Energy Definition and 999 Threads

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

    I Infinite energy from an electric field?

    Imagine an electric field between two charged plates that is so intense that its energy density is enough to produce real electron-positron pairs. These electron-positron pairs annihilate to produce photons that radiate away. Does the electric field between the charged plates regenerate so...
  2. J

    MCNP Modelling Detector Energy Response Vs Empirical Results

    Hello All, I'm trying to simulate in MCNP the energy response of a PIN diode. To do this, I have modelled a "slab" of silicon in an epoxy case at 2cm away from the source and with the F8 tally set to 25keV bin increments to 1MeV, I do as follows: Set the source energy to 33keV Run the...
  3. L

    Total Energy vs Average Energy (Thermo)

    When there is a probability involved with an energy state, e.g. with partition function, why is total energy the same as average energy (if it is). Just want to make sure - is this just a definition? Thanks
  4. ATT55

    Energy Required to Evaporate Water

    Energy required to evaporate water. Given 3 evaporating scenarios: 1. Glass filled with 50cc of water at 20C; the water is heated to 60C 2. Glass filled with 50cc of water at 20C; the water is heated to 100C 3. 50cc of water at 20C wiped over a large plate to create 50micron thickness layer...
  5. N

    What's the energy stored in a parallel plate capacitor?

    I've mentioned the problem that i'm facing in the question that I've posted. Kindly go through it.
  6. Z

    Chemistry How to understand the energy involved in mass transfer into an open system?

    The following is what is written in the book I am reading. The energy required to "push" the mass into the system is $$F\delta z=PA\delta z=PV\tag{1}$$ in which ##V## is the molar volume of the closed system, ##F## is the acting force, ##A## is the cross-sectional area, and ##\delta z## is...
  7. U

    A Equating Gas Pressure and Projectile Stagnation Pressure

    Imagine a projectile with density p, initial velocity v0 and initial stagnation pressure = 0.5p{v0}^2 being fired into the gas chamber with a final gas pressure given by Pg = nkT, for n being the final number density before (at max pressure of gas) and T the corresponding temperature of the gas...
  8. Feynstein100

    B Would isolating the Solar System prevent its heat death?

    I have a thought experiment in mind. Crudely speaking, the second law of thermodynamics implies that there is only a finite amount of change possible in the universe. Once this limit is reached, no more change can occur. The key thing here though is reversibility. If changes were reversible...
  9. Juanda

    Does energy currency make sense?

    Let me start by saying the idea isn't mine at all. It's actually somewhat usual within Science Fiction works. Maybe one of the most popular appearances of the concept is the videogame Stellaris where there is commerce with aliens so a universal currency is very convenient. In economics, money...
  10. I_Try_Math

    Distance rolled of sphere vs cylinder with same m, r, and v

    The answer from the textbook is: Use energy conservation ## \frac{1}{2}mv^2 + \frac{1}{2}I_{cyl}w^2 = mgh_{cyl} ## ## \frac{1}{2}mv^2 + \frac{1}{2}I_{sph}w^2 = mgh_{sph} ## Subtracting the two equations, eliminating the initial translational energy, we have: ## h_{cyl} = \frac{v^2}{g}...
  11. I_Try_Math

    Determine energy required to stop rolling mass

    Is it possible to solve this without knowing the radius of the cylinder? My initial thoughts were that the energy required to stop it would be the sum of its rotational and translation kinetic energy, but I'm not sure it can be calculated without knowing the radius.
  12. J

    I Tong Dynamics: cannot cancel angle from orbit energy expression calculation

    Hi, I love the lectures by David Tong. Usually I can follow his calculations (but I am not yet so far into the lectures...). But one that I just cannot do is the derivation of the energy in (4.16), the expression being ##E = \frac {mk^2} {2 l^2} (e^2 - 1)##, where l is the constant angular...
  13. M

    Conservation of String Exercise

    I'm not quite sure how to apply conservation of string to this problem, so guidance would be appreciated. Normally as long as there isn't a "sub-pulley" I can do the problem fairly easily but this one tricks me up. Thanks
  14. hongseok

    B A quick question about Planck's constant

    What does it mean to say that Planck's constant is an energy oscillator composed of imaginary and real numbers? This is an expression I saw in a book.
  15. murshiddreamengineer

    I How can we prove the kinetic energy equation

    Proof of kinetic energy work done equals a change in kinetic energy in a mechanical system. δW = F.ds W = ∫F.ds W = m∫a.ds W = m∫(dv/dt).ds W = m∫v.dv here if v and dv are in the same direction the change in kinetic energy will be the usual equation. what happens if both are in different...
  16. K

    Body attached to a block by a spring, shaped like an inclined plane

    I was taught to solve this problem by first finding the velocity of the body (of mass ##m ##) relative to the block of mass ##M ##. One way of doing this is as follows: first write $$ {v _{m _{B }}}^{2 }={v _{mx _{B }}}^{2 }+{v _{my }}^{2 } (I)...
  17. J

    A cylinder rotating on a plane with friction and then moving across a frictionless plane to a collision

    I think the angular velocity keep increasing on the plane with friction and the translational velocity keep decreasing due to friction while the total kinetic energy is conserved. When it moves to the frictionless plane, all energy converts to translational kinetic energy and it stop rolling...
  18. J

    Problem with KE and work equation (perfect rocket in space example)

    Can someone help me by taking a look at the attachment and figuring out where I am making a bad assumption? It's driving me nuts.
  19. S

    Is Work Simply a Form of Energy?

    I've found some sound and not so sound answers so far. The most interesting is one saying that work is simply one form of energy. The answer used math to support it, but I haven't checked the math yet. Adult ADD, I believe, prompted me to move on instead. Maybe I just have no patience. Idk…
  20. R

    Energy conservation equation to find equation for final velocity

    I initially thought about the different forms of energy present at each of the points: Total energy at starting point: PEA+ KEA= mgH at point D: KE_D = 1/2mv2f PED= mgD Energy at point D: PED+ KED D = mgD + 1/2 mv2f because EA= ED mgH = mgD = 1/2 mv2f mg(H-D) = 1/2 mv2f g(H-D) = 1/2...
  21. U

    I Energy transfer during gas discharge

    It is known that while the electric current flows along the wire the energy propagates through the field near that wire (and not by means of electron gas kinetic energy) and the electrons' velocity is equal approximately to 10^[-3] m/s. Meanwhile, the electrons' velocity during the gas...
  22. Frabjous

    Grow to High Energy, Nuclear and Mayonnaise, Particle Physics subforum

    PF should keep up with the times https://www.livescience.com/planet-earth/nuclear-energy/physicists-solve-nuclear-fusion-mystery-with-mayonnaise :wink:
  23. S

    B Some Questions about Electric Current/Capacitors to help my understanding

    Q 1) In electric currents, in a battery, the positive charge starts at the negative terminal and gains energy through emf which forces the charge to go to the positive terminal of the battery, with plenty of energy. This voltage is the energy difference between the terminals. For the charges to...
  24. adf89812

    What is maximum Pauling electronegativity or energy difference between orbitals?

    >-Atomic orbitals must be at the similar energy levels to combine as molecular orbitals, said Wikipedia. This is unclear. How do you quantify how "similar" means? I heard electronegative is tied to atomic radius is tied to atomic orbital energy. What are two atoms that would in theory form...
  25. Heisenberg7

    B E. Potential Energy: Uniformly Charged Hollow Sphere and Point Charge

    I was doing a problem with this one detail. It says that the electric potential energy of an uniformly charged hollow sphere and a point charge is (at the surface of the hollow sphere; both positive): $$U = k \frac{q_1 q_2}{r}$$ I guess this assumes that the hollow sphere is a point charge. Now...
  26. Heisenberg7

    A charge q approaching two stationary charges q1 and q2

    I'm only confused about one part of this problem and that is setting up the conservation of energy equation. In the solution, they just wrote this: $$\frac{mv_o^2}{2} = - k \frac{q_1 q}{r} + k \frac{q_2 q}{l + r}$$ where ##r## represents the distance at which the force created by the negative...
  27. Nabir14

    B Is there any physics topic explaining how objects break into parts?

    I am curious to know if there is any existing topic explaining how objects break into several pieces when force is applied.
  28. A

    Is there a limit on how much energy a photon might have in a FOR?

    So there was this question: The first option seems to be the only correct answer. $$\lambda_e=\dfrac{h}{\sqrt{2m(KE)}}$$. The answer would be correct if ##KE \approx eV## The option mentions that ##eV>>\phi## so ##\phi## can be ignored. But I don't think that necessarily means that the...
  29. P

    Circular motion of a Weightless rod

    I used law of conservation of energy to calculate (d theta/ dt)^2 (from:mgasin theta=1/2m(d theta/dt.a)^2+1/2mu^2(u is the velocity of the C ring at time=t)), but wasnt able to find u(velocity of C).Is there any relationship between the tangential velocity of B(d theta/dt.a) and velocity of C(u)...
  30. Sam Jelly

    Conservation of energy of a bow and arrow

    I thought the initial elastic potential energy would be converted to the arrow's kinetic energy but it also appears that the bow has some velocity.
  31. T

    Block on top of moving slab, with a rough surface - When does v_b=v_s?

    For the first question I thought of using an energy balance, there is friction ##\Rightarrow \Delta E_m = -W_f##. Both at the start and at the end, the block has no velocity. Therefore ##E_{\text{initial}}= \frac 1 2 m_s v_{s,i}^2## and ##E_{\text{final}}= \frac 1 2 m_s v_{s,f}^2##. This means...
  32. rocketwaveuk

    I Matter Anti-Matter Annihilation

    The pure energy coming from a collision and how is it measured or is it just "A Formula" and is any of that energy Dark Energy or even Dark Matter, the reason for the question is because the Dark Energy/Matter vs normal Energy/Matter seems to align with what is seen now after the Big Bang...
  33. M

    Energy analysis of the system (leaking bucket from Morin's "Mechanics"

    Hi, Can you please help me understand how the formula of energy decreasing during a sand leaking is obtained? One of possible solution to this problem, suggested in the textbook, states that when the bucket moves from x to x+dx (d is negative), there are two components responsible of energy...
  34. T

    Energy analysis of this system (curved ramp up and a spring)

    For point one it's clear that I have to use energy => ##ΔE_{AB} = W_{friction}## ; ##\frac 1 2 mv_0^2 - \frac 1 2 mv_1^2 = mgμ_d d## After that there is the path BC, but I don't know how to analyze it from an energetic standpoint. Then after BC the block will now have a different velocity, I...
  35. Bling Fizikst

    Speed of charged balls after collision

    I tried to apply energy conservation . $$\frac{-kQq}{l}=\frac{m}{2}(v_1^2+v_2^2)-\frac{kqQ}{2r}$$ Now conserving momentum : $$0=mv_1-mv_2$$ Solving for ##v_1=v_2=v'## we get : $$v'=\sqrt{\frac{kQq}{m}\left(\frac{l-2r}{2r}\right)}$$ Since the balls are elastic , so they should collide...
  36. seonjunyoo

    I I have a question about gravity -- If the value of the energy momentum tensor (Tμν) becomes zero, can it become gravitational-free?

    R μν − 1/2g μν R= 8πG/c^4T μν In this formula, if the value of the energy momentum tensor(Tμν) becomes zero, can it become gravitational-free?
  37. doke

    Rpm and power for this electric motor, gearbox and pulley system

    i got confused because if the connections btw the parts
  38. BubbaJoeLouis

    Where to Discuss a Unified Theory of Energy on Physics Forums?

    If someone believes they have developed a somewhat unified theory that brings together various physics concepts into a Unified Theory of Energy, but they are concerned about being dismissed or criticized by members who strongly adhere to conventional academic views, where in the Physics Forum...
  39. S

    B Can energy be stored in a single particle indefinitely?

    Can energy be stored in a single particle without it being lost over time? I mean, photons would be an exampld in principle, but they get redshifted as the universe expands and become less energetic as time goes by We could store that energy in form of kinetic energy for individual...
  40. adf89812

    Why isn't bond dissociation energy/bond enthaply measured in Newtons?

    I understand every bond chemically has a length and energy to break, and energy is Newton*meters. Is the Bond enthaply/Bond disassociation energy equivalent to the force needed to break the bond * the bond length? Why don't we say, to break the bond from O to H we need to put magnets on left of...
  41. Bling Fizikst

    Oscillation of system of three charges

    I tried to take angles and proceed by energy conservation But this doesn't seem to lead me anywhere . Here , the length of threads is ##l## each and ##2\theta## is the central angle. ##y_1## is the displacement of the charges attached at the extreme ends of the threads respectively while ##y##...
  42. K

    B Does energy exist?

    TL;DR Summary: does the quantity we refer to as energy actually exist energy is a property of a physical system but does it actually exist, like does an object actually lose or gain a quantity that we refer to as energy after an interaction, or is it just a mathematical concept, are those...
  43. L

    Internal energy of a gas and kinetic energy, "typical velocity"

    Source: Shankar Yale OCW physics I have three questions here: 1. K_avg is 3/2kT, sure. But isn't this the kinetic energy of one particle only? So why isn't the answer multiplied by avogadro's number (because one mole). 2. When doing the "typical velocity" derivation, I noticed that they used...
  44. T

    IPHO classical mechanics: A mass falls on an exoplanet

    Attempt: I assume that the position of the mass ##M## after it is realised its position is given by the position vectors from the origin, ##\vec m = -m(t)~\hat m## if ##m(t) > 0## or equivalently ##\vec m = m(t)~\hat m## if ##m(t) < 0## Either one we can use for energy conservation (I am...
  45. Math Amateur

    I A. P. French "Matter and Radiation: The Inertia of Energy"

    I am reading A. P. French's book: "Special Relativity". Currently I am focused on the section: "Matter and Radiation: The Inertia of Energy." Under the heading: "Matter and Radiation: The Inertia of Energy", French writes the following: In the above text by Young...
  46. L

    I It's still not clear to me what's the limit of light propagation

    Hi, I've been asking questions about light here for years, and I still don't understand the limit of propagation of light, does anyone have advanced on this field? I really would like someone to explain me how it's possible for light to propagate forever, since it's probably emitted in perfect...
  47. D

    I Can the expansion of the Universe reduce the energy of a photon to 0?

    Is it theoretically possible for the expansion of the universe to red-shift the energy of a photon all the way to zero? If so, what happens to the photon? Or does the photon's energy only approach zero as an asymptote?
  48. kingamada

    Gravity Based Electricity Generator

    Hello community, I am currently designing a gravity-based electricity generator and exploring the feasibility of different setups using a 500 kg mass. The core of my project is to harness the potential energy from this mass as it is lowered from a significant height. I am considering several...
  49. T

    B The relationship between mc^2 and mc^2 x 1/2

    Kinetic energy = 1/2 m V^2 I was thinking about this and thought another formula… E = mc^2 These look very similar except for the multiplication by 1/2. Let’s say you take a kilogram ball of uranium and accelerate it to the speed of light. I know, I know. You can’t. But let’s say you did...
  50. Martin Jediny

    B Too much energy -- thought experiment

    1/ Have a closed hydraulic circuit. Two columns of fluid. One hot, one cold, which are connected at the bottom and top. Let's have small heat loss and small hydrodynamic resistance to fluid flow and incompressible fluid. I heat the lower interconnection, I cool the upper interconnection. I have...
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