# 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. ### I 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...
2. ### Work done by gravity on a car rolling down a hill

I tried E =Fxcos0 but only ended up with 243kJ
3. ### 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...
4. ### Energy Production through Stirling Engine?

A Beta-Type Regular Low Temperature Striling Engine being used to produce mechanical energy, where hot water at 350K is being used as fuel. What power should one expect theoretically?

^^
6. ### Energy absorbed by spring

I approach this by considering the four springs in parallel each with spring constant ##k## as one spring with four times the spring constant ##k' = 4k##. The car is dropped and at the moment its tyres touch the ground I assume that the spring is in its resting position. As the car continues to...
7. ### Can energy be turned into matter in the real world, and if yes, how does that happen?

Theoretically possible, only if the process is reversible. But is it really. My knowledge base is insufficient to answer that. I need help.

36. ### Inflation era energy density

I need the theoretical energy density of inflation for my story. I seem to recall it as an enormous 1095 ergs per cubic centimeter.
37. ### 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.
38. ### 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...
39. ### 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
40. ### Energy with varying force

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...
41. ### 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...
42. ### Change in internal energy

Since the system is doing no work, would it be just 4513 J? I don't think there is any other information to use
43. ### 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.
44. ### 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...
45. ### 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.
46. ### 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...
47. ### 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...
48. ### 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.
49. ### 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 )?
50. ### 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 ×...