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. P

    A Variation of Energy for Dielectrics (Zangwill's Electrodynamics)

    Hello PhysicsForums community, I have been reading through Zangwill's Modern Electrodynamics all on my own, and I've just joined here hoping I can post some questions that come up for me. To start, I am confused about something in section 6.7.1, concerning the variation of total energy U of a...
  2. Xamaa

    Calculate the total energy required to fly a drone on this flight plan

    How much energy will drone batteries will consume if I have the following inputs, By using provided GPS flight path data I've calculated total horizontal distance using Haversine formula, and multiplying this distance with provided horizantal speed I've total horizontal flying time. By...
  3. A

    I Energy conditions and non-physical phenomena

    Is the inability to exceed the speed of light a consequence of general relativity? Is the fact that no energy is created from empty space a consequence of general relativity? Or are they both constructions deriving from the energy conditions imposed to have solutions to Einstein's equations that...
  4. J

    I How to get the energy eigenvalue of the Hamiltonian: H0+λp/m ?

    Someone says we can choose the new eigenstate: exp(-iλx/hbar)*ψ,and let the momentum operator p acts upon this new state. At the same time, so does p^2. Something miraculous will happen afterwards. My question is: how to image this point? Thank you very much.
  5. Mr_Allod

    Hund's Rule for Determining Term Symbol Energy Order

    Hello there, for the above question I have no issue finding the term symbols but I am a little unsure about employing Hund's rules to the electron configuration, particularly those referring to the energies based on the total angular momentum J. These state: - In a less than ##\frac12##-filled...
  6. Assaltwaffle

    I Question about transfer of Energy and Momentum in Ballistics

    My apologies if the prefix is too high of complexity. I don't know where this would fall, difficulty or academically speaking. While it may be surprising to some given Hollywood's portrayal of it in movies, if a person in wearing hard bulletproof armor is struck by a projectile, the person is...
  7. S

    I Microstates of an atom in an energy state

    I have a question about a sentence in the book Introduction to Thermal Physics (Daniel v. Schroeder). So in chapter 6, Schroeder talks about an atom isolated. This means its energy is fixed. The atom is in some state. The energy states of the atom have degenerated. All microstates with that...
  8. Lars Krogh-Stea

    B Energy Conservation w/ Charged Battery Time Travel

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  9. jaketodd

    B Are zero-point energy and inertia incompatible?

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  10. D

    Understanding a question about a finite energy well when E < 0

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  11. T

    I Energy of an electron in an electric circuit

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  12. LCSphysicist

    Energy change under point transformation

    How do the energy and generalized momenta change under the following coordinate transformation $$q= f(Q,t)$$The generalized momenta: $$P = \partial L / \partial \dot Q = \partial L / \partial \dot q\times \partial \dot q / \partial \dot Q = p \partial \dot q / \partial \dot Q = p \partial q /...
  13. S

    I Varying laws of high energy physics in inflation?

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  14. C

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  15. runningphysics

    Incorrect Calculation of Time for Reactor Energy Phase Change

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  16. K

    I Gravitational Field with Dark Energy: Observable Effects?

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  17. D

    Landau Energy Spectrum in the non-relativistic limit

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  18. CalvinandHobbes

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  19. V

    B How is capacitor energy the same as electrostatic potential energy?

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  20. P

    I Compressed gas energy storage formula

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  21. M

    I Understand Poynting Density of Flow of Energy w/Weak Physics Background

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  22. S

    A Does Spacetime Absorb Energy in General Relativity?

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  23. J

    B Conservation of Energy and Momentum in an Explosion

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  24. Ozen

    Fracture Energy for JH2 Model (Ceramics and FEA)?

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  25. S

    Anyone interested in Unidentified Aerial Phenomena (UAP) and how they work?

    Just wondering if anyone here finds these video/picture leaks about UAP's (Unidentified Aerial Phenomenon) interesting and inspiring enough to try and figure out how they work? Pretty big game changer of technology and would be fun to discuss. I'm hoping at least someone agrees.
  26. J

    Find largest potential energy difference between 2 loop orientations

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  27. S

    I Is it a fact that vacuum energy exists?

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  28. K

    Comsol modelling for Energy Harvester (Unimorph)

  29. T

    Conservation of energy -- Using a spring to launch a ball up an incline

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  30. J

    Find the Potential energy of a system of charges

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  31. I

    I Energy emitted by EM sources under constructive interference

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  32. Samama Fahim

    I Time Dependent Sinusoidal Perturbation Energy Conservation

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  33. F

    B Quantum entanglement and energy conservation

    As my current studies have proven conservation of energy is a universal law. How is it possible for two entangled particles to be equally or similarly affected when a force or energy is applied to a single member of the entangled pair? The production of such a pair would be invaluable to...
  34. G

    B Does Light Energy contain Heat Energy in a large scale?

    Can I understand that light energy contains heat energy? or Do I have to separate between two concepts? because light energy from the sun makes heat energy after absorption of molecular?
  35. J

    Velocity of two masses due to electric potential energy

    We can find the potential energy by finding the potential difference between the two masses. the minimum distance between the two masses is 10 cm. The maximum is 30 cm because they can be 3 string lengths apart as they repulse each other once the string is cut. So, to get potential difference...
  36. fluidistic

    I Energy flux direction in a conducting wire?

    On the Internet, I have read that the energy doesn't flow in the wire, for example in a very simple electric circuit made of a battery and a closed loop. When one computes the Poynting vector ##\vec S \propto \vec E \times \vec B##, one gets that its direction is towards the center of the wire...
  37. T

    B Understanding Nuclear Fusion and Particle Interactions

    What is the precise mechanism by which matter is converted to energy?
  38. A

    I How to interpret this definition of potential energy?

    Hello everyone, I'd like to share a doubt I am currently struggling with. So we know that ΔU=−W, where ΔU is the difference of potential energy and Wthe work done by the force to move the body from point A to point B. When analyzing this for the gravitational force, since we have U=−GmM/R, with...
  39. rudransh verma

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  40. Q

    Wanting to study energy in the human body

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  41. A

    Engineering Voltage in capacitors within an Energy Harvester

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  42. Tea_Aficionado

    Why don't we account for energy lost in collision here? SHM

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  43. Spinnor

    I Back of the envelope estimate, energy flow in a box of plasma

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  44. AlonZ

    I E=hf for massive particles, but which Energy exactly?

    Hi there, I'm a bit confused about the E=hf equation for mass particle(f for frequency), and Lorentz Invariant (E^2 -p^2c^2=m^2c^4). The question is, which energy is it? Total Energy- Kinetic plus Rest, or only kinetic energy. Now, if it's total energy, then you get that a particle at rest...
  45. rudransh verma

    B Does more energy mean more work?

    As we always say eat food and rest because tomorrow you have to go to work. When we throw a ball up, it gains a max PE at some height. More the height more energy it has which makes it do more work. Is this right to say?
  46. S

    Energy required to move an object to Earth's surface

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  47. D

    Can High Velocity Cause Hailstones to Melt?

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  48. W

    Determine how much energy the alpha particle carries

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  49. R

    I Why is momentum considered a vector and kinetic energy a scalar?

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  50. MrS

    I Open channel flow: derive function for energy slope on friction?

    I want to determine the normal flow depth in a perfectly horizontal circular conduit. The system characteristics are known (Internal pipe diameter, Mannings roughness, Discharge). However, I am not sure how to calculate the normal flow depth. When using Manning's equation one can find the normal...
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