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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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Calculation of work done by normal reaction on a falling chain

There is a chain of length L which stands vertically above the ground, such that its tip just touches the floor. The chain is let go. We are required to find the work done by the normal reaction ...
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26 views

Can change in kinetic energy be negative?

Can change in kinetic energy be negative? If yes then kinetic energy would be negative which is not possible please explain
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1answer
12 views

Energy distribution of large, falling rod striking Earth

There is a real-world concept to drop a large (say 6m x 0.30m, mass 9 tons) tungsten rod from orbit onto a ground target. A 2003 Air Force report estimates that such a weapon would strike ground at ...
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24 views

helix moving against water flow [duplicate]

There is a viral video spread showing a helix stick moving against water. For more detail there is a stick look like a spring. When water slowly pour on it. The spring like stick climbing on the ...
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3answers
103 views

Kinetic energy and conservative fields

If $W = E_1 - E_0$ for every force and a conservative field is a field such that the work on a closed path is 0, then every field should be conservative, since $W = E_0 - E_0 = 0$. I know that this ...
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0answers
22 views

How are mass, charge and energy all related?

An adequately energetic gamma ray photon can produce a pair of particles under the right conditions, one of these particles is an electron and the other one is positron. Pair production is widely used ...
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1answer
33 views

Does measured mechanical power and work change between inertial frames?

Imagine a car A is accelerating. Two observers at constant speed, B and C, analyse the change of A's kinetic energy over a same time interval. B sees A going from 10 to 30 m/s C sees A going from ...
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42 views

1D Infinite Potential Well: getting different values with different methods for $\langle H \rangle$

We have $$\psi(x,0)=3u_1(x)+4u_2(x)$$ where $u_1(x)$ and $u_2(x)$ are the ground state and first excited states of a 1D infinite potential well. I got the normalization constant. I have to calculate ...
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1answer
23 views

Can electrons jump between levels in same shell with constant $L$?

I see many cases where electrons jump from various energy levels of some $L$ value to various levels of other $L$ value. But is there any transition in same $L$ level between its different ...
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1answer
32 views

How many calories in a block of wood?

I was recently thinking about the human body and the energy we get from food. I understand we don't have the ability to properly digest certain things like stones and wood and grass, but if we did, I ...
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0answers
21 views

Energy imparted by lead aerosol

So, I'm having a friendly disagreement wherein someone and I are theorizing about the lethality of a vaporized bullet. I'm under the impression that if a bullet is vaporized, mid-flight, the ...
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2answers
30 views

Calculating Elastic Potential Energy of a Stretched Sheet

Essentially, I'm trying to determine the amount of elastic potential energy stored in a thin, elastic sheet that has gone under some type of stretching (ex. A flag of stretchy fabric waving in the ...
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0answers
17 views

Wave power calculations

In my exam solutions we try to find the Power per unit length of the wave (which is idealized as being a rectangular block moving with a velocity v, not sinosuidal). The power per unit length is given ...
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0answers
37 views

statistical definition of the temperature, the thermal energy and the pressure for an non-ideal gas

I recently realised that my definition (and understanding) of the temperature was only valid for a ideal gas: $T=\frac{2}{3 k_B}\int (u-v)^2f(v)dv$ where $u$ is the mean (or global) velocity of a ...
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3answers
63 views

If the moon produced energy like the Sun would it radiate the same energy and light on Earth as the Sun? [on hold]

I would like to know if the moon where to shine like the sun would the Earth get the same energy and light as it is getting now from the Sun?
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0answers
7 views

Excitation energies of optical transitions

If one speaks of the excitation energies of the three lowest optical transitions, does it mean that the first transition is HOMO -> LUMO and therefore the excitation energy $E_{LUMO}-E_{HOMO}$, the ...
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0answers
31 views

Does the formula for the gravitational potential energy have its limits

From what i read from my textbook , gravitational potential energy is the work done by an external agent in bringing an object from infinity to a point in a gravitational field without the ...
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2answers
24 views

Why does kinetic energy of elecron on Bohr's orbit not change if zero of potential energy changes?

If zero of potential energy is not taken at infinity, total energy and potential energy of electron change but not kinetic energy. Why is that so?
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1answer
40 views

Partitioning the kinetic energy into components in relativity

In classic physics, kinetic energy is defined as $$ KE = \frac{1}{2}m v_x^2 + \frac{1}{2}m v_y^2 + \frac{1}{2}m v_z^2 $$ So, by defining $ KE_x = \frac{1}{2} m v_x^2 $ , $ KE_y = \frac{1}{2} m v_y^2 $...
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2answers
47 views

If efficiency is more important, then why condenser is used in boiler?

What's the real reason behind condenser? some says, Without condenser, back-pressure will be created in turbine. After Expansion in turbine, entropy will be maximum so it must be cooled to reduce ...
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15answers
11k views

When a balloon pops and lets a brick fall, where does the energy come from?

Let's say a scientist attaches a 1 kg brick to a large helium inflated balloon, lets the balloon go, and then it reaches an altitude of 10 000 meters before it pops, dropping the brick. The brick ...
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2answers
85 views

Energy yield of a meteor blast - how to explain the discrepancy?

The energy yield of the Chelyabinsk meteor blast is estimated to be about 500 kT TNT, or $2\cdot 10^{15}$ joules. When compared that to its estimated speed ($\sim 2\cdot 10^4$ m/s) and mass ($1.3\cdot ...
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1answer
45 views

The observer effect, how is related to death of human brain or consciousness? [duplicate]

Are we just a measurement device that interacts with quantum wave function/photons or something more? if not then can we die/be destroyed as observer?
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4answers
56 views

How is work transferred to the system recognised?

For example, a potato initially at room temperature $25 \sideset{^{\circ}}{}{\mathrm{C}}$ is baked in an oven that is maintained at $200\sideset{^{\circ}}{}{\mathrm{C}}.$ I made potato as the system ...
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2answers
79 views

$q\mathbf{A}\cdot\mathbf{v}$ term in potential energy

In the famous Goldstein mechanics book, there is an example about a single (non-relativistic) particle of mass m and charge q moving in an E&M field. It says the force on the charge can be ...
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0answers
32 views

Work done against a force

Consider a body of $1\ \rm kg$ mass at rest at ground. Now as we know, it can be lifted by a force equal to its weight. So we have to apply a little bit more force then its weight. let $g= 10\ \rm ms^{...
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1answer
48 views

What is happening energy-wise when an object is suspended on rope from ceiling?

Several similar questions have been asked, e.g. this one, but all of them used the example of muscle work, so it was clear where the energy went. However, suppose we suspend an object on a rope from ...
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0answers
36 views

Does an electron have a frequency (and hence an energy)?

The formulation is provocative, the question is similar to the question here. There I can follow the question, but not the answers, which for me imply that an electron in a momentum eigenstate does ...
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0answers
30 views

Diagonalizing the free Hamiltonian $\hat{H}$ in terms of smeared wave packet operators

Consider the free real scalar field in (1+1)-dimensions. If the (normal-ordered) Hamiltonian is $$ \hat{H}=\int_{-\infty}^{\infty} dk\ \sqrt{ k^2 + m^2 } \hat{a}_{k}^{\dagger} \hat{a}_{k} $$ where $\...
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1answer
40 views

Is Light Energy? [duplicate]

I have been thinking about different forms of energy and ways to define them, but I realized that most elementary forms of energy are waves, such as light (a wave in the electromagnetic field). Waves ...
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1answer
61 views

Simple question about molecules leaving a liquid at a temperature below the boiling point

My textbook asks the question why a liquid cools down when partial evaporation occurs just below the surface at a temperature below the boiling point. The answer is of course that only those ...
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1answer
36 views

How does energy flow in a circuit?

I read some information on a certain blog saying that energy does not flow inside the wires of a circuit, it propagates in the form of an electromagnetic field outside the wires and straight into the ...
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1answer
37 views

Gravitational potential energy and kinetic energy

I have a question that has been bugging for quite some time now. The quesiton is from a textbook by Bostock and Chandler on Mechanics. It says : Water is being raised by a pump from a storage tank $...
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0answers
9 views

UV light air ionisation

Does 180nm UV light really create ionisation,charged particles in air? I know UV light of this wavelenght can break the dioxygen bond,but it just results in two neutral oxygen atoms,so no ionisation/...
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2answers
129 views

Bullet hits Earth at speed $0.99c$ [closed]

According to wikipedia, the speed of the neutrino is as close as we can get to the speed of light with an object with rest mass. As per the comments, since neutrinos come in different energies, let's ...
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2answers
48 views

Internal energy of dielectric

I have a plate capacitor with a given charge. It is then dipped in a dielectric fluid and I must determine which height the fluid reaches (all the necessary parameters are given). I can solve this ...
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3answers
101 views

Electric car in different inertial frames

Assume there are two inertial frames $A$ and $B$ where $B$ is moving with speed $v$ relative to $A$. Suppose an electric car is stationary in frame $A$. The car is accelerated to speed $v$ to be ...
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1answer
118 views

Why alpha particles inestead of hydrogen or duetereum?

I know atoms undergoing alpha decay emit alpha particles, but I was wondering why specifically helium nuclei? If these atoms wanted to emit the smallest unit of matter (I'm talking about hadrons here, ...
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1answer
37 views

Is the first law of thermodynamics (conservation of energy) also applicable to power?

I am familiar with the first law of thermodynamics which states that the total energy [J] of an isolated system remains constant. Based on this law one can, for instance, create a Sankey diagram of a ...
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36 views

Entropy and temperature

I have been studying the concept of entropy and I have seen many examples involving the local decrease of entropy by cooling an object. Some definitions of entropy state that low entropy is when there ...
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2answers
59 views

If electricity is made up of electrons, how can it cause the hydrogen spectrum to appear?

The hydrogen spectrum appears when an electron absorbs a photon, jumps an orbital, and then releases that photon in an effort to get back to its ground state. From what I've read electricity is ...
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0answers
26 views

Can electric energy be generated from only heat if the heat is uniformly distributed along the system? [duplicate]

I was thinking about producing energy from heat and I realized that all the methods I know require a heat gradient. Is there a deeper physical explanation as to why heat gradients are needed? ...
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0answers
12 views

Is it possible to have a vortex with more than one attachment point?

In my study of two vortexes, fire whirls and tornadoes, I have come to hear a bit about vortex attachment. As far as I can understand, a vortex is attached to its "power source" for lack of a better ...
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1answer
55 views

How can iron be forged by a hand-powered hammer?

If the compression strength of iron is 50 MPa, or 5 MPa at 1000C, how can it be forged by a falling hammer, weighing say 10 kg, 10x10 cm impact area -> 100N/0.01 m2 = 10 kPa?
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1answer
80 views

Tidal Power Gen Multiplier? [closed]

Regarding tidal power generation, the following scheme uses an outer tank and an inner tank that floats on the water of the inner tank. You fill the inner tank first (while it's at high-tide level) ...
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1answer
50 views

What is the connection between energy and motion, in a non-trivial sense? [closed]

It seems that without exception and at all scales the presense of Energy walks hand in hand with motion. It is not without reason to wonder if there is a direct connection between the two, in a deep ...
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5answers
509 views

Is PV energy part of internal energy, U?

The definition of enthalpy is $H = U + pV$. Doesn't the pressure and volume of a system get counted once in summing the internal energies, $U$? If so, why count it again to define $H$?
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1answer
43 views

Waves - Frequency and energy/Amplitude and energy

Been wondering about this for a while and I hope someone can explain it. Thanks for taking the time for reading this and perhaps answering the question :-) In my book it says: If you double the ...
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1answer
61 views

Forge-flattening

Is there an easy way to estimate how much energy is needed to forge a cube of iron into a thin square sheet with a hammer? I suppose i should integrate the force $$F = \text{yield_strength} \times \...
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0answers
40 views

Boundedness of general relativity Hamiltonian [duplicate]

When one consider a lagrangian and construct hamiltonian, we expect to be bounded below. While looking to the Hamiltonian formulation of general relativity, I have difficulties to see how it can be ...