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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56 views

Is it Possible to Directly Measure Energy?

Would it be possible to measure energy directly in joules instead of measuring what effect energy has on something? Say for example you have a car of known weight traveling a speed v. To get the car's ...
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What is the difference between the specific heat capacities of water under isobaric and isochoric conditions

Can the difference of specific heat capacity of water under isochoric and isobaric conditions be explained in terms of the internal energy of the system? Most of the videos I have watched base their ...
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The expectation value of the energy of the general wave function in the infinite square well

We know in the infinite square well that the general solution is a linear combination of the stationary states ( states with definite energy eigenvalues). Also the measurement of the energy for that ...
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3answers
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Is the heat dissipated by a resistor given by $I^2RT$ or $(V^2/R)T$? [closed]

Is the heat dissipated by a resistor given by $H = I^2RT$ or $H=(V^2/R)T$? In other words, if the resistance is reduced by a quarter, will the dissipated heat increase or decrease? The two formulas ...
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7answers
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Why is maximal kinetic energy lost in a perfectly inelastic collision?

A perfectly inelastic collision is one where both of the colliding objects stick together and move as one. My questions is, why, of all possible combinations of final velocities that conserve momentum,...
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4answers
600 views

Work term in First Law of Thermodynamics

The first law states that $\text{d}Q = \text{d}W + \text{d}U$ where $\text{d}W$ is the work done by the system on the surroundings. As far as I remember from my chemistry courses,first law in ...
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6answers
184 views

Why work done is needed when other physical quantities are available?

I have some question about work done. I understand the mathematical ways and examples which are floating all over on internet & books. But all these information not clearing the concepts of work ...
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2answers
41 views

Mechanism of heating during deformation [duplicate]

When a metal or rubber is bent and deformed, it heats up. What is the reason for this? I know that when deforming it, work is done on the object. If the object is perfectly elastic, all of the energy ...
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What is the physical meaning of ΔG = q(irr) - q(rev)?

Why is change in enthalpy ΔH equal to heat exchanged following whether reversible or irreversible path whereas TΔS is equal to the heat that would be exchanged if the process was reversible? What is ...
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4answers
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What makes an electron jump down to lower energy level?

While I can easily grasp the mechanism of energy absorption by the atom system resulting in a jump of the electron to a higher energy level, i.e. receiving energy from the outside in the form of a ...
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2answers
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Seeking a more natural proof and intuitions for $\left<T\right>=\frac12\left<\textbf x\cdot\nabla V\right>$

I am currently self-learning quantum mechanics with A Textbook of Quantum Mechanics Second Edition by P. M. Matthews and K. Venkatesan. In Example 3.15 on p. 105, the author proved $\left<T\right&...
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What is energy's basic shape/form?

I have been thinking about how earthquakes, light, sound, water tension, and movement work. I do not have a degree in physics, nor have I taken it (yet), but I am willing to learn. To summarize my ...
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1answer
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Thermodynamics Potentials Relations

I tried to find the whole connections between the thermodynamics potentials without any success, the main potentials $U,F,G,H$ and the relations between them (derivatives), for an example what is the ...
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4answers
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How can we have negative work in electrostatics, if $W=(\epsilon_0/2)\int\! E^2\ \mathrm{d}\tau$?

This question is motivated by Section 3.2.3 in Griffiths. Therein, we are considering the force of attraction between a point charge and an infinite conducting plane. One can calculate the field ...
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Potential Energy of a System of Point Charges

Is the potential energy of an isolated system of point charges placed in a bounded space in equilibrium necessarily zero?
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2answers
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What is the angular speed of mass $m$ if we pull the string? [duplicate]

Mass $m$ is rotating on a horizontal friction-less plane around a tiny hole. A massless string is connected to mass $m$ and the other end of it is held by John's hand. John pulls the string to move ...
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Difference between eigenvalues of the potential energy Hessian vs. “generalized” eigenvalues with respect to a kinetic energy “metric”

Simple version Consider if we have a Lagrangian defined by $$L(q,\dot{q}) = \frac{1}{2} g_{ij}(q) \dot{q}^i \dot{q}^j - U(q) \tag{1a}$$ where the potential energy $U(q)$ has a single minimum at $q=0$ (...
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1answer
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While freezing ice from salt water, does the salt dissolve in the ice or is it separated out?

I know that salt decreases the freezing point of water. We have freezing point (in Celsius), $\mathrm{T_f=-K_f\cdot m}$ (molality of salt). But, is it true that all of the solution doesn't freeze at ...
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2answers
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Can the Lagrangian be written as a function of ONLY time?

The lagrangian is always phrased as $L(t,q,\dot{q})$. If you magically knew the equations $q(t)$ and $\dot{q}(t)$, could the Lagrangian ever be written only as a function of time? Take freefall for ...
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1answer
193 views

Beirut 2020 explosion vs a nuclear explosion with the same energy [closed]

Port Beirut disaster (ammonium nitrate explosion, 4 August 2020) was equivalent to 1.15 kt of TNT (4.4 TJ). It falls in the range of a small nuclear device (20 t TNT for the smallest, 15 kt TNT for ...
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4answers
88 views

When does the conservation of energy hold?

This may sound trivial but I wanted to make sure I did not misunderstand. I know that the conservation of energy is always true even in cases where energy appears to be lost (when in reality it is ...
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1answer
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A question regarding Physics of sand-boarding

I was playing a game when I started thinking about this problem: Say, a person is sandboarding down a hill/dune, right before the slope begins, what are the conditions that must be met in order for ...
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1answer
35 views

Does gas of different ionisation energies affect the total impulse generated by an ion thruster? [closed]

Required to do a physics practical experiment for a year project and I decided to do something regarding ion thrusters. My idea was to create a simple DIY ion thruster (can be seen below) and put it ...
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1answer
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Electromagnetic wave associated with a single photon

To the question regarding the relation between the energy of a photon (E = hf) and the energy of the associated electromagnetic wave, I read somewhere that the energy of an electromagnetic wave is Nhf,...
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Lagrangian potential for Newtonian gravity

In the Wikipedia site for Lagrangian (field theory) the Lagrangian density for Newtonian gravity is given by $${\cal L}(\mathbf{x},t) = \frac{1}{2}\rho(\mathbf{x},t)\mathbf{v}^2 -\rho(\mathbf{x},t) \...
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1answer
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Why is it incorrect to say that KE of COM is not the same as total KE? Does this also apply to PE?

I sort of understand why KE(com) is not the same as KE(total)... correct me if I'm wrong.. its like when a symmetric body is performing rotational motion even though the COM is not moving it still has ...
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2answers
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Work done by friction on a sphere sliding down the inclined plane [closed]

If a sphere slides down from rest on an inclined plane of hypotenuse $L$ and height $H$ : Work done by friction is converted into rolling kinetic energy and as well as heat energy. If $f$ is the ...
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1answer
157 views

Why is energy quantized but mass is not? [duplicate]

According to Einstein's mass energy equivalence equation, $$ E = mc^2$$ where $m$ is the rest mass and $c$ is speed of light. According to Planck's equation, $$ E=hv$$ If Planck's equation suggests ...
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3answers
102 views

Why do we need to do work to maintain motion of a fluid?

On page 355 of principles of physics by Resnick walker and Halliday, the author writes: Work must also be done on the system (at the input end) to push the entering fluid into the tube and by the ...
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3answers
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What if we know exact position of electron?

I know many scientist gives many theory to explain structure of atom and that's Quantum mechanics explained very well. Even Schrondiger eq. to get idea where electron more probable is around a nucleus ...
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What factors does the penetrating power of a particle depend upon?

If I were given to compare three different radiations namely alpha, beta and gamma what will be the order of their penetrating powers given that they all possess the same energy (=0.5 MeV)? I have ...
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2answers
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How to convert these numbers into energy per ton-km?

From Wikipedia's energy efficiency in transport, I gathered the following data for a person riding a bike: 65 kg person riding at 4.5 m/s requires 110 kJ/km, which equates to a continuous power output ...
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1answer
25 views

Can internal energy be partitioned into well-defined components?

People often talk about different forms of energy like thermal energy, chemical energy, nuclear energy, etc. as belonging to the internal energy of a physical system. However, I can't really remember ...
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2answers
92 views

Trying to intuitively understand how temperature relates to entropy

Background I was looking into the definition of temperature and how it relates to entropy and internal energy and I came across this answer on StackExchange. According to the answer: [Temperature is] ...
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0answers
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Calculate energy in KeV from SiPM trace

I am trying to calibrate a SiPM device and I would like to know how to calculate a particle's energy given a trace from an oscilloscope. Values are returned to me as milliVolts at a given time step ...
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6answers
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Mechanical Energy Conservation (I have some problem in my basic concept)

A block of mass $2kg$ is kept at origin at $t=0$ and is having velocity $4{\sqrt 5}$ m/s in positive x-direction. The only force on it is a conservative and its potential energy is defined as $U=-x^3+...
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5answers
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Why can some electromagnetic waves heat things up while others cannot?

I have read that heat radiation happens in the form of infrared, which is an EM radiation with a longer wavelength than visible light. So the heat radiation that you can feel in an oven or under the ...
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3answers
45 views

In what direction is positive work done under a gravitational force, and what justifies the relation between work, potential and kinetic energy?

This has confused me for some time: if one integrates the gravitational force between two radii (let the motion of the particle always point toward the centre of mass of the other mass) from a radius ...
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Behaviour of Bethe formula as particle energy grows

I have the following formula: $$\dfrac{dE}{dx} = \dfrac{4 \pi e^4 z^2}{mV^2}NB,$$ where, for example, in the case of relativistic velocity of a particle, we have $$B = Z\left[ \ln \dfrac{2mV^2}{I} - \...
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How to convert GeV to AGeV? [duplicate]

The beam energy is sometimes in AGeV in collider papers. I'm stuck on how to convert AGeV to GeV.
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4answers
85 views

Inelastic Collision and Kinetic Energy

We see that our calculations always gives a loss of Kinetic energy in inelastic collisions while the net momentum of the system remains the same. Where does the lost energy go while there is no trace ...
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1answer
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What happens to energy when a car runs into a wall?

If a car drives into a wall and energy cannot escape to the environment, does all that energy that went into the wall get transferred back into the car? If so, is that why the car gets damaged?
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1answer
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Does one side of a glass slab get cooler and another side get hotter when the light passes normally through it?

Let us have a glass slab and we illuminate it by a light beam impinging normal to the slab and passing through it. Since the speed of light is slower inside the glass it loses its energy and when it ...
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0answers
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Work Enegy Conservation & Rolling Bodies [duplicate]

A disc of radius 0.1 m rolls without sliding on a horizontal surface with a velocity of 6 m/s.It then ascends a smooth continuous track. The height up to which it will ascend is​ Now I'm very confused ...
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0answers
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How do we see things? [duplicate]

What I mean is, suppose white light falls on a red object and is reflected, so when we see the reflected light reaches our eyes we see it as red (probably because its wavelength corresponds to red). ...
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1answer
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Climate Change and Degrees of Freedom of Gases

I studied a sciences of climate change module with the OU a while ago, I'm half remembering something that's bugging me and wonder whether any could help provide some clarification? I remember that an ...
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1answer
32 views

Angular momentum and collisions/pure rolling

Consider a general problem of a rigid sphere having only rotational motion placed on a rough surface where it starts rolling after some time. And final velocity is asked. Friction is just enough to ...
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1answer
53 views

Degeneracy in a Double Well Potential

Even though double well is a $\mathrm{1D}$ potential and it must not have degenerate spectrum, then why does double well have degenerate energy eigenvalue?
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5answers
1k views

Why the integral of a force gives Energy? [duplicate]

In the 10th grade, meaning few months back, I've studied the potential and Kinetic energy (I was ignorant about the importance of calculus), but When I learned calculus, and the Constant of ...
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7answers
120 views

Work done when lifting an object at constant speed

A previous post (What Is Energy? Where did it come from?) defines work qualitatively as "a process in which energy is transformed from one form to another form". And mathematically, work is ...

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