What is Energy: Definition and 999 Discussions

In physics, energy is the quantitative property that must be transferred to a body or physical system to perform work on the body, or to heat it. Energy is a conserved quantity; the law of conservation of energy states that energy can be converted in form, but not created or destroyed. The unit of measurement in the International System of Units (SI) of energy is the joule, which is the energy transferred to an object by the work of moving it a distance of one metre against a force of one newton.
Common forms of energy include the kinetic energy of a moving object, the potential energy stored by an object's position in a force field (gravitational, electric or magnetic), the elastic energy stored by stretching solid objects, the chemical energy released when a fuel burns, the radiant energy carried by light, and the thermal energy due to an object's temperature.
Mass and energy are closely related. Due to mass–energy equivalence, any object that has mass when stationary (called rest mass) also has an equivalent amount of energy whose form is called rest energy, and any additional energy (of any form) acquired by the object above that rest energy will increase the object's total mass just as it increases its total energy. For example, after heating an object, its increase in energy could be measured as a small increase in mass, with a sensitive enough scale.
Living organisms require energy to stay alive, such as the energy humans get from food. Human civilization requires energy to function, which it gets from energy resources such as fossil fuels, nuclear fuel, or renewable energy. The processes of Earth's climate and ecosystem are driven by the radiant energy Earth receives from the Sun and the geothermal energy contained within the earth.

View More On Wikipedia.org
  1. Dario56

    I Energy Flow in Circuits

    In a simple circuit consisting of a battery and a resistor, current will flow if the circuit is closed. Resistor uses the energy provided by the battery, creating heat with a power ##P = UI##, where ##U## is a voltage across the resistor and ##I## is a current through the resistor. In my...
  2. physicsmaster123

    Work done by gravity on a car rolling down a hill

    I tried E =Fxcos0 but only ended up with 243kJ
  3. jcalises

    B Virtual particles and Heisenberg

    I registered yesterday in this forum with the intention of someone clarifying me how the Heisenberg uncertainty principle can explain the existence of virtual particles. More energy implies less lifetime is only possible if ΔE Δt = h/4Pi, but that's not Heisenberg's principle, the principle is...
  4. kenobiobiwan66

    Energy Production through Stirling Engine?

    A Beta-Type Regular Low Temperature Striling Engine being used to produce mechanical energy, where hot water at 350K is being used as fuel. What power should one expect theoretically?
  5. E

    Energy absorbed by spring

    I approach this by considering the four springs in parallel each with spring constant ##k## as one spring with four times the spring constant ##k' = 4k##. The car is dropped and at the moment its tyres touch the ground I assume that the spring is in its resting position. As the car continues to...
  6. A

    Can energy be turned into matter in the real world, and if yes, how does that happen?

    Theoretically possible, only if the process is reversible. But is it really. My knowledge base is insufficient to answer that. I need help.
  7. E

    I Obtaining this form for molar energy under virial expansion (Callen)

    In his Chapter 13.3 (2nd edition), Callen gives the standard form for the virial expansion for the mechanical equation of state of a fluid as an exapnsion in powers of the molar volume ##v##: $$P = \frac{RT}{v}\left(1 + \frac{B(T)}{v} + \frac{C(T)}{v^2} + \dots \right) \equiv P_{ideal} +...
  8. Steve Rogers

    I Quasi-local mass as a measure of the gravitational energy?

    I'm self-studying the mathematical aspects of quasi-local mass, or quasi-local energy (e.g. Hawking energy), and a fundamental question has been lingering in my mind for a long time: why does quasi-local mass provide us with a measure of the gravitational energy? In general relativity...
  9. a71cj34

    Same Charge, Same Mass? | Subatomic Particles

    I converted 3.1eV into J, substituted into E = mc^2. Since the energy is the same, I got the same answer for both: 5.52*10^-36 kg. This doesn't seem quite right- I doubt that a photon and an electron have the same mass. So, when two particles have the same charge, does that mean they have the...
  10. Slimy0233

    B What is m in Kinetic Energy? Relativistic mass or Rest mass?

    note: m = relativistic mass ##m_o## = rest mass v = velocity of the object Question 1: If a particle is moving at relativistic speeds what would it's kinetic energy be? I think it's ##K.E. = \frac{1}{2} m_o v^2## and my friend thinks it's ##K.E. = \frac{1}{2} \frac{m_o...
  11. RobbyQ

    I Mass, Energy and a compressed spring

    If I take a spring with clamps and I weight that system accurately. Then I compress the spring and clamp it thus giving it potential energy. If I now weigh the clamped spring I should see an increase in mass because of the added energy. Is this the case and something that could be proved in the...
  12. Lupin

    I How much energy (in Joules) is stored in a standard 12g CO2 cartridge?

    A standard 12 gram cartridge contains both liquid and gaseous CO2 at 850psi. Assuming we are venting to atmosphere at sea level, how much energy can be extracted from the cartridge? We know it will expand to 12 grams * 22.4 liters/ 1 mole (44grams) = 6.1 liters. But how much energy did...
  13. E

    I Helmholtz entropy of ideal gas mixture is additive?

    In his classic textbook, Callen remarks that I have labelled the claims (1) and (2). I am not sure about either. For the first, I have tried to proceed as follows (all equations are from Callen's second edition and all 0 subscripts are with respect to some reference state of an ideal gas): I...
  14. new_scientist

    I Is the energy destroyed in this experiment?

    Suppose you stand on a spherical permanent magnet in space and you hold an iron ball in your hand, you can neglect the gravity force by this magnet mass. You stand and throw the iron ball upwards with some kinetic energy, the ball will eventually stops at some height because it is attracted by...
  15. E

    I How can the maximum entropy and minimum energy principles be physical?

    In Chapter 5 of his famous textbook on thermodynamics, Callen argues for the "equivalence" of the maximum entropy (Max-Ent) principle and the minimum energy (Min-En) principles. I quote from Callen first: As far as I know (though Callen never makes this explicit in what, I think, represents...
  16. Szecska8

    I Stored magnetic energy of solenoid

    Can someone explain to me what the second integral represents and what is A?
  17. P

    I Emission Spectrum & Energy: Exciting Hydrogen Vapor

    By exciting hydrogen vapors with heat or electrical discharges, it is possible to obtain the element's emission spectrum. In it, as can be seen, appear multiple wavelengths, each corresponding to a particular orbital electronic transition. From this it can, therefore, be inferred that heat and...
  18. phos19

    I Fermi energy for a Fermion gas with a multiplicity function ##g_n##

    I ran across the following problem : Statement: Consider a gas of ## N ## fermions and suppose that each energy level ## \varepsilon_n## has a multiplicity of ## g_n = (n+1)^2 ##. What is the Fermi energy and the average energy of this gas when ## N \rightarrow \infty## ? My attempt: The...
  19. milkism

    Oscillating dipoles, energy and brightness

    Problem: Attempt at solution: So "energy passing through per unit area per unit time" is equal to $$I = \frac{E_i}{A t}$$ So for a the graph will be in the form of ##y=1/x##? For b) do we have to solve the differential equation $$dI = \frac{E_i}{A dt}$$?
  20. kuruman

    Schwarzenegger on energy

    Our Governator is attending the Austria world initiative on climate change. He wants us to know that we have "2000 Gigabytes of clean energy" waiting to be developed if only the permit process can be expedited. Lucky us. Listen to the video at about t = 2:20 and rejoice...
  21. S

    B Examples of less popular pairs of physical properties with uncertainty (HUP)

    Position and momentum are the popular pairs of properties with uncertainty we often hear about, for example that we cannot know with precision where an electron is and its momentum at the same time. What are others? Such as an example of an energy and a time that we cannot know both...
  22. TonyCross

    Kinetic-Potential energy of moving Ice

    Hi, Am i correct in thinking that if we take a block of ice, moving at a constant velocity, it's then exposed to a heat source which melts the ice and turns it into water vapour, that we have simply removed any Kinetic energy, by Sublimation or converting it into heat. My question is does the...
  23. L

    Admissions DOE SULI Advice For Biology Background - Renewable Energy Development

    Hi Everyone! I am an upperclassmen undergrad of a biol background, with mainly course training in biochemistry and molecular biol, and taken chem like o-chem and a-chem, and introductory physics. I have three short term summer projects at top 5 institutions in the past and in the field of...
  24. WMDhamnekar

    Chemistry Valence Bond Theory: Energy of a system with H and Cl atoms

    Answer: The energy of a system with H and Cl atoms at varying distances can be represented by a curve that shows the potential energy of the system as a function of the distance between the two atoms. At very large distances, the potential energy is zero because there is no interaction between...
  25. C

    Work energy KE theorem for a book being lifted up in a gravitional field

    For this, From the work kinetic energy theorem, if we assume that the book and the earth is the system, and that the finial and inital speed of the system is zero, then is the work KE theorem there is no net work done on the system. However, clearly there is work done on the system is shown by...
  26. gkg

    A Force Changing With Distance (bead moving on a curved wire)

    I initially tried to solve this equation using work, but was stuck in a confusing integral that didn't make sense. I am almost sure that the utilization of energy is needed to solve this equation, but I have been flustered for the past three days at solving this.
  27. uSee2

    Kinetic Energy of a Cylinder Rolling Without Slipping

    Given that there is a cylinder rolling without slipping down an incline, the method I was taught to represent the KE of the cylinder was: ##KE_{total} = KE_{translational} + KE_{rotational}## ##KE_{total} = \frac {1} {2} mv_{cm}^2 + \frac1 2 I \omega^2## Where "cm" is the center of mass, and...
  28. StenEdeback

    I do not understand stress energy tensor for fluids

    I do private studies on my own for fun and right now I read about relativistic field theory as a preparation for later studies of quantum field theory. I simply do not understand where equation 13.78 in Goldstein's "Classical Mechanics" third edition comes from. Please explain. Please also...
  29. nav888

    Conservation of Energy when lifting a box up off the floor

    So, I cannot for the life of me write a conservation of energy statement, when an object is lifted up by a force. So in my example there is a box on the floor with v = 0, and then a force of magnitude F, where F > mg, acts on the ball, now the net force is F-mg, and hence the work done is (F -...
  30. J

    I How do we calculate the energy we used to do something?

    Usually, I like to take a physical approach to phenomena that occur in everyday life. But I feel difficult to solve problems because I don't have higher education My question stems from this question (What's the difference between running up a hill and running up an inclined treadmill?), which...
  31. kyphysics

    I Car Accident Physics - Are Wheels/Tires Good or Bad Energy Absorbers?

    Here's an applied everyday life physics question based on a MVA (motor-vehicle accident) I was involved in a few weeks ago. I was driving straight when a women hit me from the driver side (said she didn't see me due to being in her blind spot - her claim, not mine, as I don't know if I was or...
  32. Kyuubi

    Solving Orbital Speed with Energy & Angular Momentum Conservation

    I've already solved the orbital speed by equating the kinetic and potential energy in the circle orbit case. $$\frac{1}{2}mv^2 = \frac{1}{2}ka^2.$$ And so $$v^2 = \frac{k}{m}a^2$$ Now when the impulse is added, the particle will obviously change course. If we set our reference point in time...
  33. A

    Do different length ramps violate conservation of energy?

    mgh=(1/2)(m)(v^2) gh=(1/2)v^2 sqrt(2gh)=v Should have the same v, but this is not the case based on the answer and real-life experiments.
  34. E

    Express entropy density in terms of energy density (Ashcroft/Mermin)

    The starting point is the identity $$\left(\frac{\partial u}{\partial T}\right)_n = T\left(\frac{\partial s}{\partial T}\right)_n.$$ I then try to proceed as follows: Integrating both with respect to ##T## after dividing through by ##T##, we find $$ \int_0^T \left(\frac{\partial s}{\partial...
  35. chasrob

    Inflation era energy density

    I need the theoretical energy density of inflation for my story. I seem to recall it as an enormous 1095 ergs per cubic centimeter.
  36. RandiSS

    Calculating Change in Energy: A Graph Analysis

    Not a solution. This is the graph provided. I think I start with finding the magnitude of the IF vector but I’m not sure. And I don’t know where to go from there.
  37. C

    B Atom energy band formation and electron allotment

    This is actually a two-part question: 1) According to the Copenhagen Interpretation, atoms have energy bands but there's no explanation of how these bands are derived, or why they only form for protons/antiprotons. Any thoughts? 2) The Copenhagen Interpretation mentions that when an atom's...
  38. aquastor

    Thermodynamics Question Regarding Gibbs Free Energy, Enthalpy, Entropy

    Hello, is someone able to explain why these two are wrong. I am not sure how to figure out the enthalpy direction as the reaction is not changing state of matter, nor is it changing temperature. (Please solve without calculating anything) Thank you
  39. Altoid

    Energy with varying force

    Knowing that negative work occurs when the force applied to an object opposes the direction of displacement, and that the direction of acceleration vector should align with the force vector, I assumed the correct answer was that the indication of negative work comes from negative acceleration...
  40. T

    I Rewriting of Gibbs Free Energy in Peksin (Equations 13.35/13.36)

    Hey all, On page 446 in Peskin, he provides 2 different ways of writing the Gibbs Free Energy: $$\textbf{G}(M,t) = M^{1+\delta}h(tM^{-1/\beta})$$, and $$\textbf{G}(M,t) = t^{\beta(1+\delta)}f(Mt^{-\beta})$$ where ##h## and ##f## are some initial condition functions that have a smooth limit as...
  41. yinnxz

    Change in internal energy

    Since the system is doing no work, would it be just 4513 J? I don't think there is any other information to use
  42. G

    Understanding Bond Energies: X-Y Bond Energy Contribution

    Hello. Could someone please help me with this question about bond energy from an MIT course: "For two bonded atoms X and Y, a small X and large Y will result in a bond energy (E A-B) with a large __________ contribution." Thanks a lot if someone can help.
  43. O

    Understanding the energy of a dipole in a uniform electric field

    Griffith's E&M problem 4.7 asks to calculate the energy of a dipole in a uniform electric field and I ended up getting a different answer than the one given. I thought that calculating the energy/work done to construct the dipole is the same as dragging two point charges where one is d apart...
  44. A

    I How is combustion energy released?

    Is it possible that in combustion, the chemical energy of reaction is released wholly through photon emission. Say as in simple burning of wood.
  45. M

    How much energy did the Thanos snap release, approximately?

    In Avengers: Infinity Wars Thanos had the Infinity Gauntlet and when he snapped his fingers it wiped out 50% of life in the universe. Roughly, how much energy do you think the snap generated assuming the universe IS finite in size (since an infinite universe cannot have any percentage) AND the...
  46. KataruZ98

    Kinetic energy transfer from a rotating body in an inelastic collision

    The cylinder in question would have a moment of inertia of ~1.67kg*m² and rotational KE of 2.058J. At the point of impact also, assuming the body hits the sphere at a 90deg angle after traversing 90deg of displacement, it should(?) exert a force of 1.31N - enough to give an acceleration of...
  47. milkism

    Conservation of relativistic energy, collision of particles

    Question: With maximum do they mean that the speed of the pions is the same as the proton and an antiproton? Otherwise there will be two unknowns, and if I use both relativistic-energy and momentum conservation equations I get difficult equations.
  48. Ranku

    I Dark energy density proportion

    Is there a way to independently determine the proportion of dark energy density to total energy density of the universe apart from using 1 -(Ωmatter+Ωdark matter )?
  49. S

    I Potential energy of spin anti-alignment

    Hello everybody, I consider two electrons that have enough kinetic energy to reach their respective classical electron radius. This would be: 2.0514016772310431402e-13 J The corresponding speed is v = 287336682 m/s. The electric field is E = \frac{k_{e}}{R_e^2} = 1.8133774657059088443 ×...