort of energy was involved in that change. As it was realized that energy could be stored in objects, the concept of energy came to embrace the idea of the potential for change as well as change itself. Such effects (both potential and realized) come in many different forms; examples are the electrical energy stored in a battery, the chemical energy stored in a piece of food, the thermal energy of a water heater, or the kinetic energy of a moving train. To simply say energy is "change or the potential for change", however, misses many important examples of energy as it exists in the physical world.
Energy can be used not only to produce observable change, it also is used to prevent change in which case unaided observation of this kind of energy can be difficult. For example, looking at a statue holding a 10 kg weight, the presence of energy needed to do so may not be observable. However, if you are holding up the 10 kg weight instead of the statue the need for energy to accomplish this becomes apparent. You can feel the gravitational force on you both when you are moving the weight up and when you are not moving it.
Energy can be readily transformed from one form into another; for instance, using a battery to power an electrical heater converts chemical energy into electrical energy, which is then converted into thermal energy. In the previous example of holding the 10 kg weight, the work you perform to raise the weight is observed as kinetic energy of motion which is converted to potential energy. Letting go of the weight once again transforms this stored potential energy back into kinetic energy as the weight falls under the force of gravity
== Penggunaan energi ==
''Artikel utama: [[pengembangan energi]], [[kebijakan energi]]''
The way in which humans use energy is one of the defining characteristics of an economy. The progression from animal power to [[steam power]], then the [[internal combustion engine]] and [[electricity]], are key elements in the development of modern civilization. [[Future energy development]], for example of [[renewable energy]], may be key to avoiding the [[effects of global warming]].<!--- etc etc much more to be said -->
== Lihat pula ==
* [[Richard Feynman|Feynman, Richard]]. ''Six Easy Pieces: Essentials of Physics Explained by Its Most Brilliant Teacher''. Helix Book. See the chapter "conservation of energy" for Feynman's explanation of what energy is and how to think about it.
* [[Albert Einstein|Einstein, Albert]] (1952). ''Relativity: The Special and the General Theory (Fifteenth Edition)''. ISBN 0-517-88441-0
==Referensi==
{{reflist}}
== Pranala luar ==
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