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. M

    A Vacuum Energy from Correlation Functions

    In QFT the objects of interest are the n point Correlation functions which contain all the information about the theory and can be used to compute any expectation value in principle. However I cant figure out how to compute the vacuum energy from the correlation functions alone and cant find any...
  2. hello478

    Help needed in this problem involving a spring and energy balances

    i do know how to do the working but i dont understand the concept stated above...
  3. H

    I Potential energy and the gravitational field of a collapsing cloud of gas

    Imagine an empty void of intergalactic space. In this space there is a cloud of diffuse gas, mostly hydrogen and helium. The gas is non-rotating and very cold just above absolute zero. There is nothing else around this cloud, and so it has a clear center of gravity, and no other objects...
  4. F

    I Nuclear power plant vs coal power plant efficiency

    Hello, I have been reading that a coal power plant and a nuclear power plant have similar efficiencies, i.e. ~30%. This 30% refers to the conversion of thermal energy into electricity. For example, for 100 Joule of thermal energy, we only get 30 Joule out of electrical energy. How is that...
  5. NullSector

    Programs Pros and Cons of Nuclear Engineering

    Hey all. Quick questions for all you NEs and nuclear lovers out there. I am currently finishing up HS and need to decide a major for college. I am trying to decide between EE and NE, but am strongly considering NE. I have always loved nuclear energy and anything related to nuclear in general...
  6. amandela

    Elastic Potential Energy - Positive or Negative?

    So I understand that I have to integrate the negative of the force function to get the change in PE. I get -(20x^2 - 2x^3) and when I evaluate it from 0 to 2, I get -64N. But, of course, the change is positive. What am I missing? Thank you.
  7. SiRiVeon

    Conservation of energy problem: Ball rolling down inclined plane and then through a loop-the-loop

    Hello, this question may seem weird but I really need help on this. To bring the formula for the height h of the triangle above, I have to create a relation between potential and kinetic energies of the black ball with mass m (I can't find any other methods than this). For a sphere falling...
  8. A

    A How to calculate the site energy corresponding to monomer with Gaussview 6?

    how to calculate the site energy corresponding to monomer with gaussview 6?
  9. D

    Theoretical Physicist working on physics of energy systems and climate

    See also my LinkedIn page under my name, Dirk Smit. I retired recently from a big energy company where I was the corporate chief scientist. I am now affiliated with Oxford Univ and MIT working on the applied science and engineering of the energy transition in particular the role of the...
  10. martonhorvath

    Optimizing energy expenditure over a race course in cycling

    $$W = W_{gravity} + W_{friction} + W_{air}$$ Dividing by s: $$F_{total} = mg(sin(\alpha)+\mu cos(\alpha))+0.5×C_{d}A\rho×(v+v_{wind}×sin(\beta))^2$$ Then expressing v for both sections separately: headwind: $$0.5×C_{d}A\rho×v^2+v_{wind}×sin(\beta)×C_{d}A\rho×v+mg(sin(\alpha)+\mu...
  11. A

    Comparison between two Tippe tops

    I think that the second tippe top will spin on its stem first, and the first tippe top will stop spinning first due to its greater mass and lower angular velocity. Here are my ideas: They are given the same initial energy. By the conservation of energy principle, an object with greater mass...
  12. I_Try_Math

    Using energy considerations to determine speed

    $$K_i + U_i = K_f + U_f$$ $$K_i = \frac 1 2 m(15)^2$$ $$U_i = 196m$$ $$U_f = 0$$ $$K_f = K_i + U_i - U_f$$ $$=\frac {15^2} 2 m + 196m$$ $$=\frac 1 2(15^2m + 98m)$$ $$=\frac 1 2m(15^2 + 98)$$ $$=\frac 1 2m(323)$$ $$=\frac 1 2m(17.97)^2$$ $$v=17.97 m/s$$ Not seeing where I'm...
  13. Z

    Change in closed system energy with both conservative and non-conservative forces

    Thus $$\Delta K=-\Delta U + W_{nc}$$ $$W_{nc}=\Delta K+\Delta U=\Delta E_m$$ My question is about the following statement The system is closed. ##\Delta E_{system}## does not necessarily have to be zero. Where does (1) come from?
  14. M

    I Work done by electric field of an irregularly shaped conductor

    I am trying to wrap my head around something and would be grateful for some insight. Specifically, why the different electric fields along different paths to different areas of an irregularly shaped conductor don't impart different energies to inflowing particles. Say we have a negatively...
  15. C

    I I'm calculating more energy out than I put in

    Hello everyone, I'm currently working on a physics problem involving the rotation of a 5 kilogram ##M=5## solid sphere subjected to a force of 5 newtons ##F=5##, and I've encountered an inconsistency in my calculations. I'm seeking guidance or insights into where I might have gone wrong. My...
  16. petar rezek

    Calculate the mass of argon

    In the images you can see what I did I need someone to check answers and show me how to do C part of question
  17. Arasvo

    B Where does the original singularity get its energy from?

    Where does the original singularity get its energy from?
  18. P

    I Seemingly a contradiction of conservation of energy?

    I've had this question for a while now and I wonder if anyone can make sense of it. It's about two scenarios where the difference between them seems to contradict conservation of energy: Scenario 1: In a vacuum chamber, there is a robotic arm, a box, a lower platform and a higher platform. At...
  19. mister i

    B Where does the light energy lost due to expansion go?

    Sorry for my questions as an amateur interested in physics: If light changes its frequency during a long time of travel in space (vacuum?) for all observers (redshift) and therefore its energy decreases (E=hf), what "entity" absorbs this energy? I suppose the answer will be that the wavelength...
  20. heroslayer99

    Conservation of Energy with springs

    Start by finding the equilibrium position, so we have {4mgx}/{a} = mg giving us x = a/4, therefore the spring's length is 5a/4. Now the loss in EPE (and therefore gain in energy of the particle) between the bottom and the equilibrium position is clearly 4mg((a/4 + d)^2 , and then from the...
  21. Lok

    B Gravitational potential energy, a thought experiment

    Hi PF, long time no see. Hope you are all well. Recently I have come into a mental conundrum of a cosmological physical nature. After doing some napkin calculations about the energy of celestial bodies and transforming them into mass via E=mc^2 I've found that said energy is by no means small...
  22. Rayan

    Possible energy values given Hamiltonian

    So first I rewrote H as a matrix: $$ H = \begin{pmatrix} a & b \\ b & c \end{pmatrix} $$ And tried to find the eigenvalues/energies of H, so I solved $$ det (H - \lambda I ) = \begin{vmatrix} a-\lambda & b \\ b & c-\lambda \end{vmatrix} = (a-\lambda)(c-\lambda) - b^2 = ac - a\lambda -...
  23. physadict

    Alpha particle energy in MCNP6

    15- mode a 16- m1 1000. -0.111894 $MAT1 17- 8000. -0.888106...
  24. A

    I Where is the lost energy in this example?

    We connect the charged capacitor to the no-charged capacitor (consider the wires to be ideal R=0), the final energy is less than the initial energy of the system. Where is the lost energy? (see example blew)
  25. Quantum Psi Inverted

    Gravitation Potential Energy -- Questions about calculating the sign of GPE

    I believe that this is due to context of application, but now, I'm starting to doubt myself. For example, a helicopter lifting itself has positive PE change. I really don't intuitively understand how this works. Can someone kindly explain this to me?
  26. G

    B Ignoring the motion of the Earth for energy vs. momentum conservation

    Hi. If I drop an inelastic body, its potential energy first gets converted to kinetic, then to deformation energy. We use conservation of energy without taking into account the kinetic energy gain of the earth during the fall. However, at first sight conservation of momentum seems to be...
  27. robotkid786

    B Is perpetual motion a forbidden topic in the scientific community?

    I've only just clocked this to be the case. I asked chat gpt and the say big bang has a lot of merit apparently
  28. A

    B How much heat energy can give out put by ionization gas?

    I mean by accelerating electrons and ions for energy levels in atom
  29. phinds

    What are these possible heat dissipation tubes?

    In the Jan 6 Economist, there is an article about energy ("Britain needs an unprecedented expansion of the electricity grid") headed by this image. I assume that it is part of a power plant but I can't figure out what all those curved tubes are. First, I thought, maybe heat dissipation but they...
  30. W

    Troubleshooting Circular Motion: Solving the Toy Car Loop Puzzle

    I tried using conservation of energy, and using the equations for circular motion, but I can't seem to find a solution. Any help?
  31. L

    Physical Chemistry

    I tried to calculate the energy of an H-Atom with the one electron integrals. I tried to solve them numericaly but it didn‘t worked. Thats why I am hear.
  32. J

    Find out kinetic Energy using Rutherford Formula

    Hello everyone, I am working on this problem and I think I almost solved it, but then I noticed, that I do not know what values I have for dn, n and dθ. Can anyone help me with this?
  33. M

    Why kinetic energy of system of particles is not equal to p^2/2m

    It is good for the people who wants to know physics and it's beauty
  34. L

    Time crystals: They require energy added back into them?

    I was initially under the impression that time crystals are in lowest energy state and neither gain or lose energy. But other sources suggested they might require a bump in energy after a while to keep going?
  35. L

    Block on slope. Different answers using energy vs forces

    The first equation is when I use forces. The block is in static equilibrium, therefore the spring force should balance the gravitational force. The second equation is when I use energy principles. Energy before compression = Energy at compression. The height before is x * sintheta, and the...
  36. H

    B Where the extra energy comes from?

    Let's say you've created a very large balloon that doesn't explode in a vacuum and filled it with a gas lighter than air. Next, attach the balloon to a basic electric generator with a very light rope and place the balloon in a vacuum tube. Due to the balloon's weight, it will descend in the...
  37. TR094

    B How much energy do you need to split up a proton?

    is it even possible to split pu a proton and how much energy would it take to do that? i heard that it requires so much that it would make new a quark.
  38. J

    1999 AP Physics C Mech: Conservation of momentum and energy

    Why is (1/2)(mv0)^2 = 1/2(M+m0)gh not a valid equation for conservation of energy? Isn't the energy from when the dart is shot the same as when the two masses move at speed v?
  39. Bobski

    I Discover the Relationship Between Mass and Energy in Electromagnetic Fields

    When E=mc^2 is rearranged using the substitution c=1/√ε0μ0 , and making mass the subject we get m=Eε0μ0 This equation basically says that mass is directly proportional to the energy contained in an electromagnetic field. Does it not? Does this equation tell us that mass particles are made up of...
  40. T

    I Energy of spinning objects as axis of rotation moves

    Imagine an object, e.g throwing knife, spins in the air but not forced to rotate about a particular axis, i.e no rod impaling it and forcing it to spin about the rod. Then the axis of rotation converges to it's center of mass (CM) to minimize I. But there's nowhere for it's rotation energy to go...
  41. D

    I Spacecraft With Solar Mass Energy Equivalent Kinetic Energy

    Suppose in a different star system, a space shuttle sized spacecraft acquired a solar mass energy equivalent amount of kinetic energy, then passed through our solar system. While it was passing through the solar system would the craft’s gravitational effects be more similar to the space shuttle...
  42. T

    I Why is not possible to store electric energy from a Lightning?

    That question has been puzzling me recently. But I know there is a simple answer to it.
  43. Venturi365

    Solving for Masses in an Atwood Machine

    I've tried to solve this exercise but I haven't used one of the properties of the system (the displacement of the masses) so I don't know if I'm wrong about my procedure. First of all, we (obviously) know that $$ P=P $$ And since we can express the power of a force in two different ways, we...
  44. P

    I Infinite value of amplìtude of an undamped driven oscillator at resonance

    So if we set the damping constant ##\beta=0## that is if we consider an undamped oscillator the amplitude becomes infinity! What is the physical meaning of this phenomena? As we know energy fed into the system is proportional to ##A^2##. So does this mean that an infinite amount of energy is...
  45. J

    Quantum Mechanics homework question on wavefunctions

    Absolutely no clue on how to even begin this question due to the exceptionally poor quality of our lectures, who has also flatly refused to give out any solutions, which I could have used to understand what is going on. I assume the energy has to be obtained by using the eigenfunction equation...
  46. G

    I Looking for a unified expression for energy in rotating systems

    Hello, I am wondering if anyone can give me a hand. I am looking for a unified expression for energy in rotating systems or wondering if one even exists. Any help or equations you wish to share about the topic would be great. Sorry I am quite new to physics. Thanks
  47. user366312

    How can I compute energy from a protein PDB file?

    Suppose I have a protein PDB file and want to compute energy using the following formula from the coordinates in that file. How can I do that? Sometimes people suggest using the `CONECT` field for that purpose. However, some PDB files do not have this field. Example: 4OSK.pdb (Crystal...
  48. Dario56

    I Is the Poynting Vector Field the Only Factor in 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...
  49. physicsmaster123

    Work done by gravity on a car rolling down a hill

    I tried E =Fxcos0 but only ended up with 243kJ
  50. 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...
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