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.

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  1. 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...
  2. 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...
  3. 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 -...
  4. 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...
  5. 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...
  6. 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 just...
  7. 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.
  8. 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...
  9. chasrob

    What is the Theoretical Energy Density of Inflation?

    I need the theoretical energy density of inflation for my story. I seem to recall it as an enormous 1095 ergs per cubic centimeter.
  10. 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.
  11. 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...
  12. 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
  13. Altoid

    Why does the logic of negative acceleration not always indicate negative work?

    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...
  14. 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...
  15. yinnxz

    Is the Change in Internal Energy Just 4513 J When No Work Is Done?

    Since the system is doing no work, would it be just 4513 J? I don't think there is any other information to use
  16. 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.
  17. 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...
  18. 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.
  19. 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...
  20. 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...
  21. 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.
  22. 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 )?
  23. 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 ×...
  24. monsterhart

    1D Particle & Energy w/ F(x): Am I doing this right?

    A) I just did what it said to do: $$\sin\left(4x_{1}\right)=1\implies x_{1}=\frac{\arcsin\left(1\right)}{4}\ m=\frac{\pi}{8}\ m\approx 0.392699081699\ m$$ B) I modified the method from an example from the lecture the other week: $$U\left(x\right)=-\int...
  25. T

    I "Energy is not conserved" vs. energy is conserved: Friedmann Equations

    First, "Energy is not conserved" as e.g. explained by Sean Carroll in https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ . Second, the Friedmann Equations are expressed in energy conservation, e.g. https://core.ac.uk/download/pdf/25318877.pdf equation (16). Do we...
  26. Dileep Ramisetty

    B The Effect of Binding Energy on Mass

    Starting to explore quantum mechanics, I read strong nuclear force that binds protons and neutron together in nucleus of atom, gives atom its mass. More binding energy means more mass of atom. Hence the query that, for example there are two magnets having a force F1. And we have the same size...
  27. F

    Calculating pressure from a known equation of energy

    My attempted solution is as follows: Obviously the heat transfer happens during transitions 1->2 and 3->1. It's also clear that P1 = P3 V1 = V2 E2 - E1 = Integral[T dQ , from state 1 to state 2] E3 - E2 = - Integral[P dV , from state 2 to state 3] E1 - E3 = Integral[T dQ , from state 3 to...
  28. Lotto

    B What are the energy eigenvalues of a harmonic oscillator?

    Is it a total energy of a vibrating molecule? So is it a sum of potential and kinetic energy? Or it is only a total energy of a vibrational motion of the molecule? Or is it only a potencial energy, when it is related to a dissociation curve? I am confused.
  29. C

    Helium balloon energy conservation

    For this problem, How can energy be conserved if the bit highlighted in orange is true?Many thanks!
  30. T

    Average Kinetic Energy of Electron in the Conduction Band

    Hello, I've seen in a few books on solid state physics that one can deduce an expression for average K.E.: $$<\:K.E.>\:=E_c+3/2\:k_B\:T$$ from the following: $$<\:K.E.>\:=\:\frac{\int \:\left(E-E_c\right)g\left(E\right)f\left(E\right)dE}{\int \:g\left(E\right)f\left(E\right)dE}$$ I can't...
  31. O

    I Is there a Boltzmann distribution for a system with continuous energy?

    Hi. I'm not sure where to put this question, thermodynamics or the quantum physics forum (or somewhere else). For a system in equillibrium with a heat bath at temperature T, the Boltzman distribution can be used. We have the probability of finding the system in state n is given by ##p_n =...
  32. N

    A Beyond the energy equipartition, what is the uneven partitioning?

    There is a celebrated energy equipartition theorem, it works fine for many systems. But it requires the dense filling of the surface of constant energy. What if there are other conserved quantities, like momentum or angular momentum? It seems, that the energy partitioning will be uneven, with...
  33. H

    I Dark Energy Strength in Great Voids of Galaxies

    Assuming dark energy is fairly, uniformly distributed through out the cosmos, how strong is it, or how much energy is associate with it, out in the deepest, emptiest voids in space? I'm specificlaly refering to the great voids in between the great walls of galaxy clusters. I'm making the...
  34. S

    Rate of loss of potential energy

    I got answer for (a), which is 0.51 m For (b), loss of potential energy = 35 x 9.81 x 0.51 = 175 J Rate of loss of potential energy = 175 J / 1 s = 175 W But the answer key is 80 W. Where is my mistake? Thanks
  35. sinus

    I The kinetic energy of proton-electron for a black body

    I'm watching a video about " What is a black body?". That video said when the light interacts with the surface of a body, the electron and proton start oscillating. The electrons gain more transferred energy from the light that became its kinetic energy, rather than the proton because its mass...
  36. rogdal

    Deriving the kinetic energy flux in an effusion process

    I could not find any derivations in the litterature, except for the expected value of the energy flux expression itself: $$\overline{\Phi_{effusion,\epsilon}} = \overline{\dot{N_{ef}}}\overline{\epsilon_{ef}}=\frac{3Nl}{2A}\sqrt{\frac{(k_BT)^3}{2\pi m}}$$ I've started off by calculating the...
  37. Darmstadtium

    B Is the Definition of Work Done Applicable to Free Fall?

    For example, if a ball is from a certain height, the work done is 0 as there is no change in total energy the Ef =Ei. However, there is a constant force applied over a certain distance, suggesting work is being done. Which aspect am I forgetting/missing? Or is it that the definition of work done...
  38. L

    Find speed of block using energy and spring equations? Help please

    So here's what I did but it isn't right: W = (Kf + Uf) - (Ki + Ui) (2.6)(9.81)(0.45)(-0.01)=(1/2mvf^2 + 1/2kxf^2) - (1/2mvi^2 + 1/2kxi^2) -0.1 = (1/2(2.6)(vf^2) + 1/2(855)(0.02^2)) - (1/2(855)(0.03^2)) 1.3Vf^2 = 0.114 Vf^2 = 0.09 Vf = 0.3 m/s
  39. emilmammadzada

    How can I find the stopping power for a single energy value in FLUKA?

    Hello friends. How can I find the stopping power for a single energy value in the fluka? Which card should I add to the input file for this?
  40. M

    I States of equal energy are equally probable

    I'm reading "Statistical Mechanics: A Set of Lectures" by Feynman. On page 1 it says that, for a system in thermal equilibrium, the probabilities of being in two states of the same energy are equal. I'm wondering if this is an empirical observation or if it can be derived from QM?
  41. Darmstadtium

    B Conversion of Potential Energy: Sound or Heat?

    When a coin is dropped from a certain height and collides with a glass surface, is the majority of the potential energy converted to sound or heat? And how would one determine this as I only hear the sound and cannot measure the significant change in temperature?
  42. ribella

    A What is the energy dependence of the Equivalent photon approximation?

    Hi, What is the energy dependence of the Equivalent photon approximation? For this approach to be valid, what is the maximum center of mass-energy. As know, this approach is an energy-dependent approach. Can this approach be used to calculate, for example, at a center of mass energy of 100...
  43. R

    Energy Density with a Dielectric

    I am confused about how the electric field changes in this problem - is E' = E/Ke=E/2? Is E = V/d a correct usage? When I solve it this way, the answer is incorrect: change in energy density = (1/2)ε(E'2- E2) = (1/2)ε(E2/4 - E2) = (1/2)ε(-3/4)(V/2d)2. What am I doing wrong? Thanks.
  44. A

    Energy conservation law question with capacitor

    I was wondering why energy of capacitor does not equal change in kinetic energy PLUS change in potential energy where potential energy is the change in the potential energy of the charges. I believe that should be so because energy conservation = change in kinetic energy plus change in potential...
  45. KataruZ98

    Kinetic energy transfer from shockwave to secondary body

    I would guess that by multiplying the pressure exerted by the shockwave on the body, and then the resulting force - here ~69 Newtons - per the distance the shockwave passed through when traversing body A, I could get the work done but I’m not sure if it’s that easy and whether or not I should...
  46. Muu9

    Kinetic Energy / Potential Energy / Total Energy question

    W_ext is the external work done on B and C, which is 12 J Delta K_tot is the internal work, which is the work done by A on B plus the work done by A on C Delta K_tot = 5 Solving for \Delta U, we find that the change in potential energy is 7 J This answer says otherwise...
  47. J

    Expectation of Kinetic Energy for Deuteron

    Dear Forum, I am solving for the expectation value of the kinetic energy for the deuteron (Krane problem 4.3). I must be missing something since this has become far more complicated than I remember. The problem is as follows: ## <T> = \frac{\hbar^{2}}{2m} \int_{0}^{\infty}...
  48. cnh1995

    I Neil deGrasse Tyson on Nikola Tesla and electromagnetic energy

    I came across this video where Dr. Tyson talks about Nikola Tesla. Neil Tyson on Tesla. From 4:47 onwards, he says "We now send energy through wires", and talks about how bizarre it would be to walk around/stand in the way of such energy flow. Further he says the power transmission lines are...
  49. Bandersnatch

    A Black hole mass coupled to expansion -- astrophysical source of dark energy?

    Observational evidence for cosmological coupling of black holes and its implications for an astrophysical source of dark energy Comments?
  50. S

    Change in energy stored in a spherical Capacitor

    I have attached my solution. Unfortunately, after plugging in the values, my answer is 4 times more than the expected one. What am I missing?
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