# What is Dissipation: Definition and 184 Discussions

In thermodynamics, dissipation is the result of an irreversible process that takes place in homogeneous thermodynamic systems. In a dissipative process, energy (internal, bulk flow kinetic, or system potential) transforms from an initial form to a final form, where the capacity of the final form to do mechanical work is less than that of the initial form. For example, heat transfer is dissipative because it is a transfer of internal energy from a hotter body to a colder one. Following the second law of thermodynamics, the entropy varies with temperature (reduces the capacity of the combination of the two bodies to do mechanical work), but never decreases in an isolated system.
These processes produce entropy at a certain rate. The entropy production rate times ambient temperature gives the dissipated power. Important examples of irreversible processes are: heat flow through a thermal resistance, fluid flow through a flow resistance, diffusion (mixing), chemical reactions, and electrical current flow through an electrical resistance (Joule heating).

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1. ### Calculating of the heat dissipation of a LED

Hello Forum, I write you with the challenge of calculation the heat dissipation of a LED. The LED in question is http://www.seoulsemicon.com/en/product/spec/SZ5-M3-W0-00/22 a download of the datasheet is possible. The LED binning is 290lm (W4, 5000K) at 700mA, the efficacy is...
2. ### Thermal Dissipation of High-Power Electronic Subassemblies

High-power electronic subassemblies – housing CPU or power-conversion semiconductors, for instance – require significant thermal dissipation to keep their chip-junction temperatures at or below their maximum operating temperature. (As a rule-of-thumb, every 10oC increase in junction temperature...
3. ### A Dissipation function is homogeneous in ##\dot{q}## second degree proof

We have Rayleigh's dissipation function, defined as ## \mathcal{F}=\frac{1}{2} \sum_{i}\left(k_{x} v_{i x}^{2}+k_{y} v_{i j}^{2}+k_{z} v_{i z}^{2}\right) ## Also we have transformation equations to generalized coordinates as ##\begin{aligned} \mathbf{r}_{1} &=\mathbf{r}_{1}\left(q_{1}, q_{2}...
4. ### I Water Waves Over Obstacles: Higher Frequencies Grow, Not Decay

In general, it seems that higher frequencies of a wave dissipate more than lower frequencies. For sound waves, it explains why you can hear lower pitches from farther away. For a vibrating string or plate, the higher frequencies also dissipate first, with the fundamental fading last. For water...
5. ### Two definitions of material dissipation factor

Hi, I have seen two versions of definitions of material dissipation factor Df: The first one: Dk (dielectric constant) = K = relative permittivity = ɛ -jɛ’ , ɛ = energy stored and ɛ’ = energy lost. Df (dissipation factor/loss tangent) = ratio of ɛ’ and ɛ The second one: On slide 8 of webinar...
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19. ### Computational Fluid Dynamics: Dissipation Function Density Based Solver

((((As a correction dissipation function in picture should have square of divergence of U)))) Hi, first of all I am aware that we have to discretize the non linear navier stokes equations to reach the almost exact solution, and pressure based or density based algorithms are deployed for that...
20. ### Resistive dissipation and Ohm's law

Homework Statement A potential difference V is connected across a device with resistance R, causing current i through the device. Rank the following variations according to the change in the rate at which electrical energy is converted to thermal energy due to the resistance, greatest change...
21. ### Problem with Energy Dissipation

Homework Statement A 4.70-kg steel ball is dropped from a height of 11.0 m into a box of sand and sinks 0.500 m into the sand before stopping. How much energy is dissipated through the interaction with the sand? Express answer using three significant digits. Homework Equations Potential...
22. ### I General Question about Dissipation of Energy

Conceptual question here: In the case of something that dissipates energy, how do we describe the transition between the range along which the system can continue to dissipate energy more or less in the same way and the point at which, given increasing temperature, the system can no longer...
23. ### Resistance: Dissipated power in collision model

Hi. A simple model explains resistance in metals with collisions of the electrons with the stationary atomic cores. So I assume more collisions result in a higher resistance? But for the dissipated power we have ##P=U^2/R## , which is large for small resistance. I have difficulties combining...
24. ### Answer: Battery Driven Winch - 13.5 m Lift Height

Homework Statement A standard D cell can supply 10mA at 1.5 volts for about 300 hours. An alkaline D cell can do about the same. Assume - 2 - that the chemistry of the batteries will produce the same amount of energy and will maintain the same EMF regardless of the current drawn from the...
25. ### I The difference between capillary number and Laplace number?

It seems that both the capillary number and the Laplace number represents the relative effect of viscous forces and surface tension. Can anybody explain the difference between them? Thanks a lot. I have attach the Wikipedia- links to these two concepts...
26. ### I The typical and the exceptional in physics

For properly normalized extensive macroscopic properties (and this includes the center of mass operator), there is such a proof in many treatises of statistical mechanics. It is the quantum analogue of the system size expansion for classical stochastic processes. For example, see Theorem 9.3.3...
27. ### How to find power dissipation of a resistor in this circuit?

Here is an image with information: https://i.snag.gy/UMCRig.jpg I have calculated the total power dissipation as 11W. What I don't see is why R1 has a higher dissipation rate than R2/R3, and my main question is how do I calculate the power dissipation of R1? I get that R2/R3 are in series so...
28. ### A resistor with current flowing but no dissipation

If an amplifier (audio or RF) has the collector of a transistor for its output electrode and assuming the transistor operates in its active region (not saturated or cut off) the output impedance of the amplifier will be high (several 10s of kilohms). If now a tuned circuit is connected as a load...
29. ### Formula for determining the heat dissipation of a radiator

Are there any equations out there which can calculate the heat dissipation of a water-cooled radiator with a fan on it? I would assume some of the variables would include: -Volume of the radiator (240 x 199 x 45mm)=2149200mm^3 -Specific heat of water (4.187 kJ/kgK) -Specific heat of the copper...
30. ### How Does Perspiration Help Regulate Body Temperature?

I'm not sure this is the best fourm to post this, but... How much heat, preferably in watts, does perspiration dissipate under mild exertion (i.e. walking) on a 90° F day? Assume the individual is hydrated and not starving. Also assume they weigh 180Lbs. How much heat would have to be...
31. ### IP 65 rated enclosure with heat dissipation

Hi there! Is there any ip65 rated enclosure material with electronic components inside but with no holes/cutouts that can act as a heat sink without overheating while the product is operating? Enclosure will need to be wall-mounted outdoors, it could be made out of die-cast aluminium which...
32. ### Energy dissipation when a lift stops

Lifts/elevators of traditional design (i.e. not of the "gearless" type, like Kone's EcoDisc ®) are driven by a hoist that uses a worm drive gear between the electric motor and the sheave. The drum brake is always mounted on the motor shaft (high speed). As you know, worm drive gears are (except...
33. ### Fluctuation Dissipation Theorem

Homework Statement We have a system of two coupled Langevin equations dr/dt=kr-yrr+nr(t) dp/dt=kpr-ypp+np(t) where the ki,yi are constants and ni(t) are noise terms satisfying <ni(t)>=0 and <ni(t')ni(t'')>=qiδ(t'-t'') (this is zero if the two indices differ). The physical background of these...
34. ### Discretizing a Fluctuation Dissipation Theorem

Hey! I want to discretize a fluctuation dissipation theorem for the white noise ζ of a stochastic differential equation on a 2D domain (sphere). For that I integrate over "Finite Volume" elements with area A and A' (see below). \begin{eqnarray*} \int_{A} d A \int_{A'} d A'...
35. ### Alter resistance for max power dissipation?

1. The problem Given a circuit with a 30V source and five resistors. The circuit can be drawn as two boxes. Resistor 1 (R1 = 300ohm) is in series with the source. R2 (300ohm) in the middle of the two boxes, and the remaining resistors (550 ohm) in series at the other side of the box. The...
36. ### Power dissipated by resistor -- Right answer, but need insight

imgur: http://i.imgur.com/9VILpYL.jpg Now, I can easily solve this by applying the formula P_R = \frac{(\Delta V_R)^2}{R} and get all the correct answers. However, the answers feel strange to me intuitively. In the formula above, lowering the resistance increases the power that is dissipated...
37. ### Supply rate = dissipation rate in Turbulence

So in many books, authors state that the supply rate, u^3/l is proportional to the dissipation rate ɛ, in a turbulent flow i.e: u^3/l ~ ɛ where u and l are characteristic velocity and length scales of the large eddies. Many will also state that the small scale eddies have a time period or...
38. ### Power dissipation in capacitor circuit

Capacitor connected to a battery of certain internal resistance. If it is asked to calculate what is power dissipated as heat, Does -the voltage across capacitor X current through capacitor- gives heat dissipated or should we add heat dissipated in battery by considering its internal resistance.
39. ### Energy dissipation in a travelling EM wave

How is energy dissipated in a traveling electromagnetic wave ? Will there be any dissipation if it were to travel through vacuum ?
40. ### Smoke Rising in Still Air: Physics Explanation

Hi all, I have what I believe is a simple physics question. I was burning a sprig of incense in my apartment, and noticed that the smoke was rising from the burning section perfectly straight up into the air for a good meter, and then started to dissipate into the air. I know that a gas will...
41. ### Why Does Increasing the Cross-Sectional Area of a Wire Reduce Heat Dissipation?

The power dissipated in a resistor of resistance ##R## with current ##I## passing through it is ##I^2 R##. We can write ##R## as ##\rho \frac{L}{A}## where ##L## is the length of the wire and ##A## is the cross sectional area of the wire. Clearly, increasing ##A## decreases ##R##. Why is less...
42. ### Energy dissipation of steel washers

Is there an optimum number of washers to be used on a threaded bolt in respect to dissipation of energy towards the nut? And how would one calculate the dissipation rate of each washer?
43. ### Wave Power Dissipation: Solving Pozar's Book Problem

Hello Here is a problem of Pozar's Book(mircowave). and here is solution: my problem is I don't know how he obtain -40.6 because |T|^2 (T is Reflection Coefficient ) is return power and 1-|T|^2 is transfer powe but 10log none of them return -40.6 , what does -40.6 and how can calculate it?
44. ### Calculating Fan Size: Not Driven by Motor

Hey, I'm trying to find a fan that will dissipate 1471.5W. The fan is going to be driven via a belt connected to a driving spool and not driven by an electric motor. How do i go about calculating the required fan size? Is there anywhere i can look at fan specifications for fans that are not...
45. ### Aluminum structure modifications for use near a very hot fire

I'm building an frame out of solid 5/8 inch thick aluminum square bars. this frame will be around a fire temps close to melting points but there will be insulation in make sure it won't melt. I was thinking about drilling holes in the free space of the aluminum to reduce it's weight and if you...
46. ### Power dissipated by a resistor in combined citcuit

Find the power dissipated by R2 given that V a-b=120 volts, R1=1, R2=3.33, R3=2, and R4=3 I've worked this problem, just want to confirm I worked it correctly. A---------R1-----------*------------------- | |...
47. ### Dissipation function in fluctuation dissipation theory

Hello every: Actually, there are 2 questions. The first one is how to understand the dissipation function used in the fluctuation dissipation theory. I notice that for a dielectric material with a complex dielectric function, this dissipation function will be the imaginary part of the...
48. ### How Can Turbulence Dissipation Rate Be Independent of Viscosity?

I've been reading about dissipation of energy in turbulent flows. I've read in various places about Richardson's idea of Energy cascade. But one of the biggest problems I have is understanding how the books (or texts online) all refer to ϵ, the rate of dissipation of kinetic energy, being...
49. ### Fluid Mechanics Viscous Dissipation

I am trying to find an expression for viscous dissipation for burger's vortex Velocity field which only has velocity component in the V theta direction. I'm confused as to which equation for viscosity dissipation is correct. I am seeing a lot of different things tau:S, tau:delV..which one is...
50. ### Power Dissipation along power lines and Resistance value

Hello Forum, It is well known that the electrical power dissipated (in the form of heat) by a resistor of resistance R connected to a battery of voltage V is: P_diss = (V^2)/ R or equivalently P= (I^2)*(R) Clearly the smaller the resistance R, the more the dissipated power because R is at...