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{{short description|Simplification of a physical system into a network of discrete components}}
{{Technical|date=August 2019}}
{{Refimprove|date=August 2023}}
[[File:Ohm's Law with Voltage source TeX.svg
The '''lumped-element model''' (also called '''lumped-parameter model''', or '''lumped-component model''')
== Electrical systems ==
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=== Lumped-matter discipline ===
The '''lumped-matter discipline''' is a set of imposed assumptions in [[electrical engineering]] that provides the foundation for '''lumped-circuit abstraction''' used in [[Network analysis (electrical circuits)|network analysis]].<ref>Anant Agarwal and Jeffrey Lang, course materials for 6.002 Circuits and Electronics, Spring 2007. MIT OpenCourseWare ([http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-002-circuits-and-electronics-spring-2007/video-lectures/6002_l1.pdf PDF]), [[Massachusetts Institute of Technology]].</ref> The self-imposed constraints are:
# The change of the magnetic flux in time outside a conductor is zero. <math display="block">\frac{\partial \
# The change of the charge in time inside conducting elements is zero. <math display="block">\frac{\partial q} {\partial t} = 0</math>
# Signal timescales of interest are much larger than [[propagation delay]] of [[electromagnetic waves]] across the lumped element.
The first two assumptions result in [[Kirchhoff's circuit laws]] when applied to [[Maxwell's equations]] and are only applicable when the circuit is in [[steady state (electronics)|steady state]]. The third assumption is the basis of the lumped-element model used in [[Network analysis (electrical circuits)|network analysis]]. Less severe assumptions result in the [[distributed-element model]], while still not requiring the direct application of the full Maxwell equations.
=== Lumped-element model ===
The lumped-element model of electronic [[Electrical network|
The lumped-element model is valid whenever <math>L_c \ll \lambda</math>, where <math>L_c</math> denotes the circuit's characteristic length, and <math>\lambda</math> denotes the circuit's operating [[wavelength]]. Otherwise, when the circuit length is on the order of a wavelength, we must consider more general models, such as the [[distributed-element model]] (including [[transmission line]]s), whose dynamic behaviour is described by [[Maxwell's equations]]. Another way of viewing the validity of the lumped-element model is to note that this model ignores the finite time it takes signals to propagate around a circuit. Whenever this propagation time is not significant to the application the lumped-element model can be used. This is the case when the propagation time is much less than the [[period (physics)|period]] of the signal involved. However, with increasing propagation time there will be an increasing error between the assumed and actual phase of the signal which in turn results in an error in the assumed amplitude of the signal. The exact point at which the lumped-element model can no longer be used depends to a certain extent on how accurately the signal needs to be known in a given application.
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Real-world components exhibit non-ideal characteristics which are, in reality, distributed elements but are often represented to a [[first-order approximation]] by lumped elements. To account for leakage in [[capacitor]]s for example, we can model the non-ideal capacitor as having a large lumped [[resistor]] connected in parallel even though the leakage is, in reality distributed throughout the dielectric. Similarly a [[wire-wound resistor]] has significant [[inductance]] as well as [[Electrical resistance|resistance]] distributed along its length but we can model this as a lumped [[inductor]] in series with the ideal resistor.
== Thermal systems ==
A '''lumped-capacitance model''', also called '''lumped system analysis''',<ref>{{cite book | last = Incropera |author2=DeWitt |author3=Bergman |author4=Lavine | title = Fundamentals of Heat and Mass Transfer | url = https://archive.org/details/fundamentalsheat00incr_869 | url-access = limited | edition = 6th | year = 2007 | isbn = 978-0-471-45728-2 | publisher = John Wiley & Sons | pages = [https://archive.org/details/fundamentalsheat00incr_869/page/n267 260]–261}}</ref> reduces a [[thermal system]] to a number of discrete “lumps” and assumes that the [[temperature]] difference inside each lump is negligible. This approximation is useful to simplify otherwise complex [[differential equation|differential]] heat equations. It was developed as a mathematical analog of [[electrical capacitance]], although it also includes thermal analogs of [[electrical resistance]] as well.
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An early-discovered example of a lumped-capacitance system which exhibits mathematically simple behavior due to such physical simplifications, are systems which conform to ''Newton's law of cooling''. This law simply states that the temperature of a hot (or cold) object progresses toward the temperature of its environment in a simple exponential fashion. Objects follow this law strictly only if the rate of heat conduction within them is much larger than the heat flow into or out of them. In such cases it makes sense to talk of a single "object temperature" at any given time (since there is no spatial temperature variation within the object) and also the uniform temperatures within the object allow its total thermal energy excess or deficit to vary proportionally to its surface temperature, thus setting up the Newton's law of cooling requirement that the rate of temperature decrease is proportional to difference between the object and the environment. This in turn leads to simple exponential heating or cooling behavior (details below).
=== Method ===
To determine the number of lumps, the [[Biot number]] (Bi), a dimensionless parameter of the system, is used. Bi is defined as the ratio of the conductive heat resistance within the object to the [[Convection (heat transfer)|convective heat transfer]] resistance across the object's boundary with a uniform bath of different temperature. When the [[thermal resistance]] to heat transferred into the object is larger than the resistance to heat being [[diffused]] completely within the object, the Biot number is less than 1. In this case, particularly for Biot numbers which are even smaller, the approximation of ''spatially uniform temperature within the object'' can begin to be used, since it can be presumed that heat transferred into the object has time to uniformly distribute itself, due to the lower resistance to doing so, as compared with the resistance to heat entering the object.
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The equations describing the three heat transfer modes and their thermal resistances in steady state conditions, as discussed previously, are summarized in the table below:
{| class="wikitable" style="margin:1em auto; text-align:center;"▼
▲{| class="wikitable" style="text-align:center;"
|+Equations for different heat transfer modes and their thermal resistances.
|-
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|Radiation
|<math>\dot{Q}=\frac{T_{\rm surf}-T_{\rm surr}}{\left ( \frac{1}{h_rA_{\rm surf}} \right )}</math>
|<math>\frac{1}{h_rA}</math>, where<br /><math>h_r= \epsilon \sigma (T_{\rm surf}^{2}+T_{\rm surr}^{2})(T_{\rm surf}+T_{\rm surr})</math>
|}
In cases where there is heat transfer through different media (for example, through a [[composite material]]), the equivalent resistance is the sum of the resistances of the components that make up the composite. Likely, in cases where there are different heat transfer modes, the total resistance is the sum of the resistances of the different modes. Using the thermal circuit concept, the amount of heat transferred through any medium is the quotient of the temperature change and the total thermal resistance of the medium.
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where <math>R_i=\frac{1}{h_iA}</math>, <math>R_o=\frac{1}{h_oA}</math>, <math>R_1=\frac{L_1}{k_1A}</math>, and <math>R_2=\frac{L_2}{k_2A}</math>
==== Newton's law of cooling ====
{{Main|Newton's law of cooling}}
'''Newton's law of cooling''' is an [[empirical relationship]] attributed to English physicist [[Isaac Newton|Sir Isaac Newton]] (
{{Quotation|The rate of heat loss of a body is proportional to the temperature difference between the body and its surroundings.}}
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<math display="block">\text {Rate of cooling} \sim \Delta T</math>
An object at a different temperature from its surroundings will ultimately come to a common temperature with its surroundings. A relatively hot object cools as it warms its surroundings; a cool object is warmed by its surroundings. When considering how quickly (or slowly) something cools, we speak of its ''rate'' of cooling
The rate of cooling of an object depends on how much hotter the object is than its surroundings. The temperature change per minute of a hot apple pie will be more if the pie is put in a cold freezer than if it is placed on the kitchen table. When the pie cools in the freezer, the temperature difference between it and its surroundings is greater
As Newton's law of cooling states, the rate of cooling of an object
==== Applicable situations ====
This law describes many situations in which an object has a large thermal capacity and large conductivity, and is suddenly immersed in a uniform bath which conducts heat relatively poorly. It is an example of a thermal circuit with one resistive and one capacitative element. For the law to be correct, the temperatures at all points inside the body must be approximately the same at each time point, including the temperature at its surface. Thus, the temperature difference between the body and surroundings does not depend on which part of the body is chosen, since all parts of the body have effectively the same temperature. In these situations, the material of the body does not act to "insulate" other parts of the body from heat flow, and all of the significant insulation (or "thermal resistance") controlling the rate of heat flow in the situation resides in the area of contact between the body and its surroundings. Across this boundary, the temperature-value jumps in a discontinuous fashion.
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In such a situation, the object acts as the "capacitative" circuit element, and the resistance of the thermal contact at the boundary acts as the (single) thermal resistor. In electrical circuits, such a combination would charge or discharge toward the input voltage, according to a simple exponential law in time. In the thermal circuit, this configuration results in the same behavior in temperature: an exponential approach of the object temperature to the bath temperature.
==== Mathematical statement ====
Newton's law is mathematically stated by the simple first-order differential equation:
<math display="block"> \frac{d Q}{d t} = - h \cdot A(T(t)- T_{\text{env}}) = - h \cdot A \Delta T(t)</math>
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<math display="block"> \frac{d T(t)}{d t} = \frac{d \Delta T(t)}{d t} = - \frac{1}{t_0} \Delta T(t) </math>
=== Applications ===
This mode of analysis has been applied to [[forensic science]]s to analyze the time of death of humans. Also, it can be applied to [[HVAC]] (heating, ventilating and air-conditioning, which can be referred to as "building climate control"), to ensure more nearly instantaneous effects of a change in comfort level setting.<ref>Heat Transfer
== Mechanical systems ==
The simplifying assumptions in this ___domain are:
* all objects are [[rigid body|rigid bodies]];
* all interactions between rigid bodies take place via [[kinematic pair]]s (''joints''), [[spring (device)|
== Acoustics ==
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A simplifying assumption in this ___domain is that all heat transfer mechanisms are linear, implying that radiation and convection are linearised for each problem.
Several publications can be found that describe how to generate lumped-element models of buildings. In most cases, the building is considered a single thermal zone and in this case, turning multi-layered walls into lumped elements can be one of the most complicated tasks in the creation of the model. The dominant-layer method is one simple and reasonably accurate method.<ref>Ramallo-González, A.P., Eames, M.E. & Coley, D.A., 2013. Lumped Parameter Models for Building Thermal Modelling: An Analytic approach to simplifying complex multi-layered constructions. Energy and Buildings, 60, pp.174-184.</ref> In this method, one of the layers is selected as the dominant layer in the whole construction, this layer is chosen considering the most relevant frequencies of the problem.
Lumped-element models of buildings have also been used to evaluate the efficiency of domestic energy systems, by running many simulations under different future weather scenarios.<ref>Cooper, S.J.G., Hammond, G.P., McManus, M.C., Ramallo-Gonzlez, A. & Rogers, J.G., 2014. Effect of operating conditions on performance of domestic heating systems with heat pumps and fuel cell micro-cogeneration. Energy and Buildings, 70, pp.52-60.</ref>
== Fluid systems ==
== See also ==
* [[Isomorphism#System isomorphism|System isomorphism]]
* [[Model order reduction]]
== References ==
{{Reflist}}
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