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 [tag:energy-conservation] if it is specifically about its conservation.

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What came before quantum theory?

My professor explained today that the idea behind quantum theory is that particles have quantized energy states. To an undergrad like me, this is kind of strange. Wouldn't it always make sense that a ...
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Heat capacity at constant volume and internal energy

I read that heat supplied at constant volume to a system containing ideal gas changes the internal energy of system, i.e $q=nC_v\Delta T=\delta U$ But what happens if I heat a system and change its ...
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Rydberg and electrical energy

I am a high school student. This year I learned the Rydberg constant and its use of finding energy of an electron in chemistry, besides in physics I learned to calculate the energies of two loaded ...
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63 views

What is more important, energy or entropy? [closed]

What decides if something will change or remain the same? Energy or entropy considerations?
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Particle in infinite square well with $\langle E \rangle = (1/4) E_1 + (3/4) E_4$

Suppose we have a particle in an infinite square well with $\langle E \rangle = (1/4) E_1 + (3/4) E_4$. I know that I can calculate the uncertainty in the particle's position by $\sqrt{\langle E^2 ...
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What is the sign of the energy exerted by a man sitting in the back of a boat pulling a canoe?

A man sits on the back of a boat pulling a canoe while the boat travels at a constant velocity a certain distance in the positive X direction. The man feels a constant tension in the rope during this ...
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39 views

Direct comparison of energy measurements for kids

I am planning a few physics & science sessions in the woods with 14 year-olds. The theme for one of the sessions is Energy, and my aim is to show them how gravity, solar radiation, wind (air ...
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Meaning of $\frac{1}{\beta} \frac{\partial \ln{Z}}{\partial X}$ in arbitrary ensembles with X being a sharp Observable of the System

I often find the statement, that for an arbitrary ensemble with Observables $O_i$ of which only the mean value is known, and $\tilde{O}_j$ that are known exactly, the partition function ...
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1answer
41 views

Kinetic Energy: Difference in energy for observers [duplicate]

Consider a stationary 10kg bike; if I apply a force of 10J then $KE=\frac{1}{2}mv^2$ so $v=\sqrt\frac{2KE}{m}=1m/s$. If we consider the same example again, but from the point of view of a stationary ...
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280 views

How much more “work” (or energy) is required for short people to hike uphill?

Let's assume Short Person is 5 feet tall and the Normal Person is 5 feet and 8 inches tall. Let's assume the uphill hike has steps with 1 foot height. How much more work or energy does this require ...
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1answer
56 views

What is the deBroglie Wavelength in a region with potential energy?

This question was on a physics GRE: A free particle with initial kinetic energy $T$ and deBroglie wavelength $\lambda$ enters a region in which it has potential energy $V$. what is the particle's new ...
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1answer
81 views

Calculating the energy of a particle using the Time Independent Schrodinger Equation [closed]

If we have a wave function $\Psi(x,t=0)$ which is a solution to the TISE for a zero potential in an infinite square well, would calculating the energy at $t = 0$ at a position be as easy as ...
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29 views

How are probe liquids selected for Surface Energy measurements?

Why are water, ethylene glycol and diiodomethane generally used for surface energy measurement?
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1answer
64 views

How much energy is released by an evaporating black hole in the last second of its life?

From what I understand, due to Hawking radiation, black holes lose mass in the form of energy (electromagnetic radiation), with these characteristics: The larger the black hole, the less energy it ...
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74 views

Why has a molecule less energy than the uncombined atoms?

My book says, A molecule as compared to the atoms from which it is formed is more stable because it possesses energy lower than the energy of the uncombined atoms. This difference in energy is due ...
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34 views

Could we pump up (or deplete) the Earth's magnetic field?

This text calculates the energy in the Earth's field as about 10^17 Joules, and adds: A typical power station is a giga watt, and the number of seconds in a year is about 3 × 10^7 so a power ...
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1answer
49 views

What is the relation between power and photocurrent? [closed]

Question: A clean lithium surface is illuminated by 1.0 mW of light of wavelength 400 nm. The ejected photoelectrons, which have a maximum kinetic energy of 0.70eV, produce a photocurrent of 30 µA. ...
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A star starts at rest at infinity, how do you calculate the kinetic energy of the star when it crosses the event horizon of a black hole? [duplicate]

A star that begins very, very far away from a black hole (infinity) is brought in by the black hole's gravitational pull. How do you calculate the kinetic energy of the star as it passes the point ...
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207 views

What energy transformation takes place while riding a bicycle?

I have two answers in mind for this question:- 1. Muscular Energy into Kinetic Energy 2. Muscular Energy into Mechanical Energy Which answer will be more appropriate for this question?
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3answers
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Why is it said that standing waves do not transfer energy?

The author of my physics textbook writes that standing waves, unlike travelling waves, do not transfer energy. He says that this is because a standing wave is composed of two travelling waves carrying ...
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1answer
33 views

Internal energy. Mechanics and Thermodynamics

Internal energy is defined in thermodynamics as a function of state, in such a way that, in an adiabatic process, the variation of internal energy equals to the work done, regardless of the way it has ...
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1answer
63 views

Help with a bungee jumping question [closed]

Hi I have this question: Imagine that you have been given the job of desiging a new bungee jumping platform. Customers of mass 57.4 kg will step off a platform, attached to a rope of unstretched ...
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Can we in the far future have an energy source made from vacuum energy, antimatter, dark energy, Higgs boson? [closed]

Can we in the far future have an energy source made from vacuum energy, antimatter, dark energy, Higgs boson and all kind of these like stuffs?
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51 views

Deriving equipartition the (Sin Itiro Tomonaga ) way

In his book on quantum mechanics Tomonaga derives the equipartition law or energy using this integral. I am having several doubts on solving this integral! Is this solvable via this method?
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A cruise ship bumping against a lock gate [closed]

For my project, I need to create a lock gate which will be used for the Panama Canal. One of the many thing my teacher said, was that for the design of the lock gate, the gate should not fail when it ...
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Power Generation Via 2 connected objects below distinct orbits

As an example: Large weight A sits just below Earth's orbit Large weight B sits just below Moon's orbit A, B are connected by sufficiently strong material (may not exist, and if so how much ...
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Is there a maximum energy for a relativistic particle?

So I was told today that the Standard Model breaks down at really, really high energies. The lecturer mentioned particles such as electrons hypothetically having energies equivalent to that of entire ...
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123 views

Why is this specific definition of acceleration used in deriving a general equation for kinetic energy?

$$\sum_{i=0}^n m_i v_i$$ as well as $$\frac{1}{2}\sum_{i=1}^n m_i v^2_i$$ Have been proved to be conserved by Newton & Leibniz, respectively. (http://en.wikipedia.org/wiki/Vis_viva) I don't ...
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What are the limitations of velocity selective coherent population trapping?

I'm somewhat familiar with limitations of Doppler cooling, particularly with the one photon recoil limit, but what I was curious about was what is the next barrier to cooling approaching absolute ...
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32 views

Why changes in the degrees of freedom implies changes in the internal energy?

In Thermodynamics we describe equilibrium states of macroscopic systems. For those equilibrium states we make a description giving one macroscopic coordinate for each measured degree of freedom (like ...
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1answer
78 views

How can the work done by friction be positive? [closed]

I have this energy related problem that i cannot understand in this problem, I am required to find the velocity of the 4.8kg mass when it hits the ground. I easily solved this problem using newton's ...
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48 views

Could someone please answer these questions? [duplicate]

I have tried asking this question a number of different ways but I seem to keep messing it up. Please let me try another way. I know that the number of times a photon oscillates per second (it's ...
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How does electromagnetic radiations penetrate?

I am reading about the discovery of subatomic structure which includes all these things radioactivity, alpha rays, gamma rays, x rays et cetera. I read that electromagnetic radiations are energy ...
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Sound frequency and power

Although I have studied physics I am not clear on this question: Given that two (or more) sound waves have the same amplitude does the power they carry depend on frequency? What about electrical AC ...
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43 views

Why does work in the Work Energy Therom equal the Sum of the integral F*dr?

I am studying the work energy therom and I understand that 1/2(mv^2)=Wnet, however, I saw this picture below online. I understand the summation of the force times the change in distance. However, ...
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Speed of light and infinite energy [duplicate]

While browsing some physics websites, I saw that to make an object reach the speed of light, it requires infinite energy and talked about its relation with Einstein's famous equation $E=mc²$. However, ...
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1answer
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Classical Field Theory: Physical meaning of various terms in total Hamiltonian

In a classical field theory problem Lagrangian density is given as $${\cal L}=\frac{1}{2}\dot{\phi }^{2}-\frac{1}{2}\left ( \bigtriangledown \phi \right )^{2}-\frac{1}{2}m^{2}\phi ^{2}\tag{2.6}$$ ...
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Would this be correct for kinetic energy in special relativity?

In galilean relativity $$p=mv$$ and $$KE=\frac{1}{2}mv^2$$ If I understand it in special relativity the equation for momentum is $$p=\frac{mv}{\sqrt{1-\frac{v^2}{c^2}}}$$ In galilean relativity ...
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Why boundry molecules have more energy?

In a system of a solid or a liquid, why the molecules in interface have more energy compared to the bulk of solid or liquid?
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Source of energy, work of magnetic field

Suppose I have a constant magnetic field, and I switch it off (abruptly or slowly) , a ring of uniform charge would acquire same angular velocity if it was present in a plane perpendicular to B and ...
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Would this be a practical way to convert tidal energy into electrical energy?

Imagine a structure that is held above the water by pylons that are grounded on the ocean floor. In between these pylons is a pontoon that, when tides are rising holds and lifts a heavy weight. When ...
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Prove that gravitational potential energy of a body of mass m at a height h is mgh

Many introductory physics books just write that potential energy of a body of mass $m$ at a height $h$ as $U_g=mgh$. However, they never show how this was derived. I'm interested in knowing this ...
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Can we produce energy by red-shifting light

The answer on my question Does light accelerate as it nears a black hole? leads me to think that light 'gains' the gravitational energy by shifting to a frequency with a higher energy (blue). Is this ...
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1answer
37 views

Projectiles ability to do work to a box when connected by string? [closed]

I was wondering, the work-energy theorem states that KE can do work, as it is Mechanical energy. if the KE energy and thus Mechanical energy of a ball, if external...can do work on an object, ...
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1answer
58 views

Why does work done ultimately culminate as wasted heat?

Come to think of it, the work done on a body is converted into some form of energy.But why is it that it ultimately tends to produce heat? In physics we all talk about energy dissipation in the form ...
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Work energy theorem

The work energy theorem states that the work done along a path from a point (1) to point (2) is the difference of the kinetic energies between the last and the first points. But if the force was ...
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42 views

which principles govern energy transformation?

Can you point me please to the scientific principles that govern energy transformation? For example why we can't achive 100% harvesting from the various energy sources? I need the relevant theorems, ...
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Does an object lose more heat in the form of convection or radiation?

If heat transfer by convection is greater why is food wrapped in Aluminum foil? Wouldn't that result in the food losing more heat to the foil and ultimately in the air in the form of convection?
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How does the deflection of force on a tire work?

So, considering the example of a car that is turning, is all the force lost that's not along the axis of rotation of a tire, or is there some way to calculate how much of that force gets deflected ...
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Derivation of de Broglie's Equation

I came across the derivation, present all across the web, which utilized Einstein's energy mass equivalence equation and energy of a photon. It goes like this: $$ E = mc^2,\;\;E = h f \;\;[f = ...