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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26 views

Where do electrons get their energy from?

That is, free electrons in a metal conductor, in the absence of a potential difference. I'm not talking of an electron in an excited state or electrons flowing in a conductor as a result of an applied ...
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Infinities of Mass, Energy, & Velocity

My layman's understanding is that photons are particles having 0 mass, which is why they are capable of moving at c (lightspeed). Does our current model of particle physics postulate the existence of ...
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Why does internal energy depend only on a set of two states variables?

Why is the internal energy dependent only on two states variables? Why not on four, five etc. I know equation of state can reduce the number of variables by one, but that doesn't answer why does ...
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40 views

Doesn't the non-commutivity of $x$ and $p$ complicate the measurement of a quantum system's (e.g.) Energy?

A classical system is parameterized by $x$ and $p$ coordinates, and so any other observable -- such as energy -- is some function $E(x,p)$ of them. I assume, then, that to measure the energy one must ...
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What does work in thermodynamics mean versus in other parts of physics?

I just started learning about thermodynamics in an introductory physics class and am confused about the meaning of 'work' as it has been used thus far. When I was taught mechanics, my teacher said ...
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If you extracted 5 volts of energy from the human body for eight hours, how many calories would that burn?

I'm designing a device that is able to extract up to 5 volts from your body. I was wondering how many calories this would burn in eight hours? Suppose you went to sleep at night with this device ...
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25 views

Why is Total internal energy of a system or external energy only the sum of kinetic and potential?

Why is Total internal energy of a system or external energy only the sum of kinetic and potential ? Why not some some other quantity as well present. Is it because there can be only scenarios present. ...
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29 views

Torque on a spool of thread [closed]

Here is a problem I encountered in my physics class. I was thinking that the answer was a, but the answer key says that the answer is d. Could someone explain this problem to me and the concept of ...
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Relation between activity, intensity, count rate and so on considering ionizing radiation

I'm currently doing a project on waste products of nuclear power. To narrow it down, I chose to focus on gamma rays in particular. However, I generally find it hard to see the relation between ...
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Enthalpy of Ideal Monoatomic, Diatomic and Triatomic gases

We learnt today that in ideal gases (triatomic as an eg), we can find the internal energy, and their specific heats for constant pressure and volume u=3RT Differentiating with Temperature Cv= 3R Cp= ...
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How to solve this problem of the approximation in Newton’s law of cooling using Laplace transform? [closed]

I am a high school student in the process of writing my report. I found a low quality website that talked about Newton’s law of cooling with approximation of the temperature of the environment. The ...
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Independent variables of the inner energy

in thermodynamics the free energy is a function of the volume V and the temperature T. These two variables are called independent. On the other hand on can find the volume and the temperature as a ...
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What is the energy integration constant from time symmetry in general relativity?

If the metric components $g_{\mu\nu}$ are independent of the time coordinate $x^0=t$, then it's common to say there is a constant of motion called "energy" begotten from considering the ...
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Zero-point energy? (D-branes C.Johnson's book)

In Johnson's Book about D-branes (11.2) gives the zero point energy. This is given using (2.80) \begin{equation} +\frac12\omega: \text{for boson and}\;\; -\frac12\omega: \text{for fermion}. \end{...
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As the universe expands, how does it affect the mass of objects and therefore the gravity?

I think the mass of object would proportionately decrease as the universe expands, as the amount of matter is constant in accordance to the law of conservation of energy. If I am correct in this, ...
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Derivation Of Rotational Energy From Kinetic Energy

I am studying rotation, and I would like to check that my understanding is correct. I am trying to prove that: $$ \tau \theta = \frac{1}{2} I \omega^2 $$ To prove this, let's consider an object that ...
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Why enthalpy increases when we heat a gas at constant volume

I have read many answers on stack exchange about what is the physical meaning of enthalpy, and there are some wonderful explanations. one analogy i find very satisfying is it is the energy we need to ...
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Why can we multiply by $dt/dt$ to change variable of integration? Please look at equation 5-20 [closed]

I am struggling to understand why can we just multiply by $dt/dt$. I was thinking it was just a change of variables, but I cannot come up how that works. Can someone explain why we are allowed to do ...
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Amount of work: clarifications

If to lift an object of mass $m$ a force greater than $mg$ is required then why do we take the amount of work done in lifting an object as $mgh$ could someone please explain that to me?
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Dirac-delta function in Derivation of Fermi's Golden Rule

I was following along Mark Thomson's Modern Particle Physics, and got stuck on this book's derivation of Fermi's Golden Rule (On page 53): "... If there are d$n$ accessible final states in the ...
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1answer
51 views

How do I calculate the work done on the box? [closed]

A 2 kg box with some initial velocity slides up a ramp, which is at a 15 degree angle to the ground, until the box comes to rest 1 second later. The coefficient of friction is 0.2. How much work is ...
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Can a metal barrier mounted on a system of springs with a right elastic constant save a weak object from a bullet?

My question is motivated by the fact that a lot of objects or vehicles that are important can be hit by bullets.Will a barrier mounted on a system of springs efficiently attenuate the kinetic energy ...
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I have question about transformer [duplicate]

When we describe the working principle of the transformer, we say that we want to minimize the loss of electrical energy, therefore we reduce the current Q=I^2Rt, so heat reduces as well, but to keep ...
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Relativistic energy of harmonic oscillator

What is the relativistic energy of an harmonic oscillator: $$\frac{m_0 c^2}{\sqrt{(1-\frac{v^2}{c^2})}}+\frac{1}{2}kx^2$$ Or $$\frac{{m_0 c^2}+\frac{1}{2}kx^2 }{\sqrt{(1-\frac{v^2}{c^2})}}$$ I think ...
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Accessible States in the Ergodic Hypothesis

According to Wikipedia, the ergodic hypothesis is the assumption that all accessible microstates are equiprobable over a long period of time. My question is about the precise meaning of "...
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67 views

Significance of minimum energy of a proton [closed]

I have been presented with the following problem. A neutron is confined in a nucleus of size $4 × 10^{−15}$ m. Using the Heisenberg Uncertainty Principle, estimate its minimum kinetic energy. Comment ...
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35 views

Measuring potential energy and potential energy differences

In reference a discussion on gravitational potential energy $U$ (available a https://physics.stackexchange.com/questions/287292/is-energy-relative-or-absolute-does-gravity-break-the-law-of-energy-...
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Velocity of different sized balls rolling down the same rail [duplicate]

During mechanics lab we conducted an experiment in which we rolled two balls (glass and steel) down an eliptic heightened rail and measured the impact point. The lines of the rail were 0.76~cm. The ...
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56 views

Why is 'Force resolution method' and 'energy conservation principle method' giving two different answers in this problem?

A ring of mass 'm' slides on a smooth vertical rod; attached to the ring is a light string passing over a smooth peg at a distance 'a' from the rod, and at the other end of this string is a mass M (&...
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2answers
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What is the total internal energy of a liter of water? [closed]

How does one calculate the total internal energy of a liter of water? For an ideal gas, the case is simple. $E = c*n*T$, where c is the molar heat capacity, $n$ number of moles, and $T$ the ...
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Is an electron completely determined by its mass? [duplicate]

What I mean is, could a chunk of matter be "whittled down" or broken off from a larger chunk of matter to the point where it has the mass either exactly of an electron or close to that and ...
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Another way for production of positive fusion

I am wondering why energy positive fusion cannot be produced in the following manner. Large tank of hydrogen pressurized to such a high a level it would push the limits of current technology. ...
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39 views

$dT/dx=0$ always true?

In a Classical Mechanics book I found the assumption that for an arbitrary particle with constant mass in the Real line $dT/dx=0$, with T the Kinetic Energy i.e. $T=(m·\dot x^2)/2$ My hypothesis is ...
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How are Taylor and Maclaurin series applied in Newton’s law of cooling?

I am a high school student that’s having a hard time figuring out what to carry out in his report. As we know that Newton’s law of cooling shows an exponential curve in other words. This question ...
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1answer
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Understanding of what energy means [closed]

I have trying to get my head around the concept of energy to get a satisfying understanding. Energy is a signed scalar quantity that comes into two main forms: kinetic energy (motional energy) and ...
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Black Hole Event Horizon and Time [closed]

I've been thinking about a theory lately (coming from an electrical engineering background) and I was hoping maybe someone could clarify this for me. So we know black holes can absorb anything, even ...
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2answers
133 views

Does the waste heat generated by life's processes make survival more difficult in any meaningful way? [closed]

From reading Nick Lane's "The Vital Question", I recently learned that biological processes and the formation of biologically organized structures (cells, organisms, metabolism and so forth) ...
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Why energy levels of each atoms split in band theory of solid such that they are not degenerate?

I recently came across energy band theory in semiconductors and pauli's exclusion principle. But I'm not sure how energy level split and why, when the atoms are so much far from each other in solid. ...
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Does electric current through gases emit light?

Not electric spark or electric arc, just normal current through gases created by heating the air between cathode and anode as shown in the following video at 1:25. https://www.youtube.com/watch?v=pg0-...
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90 views

Find the final velocity of each electric charge [closed]

I have a system with three identical electric charges $Q=+50\cdot10^{-6}C$. The mass of each charge is $M=0.1kg$. The charges uniformly aligned along a quarter circle with radius $R=0.5m$, as shown in ...
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What is the internal energy of water vapor? OR How many degrees of freedom does $\rm H_2O$ have?

Here are the "knowns" that I'm working with. Internal Energy for a monatomic gas is $$U = (3/2)nRT$$ For a diatomic gas its $$U = (5/2)nRT$$ All of my textbooks and online sources indicate ...
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Energy of photon from superposition to ground state

Let an atom is in superposition of two eigen energy states E1 and E2. If it returns to the ground level E0 what energy should have the photon released in the process?
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Desperate help in Newton’s law cooling [closed]

I’m a high school student that is attempting to explore the math involved behind Newton’s law of cooling. I have tried to derive the equation and the idea was to apply it to a crime scene and finding ...
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317 views

Effective Field Theory vs Ostrogradsky

The low-energy effective description of a given QFT is an expansion of the form $$ \mathcal L_\mathrm{IR}\sim\sum_{n,m} \lambda_{n,m}\phi^n\partial^ m\phi $$ where we include all terms that are ...
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Why does exchange energy of electrons lead to stability?

My text book says "electrons of the same spin in degenerate orbitals tend to exchange their positions and this leads to stability". Firstly, why should the electrons exchange their positions?...
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Changes in Kinetic Energy [closed]

A body is moving with a constant speed on a circular path. What happens to its Kinetic energy? Kinetic energy is dependent on velocity but Kinetic energy on the whole is a scalar so direction does ...
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2answers
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Sunlight-powered device set to change world: will it work? [closed]

Vast areas of desert sand are being shone on upon by the sun. Now that's some ENERGY! Using this proposed apparatus: Sunlight over large area is concentrated on steam chamber, where water turns into ...
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1answer
52 views

Total Relativistic Energy of Massive Body

I have 2 different expressions for the relativistic energy of a body and I'm trying to link them both: The first one is given by $$E=\sqrt{(mc^2)^2+(pc)^2},$$ with $m$ the mass and $p$ the momentum. ...
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Feynman Lectures Chapter 4.2: Understanding the weight-lifting machine visually

Why a non-reversible weight lifting machine cannot lift higher that a reversible one is related to my current post, but everyone in that thread seems to have understood how Feynman's simple machine ...
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1answer
75 views

Equations of State [closed]

I'm having trouble coming to any meaningful answers in this question, It seems to me like all of the variables given can be put generally in terms of T. I can't seem to figure out any kind of ...

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