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
Recent contents Top users
  1. A

    Other Best journals in High Energy Physics (formal theory)

    Hello, I request inputs about selecting a journal to submit an article in High Energy Physics. This is in formal theory as opposed to Particle Phenomenology. The article is more than ten pages so PRL is not an option. How would you choose between JHEP and Physical Review D? Which is rated higher...
  2. S

    The location of the point with lowest kinetic energy

    I am not sure I understand the question. I imagine there is a horizontal table and all the diagrams are the top-view of the table. The question states that "A particle moves from P1 to P2 without acceleration along five different paths". In the first picture, I think the path is in the shape of...
  3. hairey94

    What are the assumptions for solving the charged conducting disk problem?

    Not sure how the problem set up initially as no diagram was provided in the question. Please help me to start with the solution. Your assumptions and educated guess are appreciated.
  4. hairey94

    Average Translational Kinetic Energy: P & V Given

    If this question asked for internal energy, then it is a straightforward thing, just use the formula U = (3/2) nRT = (3/2) PV. But the question specifically asked for average translational kinetic energy which I'm stuck with the value of T to plug in into the formula as the question only give us...
  5. LCSphysicist

    Collision, angular momentum and energy

    IS my solution right? Comparing with the other solutions, the answer just exchange the signals, i don't know why, THats what ifound. And here is the three equations: {i use the point which occurs the collision} Lo = Lf >> 0 = Iw + M*Vcm(block) Eg = ct> mvo² = mvf² + MVcm² + Iw² I = ml²/3...
  6. A

    A Energy needed to convert a bound proton to a neutron?

    Hey everyone, I've got a question on converting bound protons into neutrons. a. What are some methods used to achieve the proton-to-neutron conversion in atomic nuclei? I'm familiar with particle scattering off a proton in the nucleus. I'm also aware of (n,p) reactions. Are there any other...
  7. jaumzaum

    A Gravitational Energy: Field x Moment

    Hello! I was wondering if it is possible to express the gravitational energy as a product of the gravitational field by a moment, as we do with the magnetic and electric energy? Would this require the existence of bodies with negative mass? How could we relate this to the existence or total...
  8. C

    I Physical question about Phonons

    Hi, I have a question regarding Phonons and daily experience: Let's say I have a table and I hit it, does it mean Phonons were created where I've hit on the table? Meaning: By hitting the table, I'm giving energy to it, this energy goes to the motion of the table atoms, and this motion of the...
  9. J

    Energy of translation compared to the energy of rotation

    I use an example with a rack and a pinion. I suppose there is no losses from friction. I suppose the masses very low to simplify the study, and there is no acceleration. I suppose the tooth of the pinion and the rack perfect, I mean there is no gap. There is always the contact between the rack...
  10. marialovesphysics

    Gravitational potential energy formulas

    m.g.h = (GMm)/r how can we prove that mgh is potential energy and both equal to that?
  11. S

    Center of mass energy for two relativistic colliding particles

    Starting from the center of mass energy S = (E_{1} + E_{2})^2 - (\vec{p_1}+\vec{p_2}) knowing that E^2 = m_{0}c^4 + p^2*c^2 one has S = (E_{1} + E_{2})^2 - (\vec{p_1}+\vec{p_2}) = ( m_{0}c^4 + p_{1}^2*c^2) + m_{0}c^4 + p_{2}^2*c^2)^2 - p_{1}^2 - p_{2}^2 - 2p_{1}p_{2}cos \{theta} and then...
  12. GREENMONKEY

    I Why is the speed of photons different?

    Hi, I read the Feynman's book about the quantum electrodynamics and I realized, that he was talking about the different speed of photons. I know, that the light travel's "slower" in a material, but he is also talking about the different speed of photons. I read on the web, that some photons...
  13. G

    Why is the power of a particle on a wave zero in a stationary wave?

    I've marked the right answers. They mainly indicate at power carried by the particles being zero, and here is my doubt- why should it be zero? Shouldn't it have some definite value? I do understand that the kinetic energy is max at the y=0 and potential energy is max at y=A, but I don't know...
  14. L

    Calculating the elastic potential energy from a force-extension investigation

    1. The student should use a rubber band, g-clamp, a retort stand, boss and clamp, a mass hanger, 100g masses and a metre rule. The rubber band should be positioned to hang freely from the retort stand, held in place by a g-clamp to the laboratory bench. Measure the length of the rubber band...
  15. Athenian

    I Energy of Conduction and Valence Band in Opposing Directions

    Recently, I have been studying some solid-state physics and I came across this ##E-k## diagram online. Here's an image for reference to what I am referring to...
  16. K

    When does an object have kinetic rotational energy?

    When does an object have rotational energy? Is it only if it rotates around an axis within the object? Does for example a ball attached to a string with a uniform circular movement have rotational kinetic energy?
  17. iochoa2016

    Understanding Electrical Potential Energy of a charge distribution

    I quite understand the fact the EPE (Electrical Potential Energy) of a system of two charges are U = k*qQ/r, Q is fix. however when it comes to three charges i get lost. because my reasoning is : if q1 is fix then the EPE of the system when q2 is brought is U2 = k*q1*q2/r12, when q3 is brought...
  18. Y

    Understanding energy conservation in a solenoid

    So let's assume ideal wire, resistance = 0 Ohms. Also assume there is a magnetic ball 1 meter away and is attracted to the solenoid. If you have a loop of wire and run a small current through it, you get a magnetic field. This field attracts the magnetic ball, over a distance of 1 meter. If...
  19. Frigus

    Why the change in potential energy is the same in all cases?

    Definition of conservative field I use:-it is a field in which potential energy of system is independent of path taken. I understand that it is independent because whenever we take some path than all the perpendicular displacements with respect to force are not counted and if we go further than...
  20. hilbert2

    A Ground state energy of a particle-in-a-box in coordinate scaling

    The energy spectrum of a particle in 1D box is known to be ##E_n = \frac{h^2 n^2}{8mL^2}##, with ##L## the width of the potential well. In 3D, the ground state energy of both cubic and spherical boxes is also proportional to the reciprocal square of the side length or diameter. Does this...
  21. P

    Coefficient of friction/conservation of energy problem

    Not sure if I did something wrong but I am getting two different coefficients of friction for each velocity.
  22. F

    B In a nuclear decay, is all of the released energy kinetic energy?

    I'm trying to make up an example for my students to illustrate that in nuclear decay, mass-energy and momentum are both conserved. I found this problem: https://physics.stackexchange.com/questions/304277/calculate-velocity-of-radon-220-nuclear-after-decay I am trying to modify it so that they...
  23. S

    Trying to Understand Ionization

    It will try to produce a neutral atom but it depends on the amount of energy that electron is shot at the atom, if it is great enough energy for ionization it will produce cation, If it is not enough for ionization it will make a neutral atom or anion, I think.
  24. Luke2642

    Force & energy in cutting and stretching magnetic field lines?

    I have an ordinary switchable magnet for holding tools to a lathe. It's like a magnetic force gearbox, but I can't quite understand the force multiplication. When placed on a steel surface the switch force is approximately ~5N on both finger and thumb at 1.5cm radius acting over a 3cm arc...
  25. B

    I Questions about the energy released by nuclear fission

    as far as i understand nuclear fission is the splitting of a heavy nucleus cause of a added neutron the energy you get is cause of the lost mass from binding energy. but i also read that the nuclear fission products undergo beta decay which emits beta particles gamma rays and antineutrinos so...
  26. R

    Why is the kinetic energy equation multiplied by ½?

    How did you find PF?: We can derive the equation 1/2mV2. But what is the fundamental reason behind mV2(total energy) is divided by half? I find only derivative solutions to this question. But still unsatisfied.
  27. R

    Why gravitational potential energy is a system property?

    Spring has more potential energy when it is compressed or stretched from its initially balanced state. As external work is done, it stores energy in the form of potential energy. Here, we know energy is stored in spring but For the Earth-ball system, where the energy stored?
  28. A

    Temperature is a measure of the average kinetic energy of a gas?

    Why do they say temperature is a measure of average kinetic energy how is this possible. how can we take it is it applicable for all types of gases?
  29. mcastillo356

    What are the implications of considering infinite first derivatives in circuits?

    Hi I've attached the image of the circuit. My first thought before connecting it: if I connect it, I must consider the fact that current cannot circulate suddenly, because a sudden change in the amount of current should imply an infinite value in \dfrac{\Delta{I}}{\Delta{t}}, and a infinite...
  30. W

    I Conservation of energy in cosmology

    Models like Vilenkin's tunnelling from nothing model described here: https://www.sciencedirect.com/science/article/abs/pii/0370269382908668 claim the universe came from "nothing". It is claimed this doesn't violate any conservation laws because the negative energy of gravity and the positive...
  31. E

    A general potential energy for a multi-particle system

    For a system of two or more particles, it is customary to define potential energy functions ##V_{ij}## between pairs of particles, so that the total conservative force (not necessarily total) on any given particle is $$\mathbf{F}_i = \sum_{j\neq i} -\nabla_i V_{ij}$$as a sum over all other...
  32. apathetic_operator

    What does a force of zero indicate about the total force acting on a particle?

    "The force is zero" means the total force act on the particle is zero? Then there must be an external force. If not, then Fx=0 ? since Fx = d(U(x))/dx, the answer of (b) should be (i), but how about (a)?
  33. Hajarmq

    I Total energy of an isolated system

    If a closed system has kinetic and potential energy such as the total energy (the sum of the two) equals zero for all times, what does that mean? In other words, what does it physically mean that the total energy is always zero for a closed system? I think I have a small misunderstanding of the...
  34. phun_physics

    I Derivation of Average Square Energy Fluctuation in a Canonical System

    The canonical ( Boltzmann) distribution law for a canonical system is described the probability of state ##v## by ##P_v = Q^{-1} e^{-\beta E_v} ## where ##Q^{-1}## is the normalization constant of ##\sum_v P_v = 1## and therefore ##Q = \sum_{v}e^{-\beta E_v}##. Chandler then derives ##...
  35. HeisenbergC

    Energy of n-Photons: Find Equation

    Hi, can anyone help me with this problem? Find equation for energy of n-photons Thank you.
  36. F

    Total Energy, Period and frequency of a masspoint

  37. Franklie001

    The average rate of energy loss resulting from internal friction

    Summary:: Having the following dates:
  38. I

    A Zero-point Energy Contribution Below the Planck energy cutoff

    Is it possible to determine the amount of zero-point energy contributed by all fields below the Planck energy cutoff?
  39. A

    Energy gaps for quasi-free electrons in a 2D lattice

    Hi! Situation: quasi-free electron in a 2D lattice, considering atomic potential V(r) = exp{-|r|/b} (r is the distance from the atom) I'm trying to compute the first five energy gaps at point (10), firstly I don't understand the meaning of calculated 5 energy gaps at one point and usually we...
  40. G

    A rocket and its gas exhaust velocity

    This isn't right, is it? -\dfrac{GM}{R}+\dfrac12 v^2=-\dfrac{GM}{R+h} v=\sqrt{\dfrac{GM}{R}}\left( 1-\sqrt{\dfrac{R}{R+h}}\right) He's doing energy conservation. The mechanical energy at the Earth's surface is equal to the energy when the speed is 0.
  41. J

    Calculation of the forces and the work of these forces

    Like AT asked I post a new thread because it is a different example, here there is no friction and I use a needle and an elastic to have the forces. The circle moves in horizontal translation, the circle doesn't rotate around itself, the red wall rotates around A0, and A0 is fixed to the...
  42. Tanmoy

    Ground State Energy: Explanation & Calculation

  43. Marilyn67

    A R. Mallett's Device: Exploring Weak Energy Possibilities

    Hello, I am sorry to raise this old subject concerning the device of R. Mallett, in particular, one is right to quote has violation of the condition on the weak energy which is not respected. Okay, let's give up this violation of the weak energy condition for now. In my opinion, the big...
  44. Haorong Wu

    I Can atoms absorb photons with energy higher than transition energy?

    Suppose a atom has two energy levels ##\hbar \omega _ a##, and ##\hbar \omega _b##, and let ##\hbar \omega _0 =\hbar \omega _b -\hbar \omega _a ##. Also, there is a phton with enerigy of ##\hbar \omega##. If ##\hbar \omega =\hbar \omega _0##, then a atom in level ##\hbar \omega _ a## can absorb...
  45. F

    Bohr Model - Absorbing a Photon with Enough Energy to Ionize the Atom

    I just want to confirm something. You need about 13.6 eV of energy to ionize a hydrogen atom in the ground state. Can the atom absorb a photon with 15 eV of energy? I think it can. This would free the electron, and the freed electron would move off with a kinetic energy of 15 minus 13.6 eV...
  46. U

    Efficiency and Energy to move a box on a conveyor belt

  47. U

    Calculating Kinetic Energy Using Uniform Velocity and Energy Equations

    For: a) Avdol did the work because he is the force that is causing the displacement, right? b) Is there another formula we would have to use? I am confused at how this would work out and what the answer would be.
  48. A

    Speed and rest energy of a particle collision

    I honestly have no clue where to start, any help would be great.
  49. P

    Case when the potential energy of the 1st excited state is zero

    I know how to solve this problem when the energy at ground state is zero but I don't know how to deal with 1st excited state energy as zero. According to me since the potential energy is zero therefore the kinetic energy must be 13.6eV according to conservation of energy. I also know that the...
Back
Top