Questions tagged [dissipation]
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Greiner's Green's function for diffusion
I am reading Greiner's "Quantum Electrodynamics". In example 1.5 he derives the Green's function for diffusion. I am stuck on a step in the derivation.
He has the defining differential equation as
$$
...
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3answers
61 views
Why do energy transfers always result some heat loss?
explain that energy transfers and transformations in mechanical systems always result in some heat loss to the environment. Like why is it necessary for a heat loss?
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2answers
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How is the simple damper equation derived?
I know the spring is modeled as $F_{\text{elastic}} = k\cdot x$ when the displacements are small since this is empirically based, but what happens with $F_{\text{damping}}=c\cdot\dot{x}$? It is the ...
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2answers
91 views
Harmonic motion with friction [closed]
Consider an undamped mass and spring system with friction. The friction is not proportional to the velocity of the mass. If we solve the corresponding differential equation,the spring will oscillate ...
1
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1answer
67 views
Explanation of a spring using “Artistotle's law of motion”, $\vec{F}=m\vec{v}$
So I was watching Susskind's Classical Mechanics lectures and I didn't understand something in the second lecture.
He was telling about Aristotle's Law of motion which is $$\vec F = m\vec v.$$
He ...
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1answer
51 views
Exact form of the damped wave equation
The undamped wave equation has the standard form
\begin{equation*}
\frac{\partial^2 \psi}{\partial t^2}=c^2\nabla^2\psi
\end{equation*}
while the damped wave equation is frequenly found written in ...
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2answers
1k views
How much energy is wasted by a noisy refrigerator?
I recently bought a new refrigerator for my kitchen. The feet are adjustable but I've been lazy.
Whenever the motor runs and the feet aren't all touching the floor there is a loud buzzing noise. As ...
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1answer
27 views
Potential energy-Conservative forces
Does potential energy exist because conservative forces exist?
Can a system of objects that interact with dissipative forces have potential energy?
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3answers
55 views
Is there an intuitive reason the resistance that maximizes power dissipation in this simple circuit has a simple form?
Consider the following circuit:
A textbook problem[2] asks to find the resistance $R$ such that the power dissipated in $R$ is maximized (assuming $R_1$ and $R_2$ are fixed).
I found that $R$ should ...
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1answer
28 views
Damped mechanical wave
a string with density $\rho$ and tension $T$ is bound at it's two ends at $x=0$ and $x=L$. there is a force acting on the string proportional to the velocity $F(x,t)= -2\gamma \rho \dot \psi(x,t)$ ...
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3answers
55 views
If the molecular collisions are elastic will there be any dissipation in a fluid?
Viscosity arises due to collisions of the molecules of one layer of a fluid with another in contact. But viscosity is a dissipative element leading to heating and dissipation. Where does it heat come ...
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1answer
32 views
Airflow velocity and volume loss due to friction
I have a vacuum that the manufacturer claims capable of moving 20,000 cfm of air. In my application we often connect 150 feet of 6 inch diameter, smooth bore, flexible tubing to the vacuum. I know ...
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1answer
59 views
Energy dissipation for force-free, incompressible Navier-Stokes equation with tangent boundary conditions on bounded domains
I consider the Navier-Stokes equation for uniformly incompressible, force-free, Newtonian fluids with constant viscosity. The equations describing the situation are:
$\partial_tv-v\times \text{curl}(...
2
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1answer
59 views
Is the violation of time-reversal always associated with the violation of time-translation and vice-versa?
Is the violation of time-reversal symmetry always associated with the violation of time-translation symmetry? What about the converse? Is it possible for one to be violated while the other remaining ...
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2answers
70 views
Significance of the complex component in the underdamped harmonic motion equation [closed]
The following differential equation represents the motion of a body of mass $m$ and displacement $x$ from the mean position, that is attached to a spring of force constant $a$ and viscous damping ...
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1answer
35 views
Why do we need to have a low voltage drop over long distances in transmission of electricity? [closed]
It is said that AC is preferred over DC because of the negligible voltage drop over long distances. How does that work?
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2answers
51 views
Why does height affect bounce efficiency?
I recently conducted an experiment, where I dropped tennis balls from various heights and recorded their rebound heights. The higher the drop, the higher the rebound height, obviously. However, the ...
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0answers
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Motor/energy considerations
A motor is a device into which energy is transferred by electrical transmission while energy is transferred out by work.
we know that the power delivered by an emf source is P=I^2*R.
now coming to ...
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4answers
57 views
How do charges lose electrical potential energy when going through a resistor?
I can understand that the charges need to do work against the resistance - which transfers energy to forms such as light, heat, etcetera - using the electrostatic force provided by the battery ...
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2answers
94 views
Exact equation of exponential curves of underdamped harmonic motion
I was studying the underdamped harmonic motion and got curious about the fact that the decreasing exponentials $\pm Ae^{-\gamma t}$ are good approximations only for light damping $(\gamma<<\...
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4answers
131 views
Why do we use the RMS but not the fourth root mean quad?
Why do we use the power of $2$? What is the relation between this and having the same heat energy in both AC and DC?
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0answers
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Would it be appropriate to say that in presence of Non-Conservative Forces, the Entropy of System Always Increases?
Would it be appropriate to say that in Presence of Non Conservative Forces the Entropy of a system would always increase? Can we relate Non-Conservative Forces and Entropy in some way? I intuitively ...
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Numerical Simulation of Stochastic Master Equation using Stochastic Schrödinger Equation (Wave Function Monte Carlo)
Consider a time independent system coupled to a Markovian bath, the equation of motion for the density matrix of the system has to take the form
\begin{equation}
\dot{\rho} = - i \left[H,\rho\right] -...
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3answers
84 views
Do power supplies have to heat up?
Is there a fundamental reason why a power supply, say, from 120v AC to 12v DC, has to lose some power to heat dissipation?
I am not asking why current engineering solutions do; I'm aware that ...
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1answer
103 views
Maxwell's wheel and friction
We experimented with the Maxwell's wheel today and at the end we were asked about why does this apparatus stop since there is conservation of energy.
I did some research and apparently there is a type ...
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1answer
61 views
PT-symmetric hamiltonians: Transition from non-dissipative to dissipative system
It is known that Spectrum of a non-hermitian hamiltonian is complex ($E_n-i\gamma_n$) and they represent the dissipative system. eg, Damped harmonic oscillator(DHO), where $E_n$ are the energies of ...
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1answer
110 views
Noether's Theorem, Boundary-Bulk, and Open Thermodynamic Systems
Before going any further, I should emphasize that I know we cannot use the action principle for locally dissipative systems or even Noether's theorem for that matter. There are plenty of stackexchange ...
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1answer
313 views
How is the rate of decay of a Newton's Cradle affected by the number of spheres in the system?
How does the number of spheres in a Newton's Cradle affect its rate of decay? (assuming only 1 sphere is released initially)
From experiment, I observed that the rate of decay of a 5 sphere Newton's ...
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2answers
43 views
Force, work and the apparent disappearance of Mechanical Energy [duplicate]
A man exerts force on a wall of bricks. The man must have consumed the energy he possessed (mechanical energy?) to exert the force. The man sweats and tires himself out but the wall does not move.
...
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1answer
26 views
Large Damped Harmonic Oscillator misunderstanding
So I'm confused, here with what is highlighted. When the book says of "order $1/y_-$" you will reduce the displacement by a factor of $1/e$. Does of order mean when the time is equal to $1/y_-$, if ...
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0answers
26 views
Principle of least action for dissipative systems [duplicate]
I am trying to solve a problem with involves friction, and am having trouble solving it with $F=ma$. I am considering to use Lagrangian mechanics, but I am puzzled. There doesn't seem to be a ...
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2answers
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Can a water vortex (whirlpool) be used to efficiently store energy?
Either a large, constrained cylinder of water or a large volume of open water (perhaps in the ocean) could be persuaded to form a vortex by pumping energy in. The body of water would be acting as a ...
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3answers
99 views
When you have engine at idle and turn your car, where does the kinetic energy go?
When you turn the front wheels to the right for example, your car will tend to continue in its original forward direction. However, there is friction between the wheel and the ground that prevents it ...
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3answers
91 views
What happens to the heat generated in an irreversible process?
I know that a reversible process does more work than an irreversible process and that extra work is turned into heat. But what does that heat do? Does it increase the temperature?
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3answers
79 views
Kinetic Energy with constant velocity
I am sorry for the question, but I am a noob in Physics.
I don't understand why in the kinetic energy's formula the distance is out of the equation.
Kinetic Energy formula:
$$K = \frac{1}{2}m v^2 = ...
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4answers
97 views
In a simple battery + resistor circuit, what form of energy is lost from the electrons upon exiting the resistor?
I will give this question a little context. Firstly, as I understand it, as soon as I "close the switch" on a circuit, electric current pretty quickly establishes a steady state where, at any given ...
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1answer
103 views
Magnetic Turning Wheel [duplicate]
So a few years ago I came up with the idea of a magneticly powered wheel that could be attached to a generator and power it, it would not be infinite as over time magnets lose their power yet I think ...
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1answer
2k views
What is the electrical difference between rolled and unrolled extension cables? [closed]
I am talking about the extension cord of length 10 meter or more as follows.
The clerk said that it is better to unroll it when using. So what is the difference? I know for alternating current there ...
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0answers
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Potential gravitational energy versus potential spring energy [duplicate]
My textbook points out the following predicament, and I do not know the answer, despite a lot of thinking:
A spring is hanging of a ceiling and a mass is attached. As a result the spring descends to ...
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3answers
90 views
Where does the energy go if the current is induced in a perfect conductor?
Let us rotate a closed circuit made of a copper coil in front of a magnet. This will generate an induced current and all the mechanical energy will be converted into heat through Joule effect. If the ...
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2answers
127 views
Why is sound produced when we hit a metal? [duplicate]
We all know that metal are sonorous and produce sound on hitting but why is sound produced
I thought that it might be a way of energy released on inelastic collision but then why not heat as heat is ...
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1answer
50 views
Guitar string - feedback from the body
This is a question about the frequencies that remain on the string during the decay of vibrations.
As I understand it decay occurs because of the:
a- Internal damping of the string. This is due to ...
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2answers
99 views
Is there a fundamental reason why “dynamic energy storage” is lossy?
Consider springs and moving masses - both can be used to store energy, the spring via tension $E=\frac{1}{2}kx^2$ and the mass via kinetic energy $E=\frac{1}{2}mv^2$. But day to day experience tells ...
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0answers
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Definition of Ohmic bath and damping force
I have read the Wikipedia article on quantum dissipation where it is talking about the bath spectral function.
The bath spectral function provides a constraint in the choice of the coefficients $...
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1answer
41 views
Phase shift of damped mass on spring with moving source [closed]
I have a setup with a moving (upwards, downwards) board on which there is a spring mounted on the edge. The spring has a mass $m$ on its end and is damped in some sort of liquid. What methods could I ...
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1answer
30 views
Is there a temperature limit for the magneto caloric effect?
Suppose a thermally insulated box which has to keep its inner temperature below a certain limit, and suppose a high temperature outside of the box, such as a fire at 1000C.
Would it be possible to ...
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1answer
206 views
How does temperature affect sound wave dissipation in air?
Sound waves dissipate in any material such as air, but if temperature decreases or increases; how would the increase or decrease in temperature affect the dissipation?
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1answer
159 views
A damped harmonic oscillator is NOT a dissipative system?
I know this sounds rather insane, but it says so in my book.
The argument is the following:
Given a damped harmonic oscilator
$$\ddot{q}+\frac{b}{m}\dot{q}+\frac{k}{m}q=0 \tag1 $$
this system can be ...
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2answers
249 views
Why does a ball rolling without slipping stop due to friction? [duplicate]
I'm confused on a scenario. If we roll a ball on a horizontal surface and it rolls without slipping it should eventually stop due to friction. However rolling without slipping means the velocity at ...
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1answer
52 views
What is the energy dissipation of an ideal launch loop?
Dynamical structures for space access use a stream of electromagnetically accelerated and levitated pellets that moves faster than escape velocity to maintain megascale structures and transmit force ...
