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veličina koja karakterizira gibanje, mirovanje ili položaj tijela, tekućine, čestice ili sustava čestica, ukupna količinska mjera gibanja i međudjelovanja svih oblika tvari te veličina za opis čestica polja koje prenose prirodne sile i međudjelovanja čestica. U prirodnofilozofskom smislu, energija zajedno s tvari daje pojam materije. Energija se u prirodi, tehnici i industriji pojavljuje u različitim oblicima, koji se pretvaraju jedan u drugi po <a href="https://www.enciklopedija.hr/clanak/zakon-ocuvanja-energije">zakonu očuvanja energije</a>: ona se ne može potro&scaron;iti ni stvoriti, već samo promijeniti svoj oblik. U gravitacijskome polju Zemlje, da bi se tijelo pomaknulo po nekome proizvoljnom putu, treba obaviti rad dizanja. Budući da je gravitacijska sila konzervativna, rad dizanja, koji ovisi samo o visinskoj razlici između početne i krajnje točke puta, nije izgubljen, nego odgovara razlici potencijalnih energija u početnoj i konačnoj točki puta. Dakle, <span style="color: #000000"><i><a href="https://www.enciklopedija.hr/clanak/potencijalna-energija">potencijalna energija</a> </i></span>ovisi o položaju tijela <span style="color: #000000"><i>h</i></span> (visini) u gravitacijskome polju, <span style="color: #000000"><i>E</i></span><sub>p</sub>&nbsp;=&nbsp;<span style="color: #000000"><i>mgh</i></span>, gdje je <span style="color: #000000"><i>g</i></span>&nbsp;=&nbsp;9,806&nbsp;65&nbsp;m/s&sup2;, normirano <a href="https://www.enciklopedija.hr/clanak/gravitacijsko-ubrzanje">gravitacijsko ubrzanje</a>. Kada tijelo pada, njegova se potencijalna energija smanjuje i pretvara u <span style="color: #000000"><i><a href="https://www.enciklopedija.hr/clanak/kineticka-energija">kinetičku energiju</a></i></span>. Općenito se pod djelovanjem neke sile tijelo mase <span style="color: #000000"><i>m</i></span> ubrzava na nekom putu. Promjene brzine tijela, zbog rada &scaron;to ga obavi tijelo ili rada utro&scaron;enog na tijelu, izražavaju se kinetičkom energijom, <span style="display: inline-block;"><span style="color: #000000"><i>E</i></span><sub>k</sub>&nbsp;=&nbsp;<span style="display: inline-block; vertical-align: middle;"><span style="text-align:center; display: block;">1</span><span style="display: none;">/</span><span style="text-align:center; display: block; border-top: thin solid black">2</span></span>&nbsp;<span style="color: #000000"><i>mv</i></span>&sup2;</span>. Dakle, utro&scaron;eni rad ubrzanja d<span style="color: #000000"><i>W</i></span> jednak je promjeni kinetičke energije d<span style="color: #000000"><i>E</i></span><sub>k</sub>&nbsp;tijela mase <span style="color: #000000"><i>m:</i></span></p><p>\[{\rm d}W=F\cdot{\rm d}s={\rm d}\left(\frac12mv^2\right)={\rm d}(E_{\rm kin}).\].</p><p>Pojmove potencijalne i kinetičke energije u mehaniku su uveli <a href="https://www.enciklopedija.hr/clanak/galilei-galileo">Galileo Galilei</a>, <a href="https://www.enciklopedija.hr/clanak/newton-isaac">Isaac Newton</a> i <a href="https://www.enciklopedija.hr/clanak/leibniz-gottfried-wilhelm">Gottfried Wilhelm Leibniz</a>, a formulacije zakona očuvanja energije dugujemo <a href="https://www.enciklopedija.hr/clanak/lagrange-joseph-louis-de">Joseph Louis de Lagrange</a> i <a href="https://www.enciklopedija.hr/clanak/helmholtz-hermann-ludwig-ferdinand-von">Hermann Ludwig Ferdinand von Helmholtz</a>. Mehaničku narav topline u XIX.&nbsp;st. dokazali su <a href="https://www.enciklopedija.hr/clanak/joule-james-prescott">James Prescott Joule</a> i <a href="https://www.enciklopedija.hr/clanak/mayer-julius-robert-von">Julius Robert von Mayer</a>. U konzervativnim poljima sile (gravitacijska sila, sila opruge, kulonska sila) ukupna je energija očuvana, <span style="color: #000000"><i>E</i></span><sub>u</sub>&nbsp;=&nbsp;<span style="color: #000000"><i>E</i></span><sub>p</sub>&nbsp;+&nbsp;<span style="color: #000000"><i>E</i></span><sub>k</sub>&nbsp;=&nbsp;konst., dok kod nekonzervativnih polja (sila trenja) to nije zadovoljeno jer se rad pretvara u toplinu. Konačnu formulaciju zakona očuvanja energije dobio je <a href="https://www.enciklopedija.hr/clanak/einstein-albert">Albert Einstein</a> (1905): &raquo;<span style="color: #000000"><i>Ako tijelo predaje energiju u obliku zračenja, masa mu se umanjuje za E</i></span>/<span style="color: #000000"><i>c</i></span>&sup2;&laquo;, &scaron;to je slavna formula <span style="color: #000000"><i>E</i></span>&nbsp;=&nbsp;<span style="color: #000000"><i>mc</i></span>&sup2; za ekvivalenciju mase i energije, u kojoj je <span style="color: #000000"><i>c</i></span> <a href="https://www.enciklopedija.hr/clanak/brzina-svjetlosti">brzina svjetlosti</a> u vakuumu.</p><p>Istraživanjima primjene energije u svakodnevnom životu, <a href="https://www.enciklopedija.hr/clanak/energetski-izvori">energetskim izvorima</a>, i utjecajima uporabe energije na okoli&scaron; bavi se <a href="https://www.enciklopedija.hr/clanak/energetika">energetika</a>.</p><p>Zakonite mjerne jedinice energije jesu: <a href="https://www.enciklopedija.hr/clanak/dzul">džul</a> (J), <a href="https://www.enciklopedija.hr/clanak/elektronvolt">elektronvolt</a> (eV&nbsp;&asymp;&nbsp;1,602&nbsp;177&nbsp;33&nbsp;&times;&nbsp;10<sup>&ndash;19</sup>&nbsp;J) te umno&scaron;ci zakonitih jedinica snage i vremena, npr. <a href="https://www.enciklopedija.hr/clanak/kilovatsat">kilovatsat</a> (kWh&nbsp;=&nbsp;3,6&nbsp;MJ). Zastarjele su mjerne jedinice energije: <a href="https://www.enciklopedija.hr/clanak/18238">erg</a> (erg&nbsp;=&nbsp;10<sup>&ndash;7</sup>J), kilopondmetar (kpm&nbsp;=&nbsp;9,8066&nbsp;J), te za toplinu i energijsku vrijednost hrane <a href="https://www.enciklopedija.hr/clanak/kalorija">kalorija</a> (cal&nbsp;=&nbsp;4,1868&nbsp;J). U anglofonim zemljama se za velike vrijednosti energije rabi nenormirana jedinica <a href="https://www.enciklopedija.hr/clanak/kvad">kvad</a> (prema engl. <span style="color: #000000"><i>quadrillion British thermal units</i></span>), vrijednosti bliske eksadžulu, tj. kvad&nbsp;=&nbsp;10<sup>15</sup> Btu&nbsp;&asymp;&nbsp;10<sup>15</sup>&nbsp;&middot;&nbsp;1055,06&nbsp;J&nbsp;&asymp;&nbsp;10<sup>18</sup>&nbsp;J&nbsp;&asymp;&nbsp;EJ.</p> </div> </div> </div> <div class="container citiranje"> Citiranje: <p> energija. <i>Hrvatska enciklopedija</i>, <i>mrežno izdanje.</i> Leksikografski zavod Miroslav Krleža, 2013. – 2024. 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