Questions tagged [energy]

Energy is the conserved quantity associated to time-translation invariance and represents the work a system is capable of doing. Use this tag for questions about energy, and consider adding the [energy-conservation] tag if it is specifically about its conservation.

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Energy neither be created nor be destroyed. Then where energy comes from?

We are listing that energy neither be created nor be destroyed then where it comes form?
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Energy in hamiltonian formalism from phase space evolution

The hamiltonian for a free falling body is $$H = \dfrac{p^2}{2m} + mgy$$ and since we are using cartesian coordinates that do not depend on time and the potential only depends on the position, we know ...
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Why is the kinetic energy for non-relativistic velocities not described by $KE=mc^2$?

Today, I learned that the kinetic energy of an object can be described by $$KE=\gamma mc^2=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}mc^2.$$ I have been taught in school that the kinetic energy of an object ...
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Lagrangian of a free particle in Special Relativity and equivalence between mass and energy

I am a bit confused on the way Landau derives the Lagrangian of the free particle in SR (L. Landau, E. Lifshitz - The Classical Theory of Fields) and his conclusions about the equivalence between mass ...
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Deriving magnetic field energy per unit volume for a wire

The magnetic energy per unit volume for an inductor can be derived from the formula: $U=\frac{1}{2}L{I}^{2}$ But how to derive formula for energy per unit volume for a (long) wire. Since there is no ...
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Does it always take the same amount of energy to get from A to B?

Imagine you have a point A and a point B. Does it takes the same amount of joules to travel from A to B no matter how you do it (if in neutral conditions e.g. no wind)? For example, walking, riding a ...
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Won't decrease in kinetic energy decrease the momentum as well?

My doubt stems from the question attached. If some kinetic energy gets converted into spring potential energy, shouldn't the velocity of be reduced and subsequently momentum? I also considered the ...
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How an inductor lose energy when pulling iron

I'm confusing with a problem. Consider an ideal inductor with a shorted circuit and no resistance and having a current through it. we know this current is constant while the inductor has no resistance ...
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COM frame for photons

We know that there is no frame in which the energy of a photon is zero and hence it has zero momentum. But can we find a frame in which the energy of two head on photons is zero, or the center of mass ...
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Kinetic energy of a rolling disc when slipping

To derive kinetic energy expression for a rolling body, we consider the instantaneous center of rotation and write the angular energy about that point. After some algebraic simplification, we conclude ...
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Can hypervelocity impacts be an efficient EUV source?

Imagine that two small pieces of rock hit each other at a very high velocity (for example, small meteoroids moving at orbital velocity in opposite directions). How much of the impact energy could be ...
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What actually is a force?

First law of motion clearly says that, anything that creates a state change is a force. So, in our thermodynamic systems thermal energy, hence kinetic energy of constituent particles change when ...
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Thermal Energy and Forces on a particle

Suppose there is a container (filled with vacuum) in which there is a particle (only one).Let the initial temperature be $0K$ so that the particle will have $0$ Thermal energy. Now as we increase the ...
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PMDC Motor Characteristic Graph for Given Parameters

TL;DR Need to make a graph which showcases all parameters of a PMDC motor against the torque input. I am trying create a graph similar to as shown below. However, I am confused as to how the torque is ...
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Can gravity be utilized to generate hydropower in a clean, repeatable fashion without rivers or lakes? [closed]

I came up with this idea today and am just wondering if it is feasible. It is much easier to communicate via images so I drew the attached image to convey it. The key concept is rolling a ball in ...
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Clear up confusion regarding energy dissipation by sliding friction on a rigid body [closed]

Does anyone have a relatively simple approach for evaluating the heating for a solid non-rigid body? For example, in fluid mechanics the mechanical energy equation sometimes includes a friction loss ...
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Does the universe lose energy when spacetime expands?

When reading about the Big Bang you’ll commonly hear that the universe began “cooling down” soon after. However, if we assume the universe is a closed system, would that thermal energy not continue to ...
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How much energy is in the Universe as photons? [duplicate]

The mass of Universe in kilograms is about 10 power 53. But how much energy exists in the Universe in form of photons? And if they would all be converted into mass, how much mass that would make? Dark ...
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Exergy of a vacuum in a cylinder with a piston

So my system cylinder with volume V and rigid walls, except one of them is movable (piston). Outside I have pressure P0 and temperature T0. What is the exergy of my system? So I've been told that ΔB = ...
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Poynting vector on the surface of an isolated system

We've an isolated system inside which currents and charges reside. If the Poynting vector on the surface of the system is non zero, then there is energy exchange between the system and the ...
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Work and energy: law of conservation of energy? [duplicate]

According to the law of conservation of energy: Energy can neither be created nor be destroyed Then how did energy come into existence?
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Can human created heating increase the temperature of the planet surface/atmosphere?

If we ignore the greenhouse effect and imagine all the worlds energy is produced by nuclear or other power plants would the energy needed by humanity to heat be enough to measurably increase the ...
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Does an electron change its energy when the spin projection is changed?

When an electron changes its spin projection in a magnetic field does it receive or take energy? When the initial spin projection is indefinite (as is according to QM) how much is that energy?
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Estimate Calories Burned from Arbitrary Limb and Body Movements [duplicate]

I'm looking to create a model using a reasonable physics-based approach to estimate calories burned for arbitrary movement. To keep the scope fairly limited and tractable, I am looking at the ...
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Estimate Calories Burned from Arbitrary Limb Movement

I'm looking to create a model using a reasonable physics-based approach to estimate calories burned for arbitrary movement. To keep the scope fairly limited and tractable, I am looking at the ...
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1answer
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How do I derive this internal energy equation?

So my question is probably quite basic, but I have found this paper online. http://www.astro.lu.se/~david/teaching/SPH/notes/annurev.aa.30.090192.pdf And it defines the rate of change of thermal ...
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Why do inelastic collisions occur in theoretical calculations?

When solving collision problems related to the conservation of momentum in my applied maths course, the question of whether the collision was elastic or not is often asked. A lot of them time (such as ...
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Where does the extra energy come during superelastic collison

It is known that in superelastic collison the energy of particles after collison is more than the original particles but how is it possible . I barely could find suitable information on net for a high ...
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Producing Pressurized Hydrogen Underwater Then Using it To Do Work

This is a bit of a weird one but I didn't know where else to ask this. I was recently talking to my brother and he told me about an idea he had talked with his boss about how to create compressed ...
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Energy-conservation for exothermic reactions

How do exothermic reactions not violate energy conservation if e.g reactants were given 2200kJ/mol of energy and product released 2300kJ/mol of energy, where has this +100kJ/mol come from?(2200kJ/mol ...
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Can particles gain energy during inelastic collision?

We know that when two particles collides inelastically, the net energy equation is given by $$E_i=E_f+Q$$ where $E_i$ and $E_f$ are initial and final mechanical energies of the particles and $Q$ is ...
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Total Energy Equaling Potential Energy of Finite Well

I know that bounded states in quantum mechanics mean that the total energy of the state E is less than the potential $V_0$ ($E < V_0$ : finite potential well). and scattering is a result of $E>...
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Is direct, complete conversion of matter to energy theoretically possible?

I know that small amounts of matter can be converted to energy via chemical and nuclear reactions, and that complete conversion is possible if matter meets anti-matter. Other types of conversion might ...
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Energy measures and probability of measuring them

We have the following Hamiltonian $\hat{H}=a|u_{1}\rangle\langle u_{2}|+a|u_{2}\rangle\langle u_{1}|$ with a $\in \mathbb{R}$ and $|u_{1}\rangle,|u_{2}\rangle$ an orthonormal system The matrix ...
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How is the fabric of space-time curved? It bends due to energy or mass, but what causes bending? [duplicate]

When we experiment with General Relativity on Earth, a tissue bends according to the experiment due to the placement of a mass, but of course there is a gravitational pull that causes bending. If we ...
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In Franck-Hertz experiment, why mercury emits UV light while neon emits visible light despite neon having greater excitation energy?

The reference that I checked shows that Neon has an excitation energy of 18.2 eV, while mercury has 4.9 eV. However, the reference also shows that the wavelength emitted by the mercury is at the ...
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Does mechanical energy remain constant during an inelastic collision

Suppose two bodies collide inelastically, does the total loss in kinetic energy get converted into potential energy associated with the deformation? Or does the energy also transform into other forms ...
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Real world comparisons to the energy lost when a ball is dropped from a certain height? [closed]

Are there any real-world applications that can be related to the efficiency of a ball when dropped from a fixed height onto a flat surface (with no downwards force applied) My teacher was thinking of ...
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How much voltage do you need to force charge into a material?

If I'm using electricity not to induce steady-state current, but instead to force as much charge as possible into a given material, given I know the relevant properties of the material, how do I ...
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What is the relation between the energy and the momentum?

(Please excuse my English) One day, I realized that I had not clearly learned about the relation between the energy and the momentum, though I am an educated engineer. I just thought the energy is a ...
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Uncertainty principle in relativistic quantum mechanics

In Walter Greiner book about relativistic quantum mechanics, he writes the uncertainty relations for 4-position and 4-momentum in a neat way as: $$[p^\mu, x^\nu] = i\hbar \eta^{\mu\nu}{\bf 1}$$ with ...
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Why are drag and kinetic energy proportional to velocity squared? [duplicate]

For example, it takes a car four times the kinetic energy to travel the same distance at twice the speed, which quadruples the drag experienced by the car. This observation is described by the ...
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Question about Dirac notation (using the Braket notation) [duplicate]

How do I add two energies and calculate the expectation value given a set state in two $|nlm\rangle$ notation? $|\psi\rangle=\sqrt\frac{1}{2}|n_1l_1m_1\rangle+\sqrt\frac{1}{2}|n_2l_2m_2\rangle$ I know ...
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1answer
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How does physics deal with perpetual motion in orbits?

This is a bit a bit of a weird question. Simply put, if a body orbits something such as a black hole there is no internal issue (that I know of) that would cause it to eventually stop (by internal I ...
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How do you derive the equation that the total energy is equal to the sum of potential plus the kinetic energy?

In this stack-post, an equation relating the external work done and the change in potential and kinetic energy of the system was derived by the user Biophysicist. Given: $$ W_{ext} = \Delta K + \...
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1answer
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Minimum velocity for a pendulum to rotate a quadrant [closed]

Suppose a pendulum is allowed to rotate in a vertical plane(z-x) shown below: What is the minimum velocity that it must start with such that it rotates an angle of $ \frac{\pi}{2}$ degrees? In the ...
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Why is the energy function not always equal to total energy? [duplicate]

Why is the energy function $h = \dot{q_i}\frac{\partial L}{\partial \dot{q_i}} - L $ not always equal to total energy $E = T + V$? Here $T$ is Kinetic Energy and $V$ is Potential Energy. I've read ...
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Conservation of energy in explosion

Suppose a shell rested on a smooth surface fires a bullet horizontally ,generating some energy of explosion say E. I know that initially system(shell+bullet) has no energy. When the internal forces do ...
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Why is there a need for the concept of energy if we have the concept of momentum?

We have two concepts that are energy and momentum. To me, momentum is more fundamental than energy and I think that momentum was the thing which we wanted to discover as energy. Now momentum can ...
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Why is work done not equal to force times time?

I am having an issue with the definition of work. I know there are several questions already posted which seem the same as this one but the issue is that all of them either haven't been answered or ...

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