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 stored in the electric field of an electron

Energy stored in the electric field or self energy of a solid sphere of radius $r$ and uniform charge distribution $q$ would be $$\frac{3}{5}\frac{1}{4\pi\epsilon_0}\frac{q^2}{r}$$ This result could ...
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Relativistic energy and Momentum understanding

After discussing with some people, it appears to me that the way to define quantities like "energy" and "momentum" in general, is to just treat them as "conserved quantities&...
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Significance of resemblance of Electrical energy with Potential energy of spring

In a spring-mass system the total mechanical energy is represented as: $TE= 1/2kx^2 + 1/2mv^2$ $k$ being the spring constant and $x$ being the displacement. In L.C Oscillations we find the total ...
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Power Demand for Motion of Vehicle on a Road and Plain in Air with Air Resistance

Let's say we have an ideal motorcycle without frictional losses (no tire slip, ideal machine without losses). But we want to consider wind force which is proportional to the square of speed $F = k \...
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The energy of the photon $hf$, so what is the energy of other masses such as the Earth?

The energy of the photon $hf$, so what is the energy of other masses, and of course I do not mean to write in frequency $f$, what is the relationship of the energy of other masses
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space, flow of energy and prime numbers [closed]

It has been some time now that i have been trying to find a physical shape of numbers (do not ask why, i am not able to explain). I always start with everything is Energy and the First Law of ...
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Deriving $K_{max}=\frac{1}{2}m{v_{max}}^2=eV_0$

In deriving the maximum kinetic energy of photoelectrons in the photoelectric effect, Young & Freedman says that "As an electron moves from the cathode to the anode, the potential decreases ...
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How much of Earth's mass is created by the energy of the core?

I've read that higher energy means higher mass, and in atomic systems, the kinetic energy and potential energy actually contributes more mass than the actual particles themselves (or so I've read). So,...
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Minimum energy eigenvalue [duplicate]

Why is the energy eigenvalue is always greater than minimum potential for a particle moving in a certain potential?
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What electronic configuration leads to the lowest work function In a molecular system? [closed]

The work function of several elements is freely available in several documents online, but those table have only data of the element in its free state and in the surface and phase at STP, which ...
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What is a Joule in Work [closed]

I understand that in terms of work done, 1 Joule is 1 newton exerted through a distance of one meter, following that logic, 2 Joules would equal 2 newtons exerted over a distance of two meters, right (...
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Why momenergy has magnitude equal to the mass?

The mom-energy of a particle is a 4-vector: Its magnitude is proportional to its mass, it points in the direction of the particle's spacetime displacement, and it is reckoned using the proper time for ...
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How can $\Delta U=\Delta H$ at constant volume, if gases are involved?

Note: This is not a homework problem; I am not asking how to solve this problem; and not even why my solution is wrong, in a sense. Just ignore the numbers in the "background" part. I ...
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A ball of mass 100g is falling from a building of height 1000m. What will be the kinetic energy of the ball after 10 seconds? [closed]

Mass=100g Height=1000m=1km Initial Velocity=0m/s Change in time= 10sec Find Kinetic Energy of the ball after 10 seconds?
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A cat weights 5kg. 1 Catpower is equal to only 196 watt. The cat climbs a tree vertically. What is the maximum speed the cat can climb? [closed]

The Problem: We are creating something called Catpower similar to Horsepower. Our cat is very weak due to malnutrition and only weights 5kg. 1 Catpower is equal to only 196 watt. We want our cat to ...
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Looking for a rigorous derivation of the work - energy theorem

I'm currently trying to derive the work - energy theorem from the definition of the line integral of a vector field, but I'm having some (probabiliy silly) trouble with variables. It is known to me ...
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Recoil energy when collision occurs between 2 hard spheres

My Project I am working with a project where I want to simulate an elastic collision between a hard sphere with initial velocity $U_0$ and another hard sphere that is stationary (in 2D). I have ...
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True or False: energy is conserved in all collisions [closed]

Using introductory physics, how would you answer this question? (I have a disagreement with my instructor and I’m curious to hear your input) One of us says true because the question doesn’t specify “...
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Can massless particles travel slower than the speed of light in vacuum?

We know an object with positive mass cannot be accelerated to the speed of light because this would require an infinite amount of energy. My question is: Is there anything in the universe that can ...
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Why is the kinetic energy of moving objects growing quadratically if the speed increases linear according to $(1/2)*m*v^2$? [duplicate]

My main problem of the formula "1/2 * m * v^2" is, that the kinetic energy does not grow proportional to the speed of the object. I do know its derivation and the formula does work normally ...
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Formula for Energy Of black body radiation

The formula for energy for blackbody radiation on some trusted websites is given by: $$E_{\lambda}=\frac{8\pi hc}{\lambda^5\cdot\left(\exp\left(\dfrac{hc}{\lambda\cdot k\cdot t}\right)-1\right)}\tag1$$...
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What is self-energy? And how it developed in a charged body? how this energy is converted into field energy of the charge body? [closed]

What is self-energy? How this self-energy is developed on the body? I don't get the concept please explain?
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Infinite bouncing in finite time?

Is it possible to calculate the time in which a bouncing ball with restitution coefficient $k$ which first hits the ball with velocity $v$ will be considered stopped? Note that each collision lasts $\...
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1answer
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Can average power be non zero, but instantaneous power be zero

Q. A wind-powered generator converts wind energy into electric energy, Assume that the generator converts a fixed fraction of wind energy intercepted by its blades into electrical energy. For wind ...
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Kinetic energy and work-energy theorem [closed]

I don't understand how this step has done which i have marked so please help me [!]1
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What qualifies as “Energy” in the Einsteinian sense of the word?

As an absolute beginner to special relativity (and all the 1900s Einstein stuff), I find it hard to grasp the real meaning of the term energy used in the popular equations. I’ve heard it is possible ...
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Energy conversion of electrons in cathode rays

From the principle of conservation of energy, for each electron: Electrical potential energy = Kinetic energy $$eV = \frac{mv^2}{2}$$ $$v = \sqrt\frac{2eV}{m}$$ where e is the charge of an electron, V ...
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Does an infinite wire of charge have an infinite potential energy per unit length?

I was doing a physics problem in Purcell's E&M book when I encountered a problem that asked to find the work needed per unit length to assemble an infinite wire charge of radius $a$, by bringing ...
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1answer
123 views

How can an object lose energy but still move at the same speed with constant momentum?

I'm studying collisions, and while doing it, the conservation of momentum seems to strike me in a weird way. If the momentum in a system is constant but energy can still be lost through thermal ...
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Deriving energy from action symmetry [duplicate]

I am trying to derive the energy of a system directly from the action principle without explicitly using the Noether's theorem. So I tried considering a time translation on the action as following: $$...
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Why is there more energy in a gamma wave vs. lower frequencies?

If energy is a function of frequency and amplitude, why are microwaves and lower frequency waves considered “less energetic” than gamma waves only as a function of their frequency and wavelength?
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What is the relation between Time and Energy? [closed]

According to the formula $E = hf$, it can be re-arranged like $E = h/t$ where, $t =$ time, $E =$ energy, and $h =$ Planck's Constant. So from this, it would be reasonable to derive a relationship ...
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Why do outermost particles on a rotating rigid body gain more kinetic energy?

This is something which I know from my $9^{th}$ standard that on a rigid body rotating about a fixed axis , the velocity of the outermost particle is the maximum and it should be true so that the ...
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1answer
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Odd function (probability density function) describing the measurable quantity of a quantum particle [closed]

This question is in reference to Quantum Physics of Atoms, Molecue, Solids, Nuclei and particles by Robert Eisberg and Robery Resnick. The setup for an infinite square quantum well with the potential ...
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Doubt in energy of electron in first shell of hydrogen atom [closed]

Let us calculate the energy of the electron in the first shell of the hydrogen atom. Since hydrogen is a single electron species we can use Bohr's postulates. The kinetic energy (KE) of an electron in ...
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1answer
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Why don't we add energy wasted as work to the heat measured by calorimeter in calculation of $\Delta H$?

Lets say a exothermic reaction is happening in a constant pressure calorimeter. This reaction releases energy as heat($q$) and work($w$) (done to expand its volume against atmospheric pressure). The ...
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Work done by a pump to empty a well [closed]

I've come across problems that ask to calculate work done by a pump to empty a full cylindrical well. The result obtained through integration is $mgh/2$, where $m$ is the mass of water in the well and ...
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Do double pendulums violate the law of the conservation of energy?

When I first saw a double pendulum I thought it was fake because it just doesn't look real the way it moves. Then I searched for more videos and found out that it's real. Can someone explain why they ...
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Question about the work done by a frictionless rod on a ball on it (Mechanics) [closed]

Please refer to the question, everything is mentioned there. The answer is option (b) which is $mv^2$ I tried solving this, but could not do it. The problem here is the plane we are using, if it was ...
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1answer
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What puts a limit on lowering $\langle T\rangle$ low avg in heat engines?

I have a simple question which is: We know that the thermal effeciency in heat engines increases when we lower the average temperature at which heat is rejected. But the problem is that there is ...
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Why do lines become weaker in intensity as wavelength decreases?

In 1885 the first series was observed by Swedish school teacher Johann Jakob Balmer in the visible region of the hydrogen spectrum. This series is called the Balmer series. In Balmer series as the ...
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Why does higher temperature lead to lesser efficiency of photovoltaic cells?

I have understood that band gap decreases but why does that decrease lead to lower efficiency?
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“Conservation of energy, or lack thereof,” in quantum mechanics

In answering another question(1) on this site, I started to consider the conservation of energy in Quantum Mechanics. Doing some research, I came across this recent paper.(2) The abstract of the paper ...
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How did Feynman derive the potential energy of the ballistic galvanometer?

In the Feynman Lectures Vol 1. Chapter 41 - The Brownian movement, the following statement is present under Section 1 - We know the formula for the kinetic energy of rotation—it is given by Eq. (19.8)...
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P-N Junction Diode (Semiconductor)

I have read that in a p-n junction, free electrons from n side diffuse to the p side and then recombine with the holes of p side. Here, it does not refer regarding energy bands. However, free ...
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If a robot swings a bat from point A to point B and hits a ball at point B then where does that kinetic energy go if there is no ball at point B?

EDIT: After thinking about this I have concluded that my hypothetical experiment isn't possible, because if the robot stops the bat at exactly point B on each swing, then the ball doesn't move since ...
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1answer
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Energy turns into mass [closed]

I studied on YouTube that when mass is converted into energy, energy is converted into mass somewhere, but when I searched for this theory, I did not find it. Does anyone know what the name of this ...
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What happens to an electron if given quantized energy to jump to a full orbital?

Let's consider the element neon. Its ground-state electron configuration is: $1s^2 2s^2 2p^6$. What would happen if enough energy was given for one electron in the $1s$ orbital to jump to the $2s$ ...
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Why don't we take kinetic energy into account when finding the change in potential energy?

An engine draws water from depth of $10 \;\text{m}$ with constant speed $2 \;\text{m/s}$ at rate of $1 \;\text{kg/s}$. What is the power of the engine? While solving the question, I found that we ...
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8answers
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Why don't we define temperature as a kind of energy? [duplicate]

Currently, the temperature $T$ is defined in relation to the energy $E$ via Boltzmann's constant $k_B$ like so $$ E \: [\mathrm{J}] = k_B \: [\mathrm{J/K}] \cdot T \: [\mathrm{K}]$$ I'm now proposing ...

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