Questions tagged [dissipation]

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16
votes
4answers
3k views

Why can't we ascribe a (possibly velocity dependent) potential to a dissipative force?

Sorry if this is a silly question but I cant get my head around it.
14
votes
2answers
4k views

Lagrangian and Hamiltonian EOM with dissipative force

I am trying to write the Lagrangian and Hamiltonian for the forced Harmonic oscillator before quantizing it to get to the quantum picture. For EOM $$m\ddot{q}+\beta\dot{q}+kq=f(t),$$ I write the ...
9
votes
1answer
3k views

How do non-conservative forces affect Lagrange equations?

If we have a system and we know all the degrees of freedom, we can find the Lagrangian of the dynamical system. What happens if we apply some non-conservative forces in the system? I mean how to deal ...
10
votes
2answers
3k views

How do you define the resonance frequency of a forced damped oscillator?

Consider a forced, damped harmonic oscillator $$\ddot{\phi} + 2\beta \dot{\phi} + \omega_0^2 \phi = j(t) \, .\tag{1}$$ If I pick a sinusoidal driving force $j(t) = A \cos(\Omega t)$, I find $$\phi(...
10
votes
5answers
1k views

A conceptual doubt regarding Forced Oscillations and Resonance

While studying about the Resonance and Forced Oscillations, I came across a graph in my textbook that is given below:- Now, the author writes As the amount of damping increases, the peak shifts ...
15
votes
4answers
2k views

What exactly makes a force conservative?

I get that forces can be classified as either conservative or non-conservative, depending on whether the work done in a round trip is zero or non-zero. What property of the force makes it to be, ...
8
votes
4answers
21k views

Would a pendulum swing indefinitely in a frictionless vacuum?

I am attempting to settle a friendly bet. Would a pendulum swing indefinitely in a hypothetical vacuum (i.e. no air resistance) having a hypothetical frictionless bearing (i.e. no energy lost due to ...
7
votes
3answers
3k views

An example of non-Hamiltonian systems [closed]

I am preparing for the exam. And I need to know the answer to one question which I can't understand. "Give an example of non-Hamiltonian systems: in case of infinite number of particles; for a finite ...
1
vote
6answers
4k views

What happens to half of the energy in a circuit with a capacitor?

For a simple circuit with a battery supplying a voltage V to a capacitor, let us assume that the charge on the capacitor is Q. Now, the work done by the battery or the energy supplied is given by the ...
4
votes
3answers
2k views

If string is stretched just by weight, where does the gravitational potential energy goes if only half is converted to elastic potential energy?

If a spring is stretched by a weight of mass m, (so the extension is $\Delta x$) then $ k = \frac W{\Delta x} = \frac {mg}{\Delta x}$. So $ k\Delta x= mg $. When the Spring is stretched by distance $\...
2
votes
2answers
400 views

What is the origin of non-conservative force? [duplicate]

My understanding about conservative force is a force that its work is independent of path such that we can construct another form of the work called potential to make our life easier. For friction, ...
61
votes
6answers
7k views

Why does a system try to minimize its total energy?

Why does a system like to minimize its total energy? For example, the total energy of a $H_2$ molecule is smaller than the that of two two isolated hydrogen atoms and that is why two $H$ atoms try to ...
9
votes
1answer
374 views

Caldeira-Leggett Dissipation: frequency shift due to bath coupling

I am trying to understand the Caldeira-Leggett model. It considers the Lagrangian $$L = \frac{1}{2} \left(\dot{Q}^2 - \left(\Omega^2-\Delta \Omega^2\right)Q^2\right) - Q \sum_{i} f_iq_i + \sum_{i}\...
10
votes
3answers
819 views

Is a superconductor really a super conductor?

It is known that a superconductor is a material with electrical resistance zero. My question is, it is exactly zero, a theoretical zero, or for practical realistic reasons it is effectively zero?
0
votes
3answers
432 views

RLC circuit - calculating resonant frequency

If I take a series RLC circuit connected to a battery, the impedance is minimized when $\omega = \frac{1}{\sqrt{LC}}$. I also know that the series RLC circuit is analogous to a damped driven harmonic ...
1
vote
4answers
551 views

Friction during pure rotation

I have already read Role of friction during pure rolling What is the direction of the friction force on a rolling ball? Rolling resistance and static friction and many others But I still have the ...
1
vote
0answers
504 views

Friction in Lagrangian formulation

We know the Lagrange equations are: $$\frac{\partial \mathcal{L}}{\partial q_i}-\frac{d}{dt}\left(\frac{\partial \mathcal{L}}{\partial \dot{q_i}}\right)=0.$$ Then, when we add friction in there, we ...
6
votes
2answers
1k views

What are the reasons for leaving the dissipative energy term out of the Hamiltonian when writing the Lyapunov function?

I have a problem with one of my study questions for an oral exam: The Hamiltonian of a nonlinear mechanical system, i.e. the sum of the kinetic and potential energies, is often used as a Lyapunov ...
6
votes
3answers
5k views

Liouville's theorem and conservation of phase space volume

It can be proved that the size of an initial volume element in phase space remain constant in time even for time-dependent Hamiltonians. So I was wondering whether it is still true even when the ...
6
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2answers
25k views

Why does friction produce heat?

What causes two objects sliding against each other to produce heat? Why don't they generate visible light or something else?
5
votes
2answers
545 views

What physical processes may underly the collisional term in the Boltzmann equation, and how do they increase entropy?

Consider particles interacting only by long-range (inverse square law) forces, either attractive or repulsive. I am comfortable with the idea that their behavior may be described by the collsionless ...
4
votes
1answer
641 views

Saving energy while charging capacitor

While charging a capacitor by a DC source, half energy is wasted as heat. Can't we save that energy? Here I am talking about $$(1/2)CV^2 $$ which is wasted while supplying $$CV^2$$
2
votes
4answers
396 views

Why current loses its energy?

Can someone explain or give a link to explanation why current loses its energy? For example in simple circuit with lamp. I understand that the energy is spent for heating and lighting. But how exactly ...
4
votes
0answers
136 views

Are gravitational waves dissipated and what is the mechanism? [duplicate]

I was amazed by the fact that we have been able to detect gravitational waves created thousands of year before they were observed. My surprise is that any wave phenomena I know it is always dissipated,...
2
votes
2answers
108 views

Why is a bath necessary for nonconservative forces in quantum mechanics

In classical mechanics, it's straightforward to include nonconservative forces. For a particle in 1D, Liouville's equation becomes, $$\partial_t \rho + \dot{q}\partial_q \rho + \dot{p}\partial_p \rho ...
1
vote
2answers
628 views

If I charge 3 capacitors in series, then connect themin parallel, where does the remaining energy go? [duplicate]

I'll get into the math straight away $C_1, C_2, C_3$ each are charged to $q$. The energy is $$\frac{q^2}{2\left(\frac{1}{\frac{1}{C_1}\frac{1}{C_2}\frac{1}{C_3}}\right)}$$ $$\frac{1}{2}\left(\frac{q^2}...
-1
votes
3answers
109 views

Why do energy transfers always result some heat loss?

explain that energy transfers and transformations in mechanical systems always result in some heat loss to the environment. Like why is it necessary for a heat loss?
4
votes
1answer
126 views

Why can't friction in an accretion disk radiate away enough of the angular momentum do create inward spiral?

As almost every source that I'd found explains, it was a challenge for scientists to understand the mechanism by which the total angular momentum in the accretion disc was conserved, while taking the ...
4
votes
2answers
270 views

Orbit in the vacuum

As the space is a vacuum and there is no friction in space, Can we assume that, if we place an object in gravity in exactly the right distance from a planet with gravity and in the right acceleration, ...
3
votes
3answers
661 views

Dissipation and first law of thermodynamics

Consider the following situation: a certain gas is contained in a well-insulated cylinder with a well-insulated piston head. Now, in this case the piston is not frictionless. In order for the piston ...
1
vote
1answer
50 views

Can a Peltier/Seebeck cell transfer energy from a “thermally insulated” system converting (a part of) thermal energy into electrical energy?

Suppose you have a closed system, as per the schema attached below, which walls are thermally insulated. Suppose you want to reduce the temperature of the fluid inside the closed system, without ...
1
vote
1answer
1k views

What slows down a fidget spinner more: air resistance, or friction from the bearing?

So "fidget spinners" are now massively popular. I got one, and it occurred to me that I'm not sure which force is more important in slowing them: friction in the bearing, or air resistance acting on ...
1
vote
3answers
198 views

Energy doesn't get lost? Basic understanding, please [duplicate]

If its true that energy isn't lost, its just transferred, where did the energy go from a falling object that hits the floor and stays there? It started with the most gravitational potential energy, a ...
1
vote
1answer
80 views

Help with my bouncy ball lab (I know the factors just not how to approach them) [closed]

In my physics lab we need to determine the factors that account for the energy "loss" during a high bounce ball bounce. I know that energy is "lost" (not really) to heat, air resistance, and sound. ...
1
vote
8answers
494 views

Why not use our own light production to produce new energy instead of wasting it?

Why don't we use our own light production at night (I mean home, buildings, streets,..., lighting) to charge photovoltaic panels instead of wasting it?
0
votes
0answers
171 views

How to debunk this perpetual motion machine? [duplicate]

I know it's impossible. I normally find the flaw behind the principle, but this one is hard to crack. The two wheels are geared together, so that no magnet on one wheel "backlashes" the wheel in a way ...
-2
votes
1answer
1k views

How to plot forcing ocillation with damping correctly? [closed]

The forced oscillation with damping is described as $$\frac{d^2x}{dt^2}+\gamma\frac{dx}{dt}+\omega_0^2x=\frac{F_0}{m}\cos{\omega t}$$ Its solution is $x=A\cos{(\omega t-\delta)}$, with $$A(\omega)=...