# Lumped element model

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The lumped element model (also called lumped parameter model, or lumped component model) simplifies the description of the behaviour of spatially distributed physical systems into a topology consisting of discrete entities that approximate the behaviour of the distributed system under certain assumptions. It is useful in electrical systems (including electronics), mechanical multibody systems, heat transfer, acoustics, etc.

The topology of an electronic circuit is the form taken by the network of interconnections of the circuit components. Different specific values or ratings of the components are regarded as being the same topology. Topology is not concerned with the physical layout of components in a circuit, nor with their positions on a circuit diagram, similarly to the mathematic concept of topology it is only concerned with what connections exist between the components. There may be numerous physical layouts and circuit diagrams that all amount to the same topology.

An electrical network is an interconnection of electrical components or a model of such an interconnection, consisting of electrical elements. An electrical circuit is a network consisting of a closed loop, giving a return path for the current. Linear electrical networks, a special type consisting only of sources, linear lumped elements, and linear distributed elements, have the property that signals are linearly superimposable. They are thus more easily analyzed, using powerful frequency domain methods such as Laplace transforms, to determine DC response, AC response, and transient response.

Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter.

## Contents

Mathematically speaking, the simplification reduces the state space of the system to a finite dimension, and the partial differential equations (PDEs) of the continuous (infinite-dimensional) time and space model of the physical system into ordinary differential equations (ODEs) with a finite number of parameters.

In physics and mathematics, the dimension of a mathematical space is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it – for example, the point at 5 on a number line. A surface such as a plane or the surface of a cylinder or sphere has a dimension of two because two coordinates are needed to specify a point on it – for example, both a latitude and longitude are required to locate a point on the surface of a sphere. The inside of a cube, a cylinder or a sphere is three-dimensional because three coordinates are needed to locate a point within these spaces.

In mathematics, a partial differential equation (PDE) is a differential equation that contains unknown multivariable functions and their partial derivatives. PDEs are used to formulate problems involving functions of several variables, and are either solved by hand, or used to create a computer model. A special case is ordinary differential equations (ODEs), which deal with functions of a single variable and their derivatives.

In mathematics, an ordinary differential equation (ODE) is a differential equation containing one or more functions of one independent variable and the derivatives of those functions. The term ordinary is used in contrast with the term partial differential equation which may be with respect to more than one independent variable.

## Electrical systems

### 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. [1] The self-imposed constraints are:

Electrical engineering is a technical discipline concerned with the study, design and application of equipment, devices and systems which use electricity, electronics, and electromagnetism. It emerged as an identified activity in the latter half of the 19th century after commercialization of the electric telegraph, the telephone, and electrical power generation, distribution and use.

A network, in the context of electronics, is a collection of interconnected components. Network analysis is the process of finding the voltages across, and the currents through, all network components. There are many techniques for calculating these values. However, for the most part, the techniques assume linear components. Except where stated, the methods described in this article are applicable only to linear network analysis.

1. The change of the magnetic flux in time outside a conductor is zero.

${\displaystyle {\frac {\partial \phi _{B}}{\partial t}}=0}$

2. The change of the charge in time inside conducting elements is zero.

${\displaystyle {\frac {\partial q}{\partial t}}=0}$

3. 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. The third assumption is the basis of the lumped element model used in network analysis. Less severe assumptions result in the distributed element model, while still not requiring the direct application of the full Maxwell equations.

Kirchhoff's circuit laws are two equalities that deal with the current and potential difference in the lumped element model of electrical circuits. They were first described in 1845 by German physicist Gustav Kirchhoff. This generalized the work of Georg Ohm and preceded the work of James Clerk Maxwell. Widely used in electrical engineering, they are also called Kirchhoff's rules or simply Kirchhoff's laws. These laws can be applied in time and frequency domains and form the basis for network analysis.

Maxwell's equations are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric circuits. The equations provide a mathematical model for electric, optical, and radio technologies, such as power generation, electric motors, wireless communication, lenses, radar etc. Maxwell's equations describe how electric and magnetic fields are generated by charges, currents, and changes of the fields. An important consequence of the equations is that they demonstrate how fluctuating electric and magnetic fields propagate at a constant speed (c) in a vacuum. Known as electromagnetic radiation, these waves may occur at various wavelengths to produce a spectrum of light from radio waves to γ-rays. The equations are named after the physicist and mathematician James Clerk Maxwell, who between 1861 and 1862 published an early form of the equations that included the Lorentz force law. Maxwell first used the equations to propose that light is an electromagnetic phenomenon.

In electronics, steady state is an equilibrium condition of a circuit or network that occurs as the effects of transients are no longer important.

### Lumped element model

The lumped element model of electronic circuits makes the simplifying assumption that the attributes of the circuit, resistance, capacitance, inductance, and gain, are concentrated into idealized electrical components; resistors, capacitors, and inductors, etc. joined by a network of perfectly conducting wires.

Capacitance is the ratio of the change in an electric charge in a system to the corresponding change in its electric potential. There are two closely related notions of capacitance: self capacitance and mutual capacitance. Any object that can be electrically charged exhibits self capacitance. A material with a large self capacitance holds more electric charge at a given voltage than one with low capacitance. The notion of mutual capacitance is particularly important for understanding the operations of the capacitor, one of the three elementary linear electronic components.

In electromagnetism and electronics, inductance describes the tendency of an electrical conductor, such as coil, to oppose a change in the electric current through it. When an electric current flows through a conductor, it creates a magnetic field around that conductor. A changing current, in turn, creates a changing magnetic field. From Faraday's law of induction, any change in total magnetic field through a circuit induces an electromotive force (voltage) across that circuit, a phenomenon known as electromagnetic induction. From Lenz's law, this induced voltage, or "back EMF" in a circuit, will be in a direction so as to oppose the change in current which created it. So changes in current through a conductor will react back on the conductor itself through its magnetic field, creating a reverse voltage which will oppose any change to the current. Inductance, , is defined as the ratio between this induced voltage, , and the rate of change of the current in the circuit.

In electronics, gain is a measure of the ability of a two-port circuit to increase the power or amplitude of a signal from the input to the output port by adding energy converted from some power supply to the signal. It is usually defined as the mean ratio of the signal amplitude or power at the output port to the amplitude or power at the input port. It is often expressed using the logarithmic decibel (dB) units. A gain greater than one, that is amplification, is the defining property of an active component or circuit, while a passive circuit will have a gain of less than one.

The lumped element model is valid whenever ${\displaystyle L_{c}\ll \lambda }$, where ${\displaystyle L_{c}}$ denotes the circuit's characteristic length, and ${\displaystyle \lambda }$ 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 lines), 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 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.

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 capacitors 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 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, [2] 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 heat equations. It was developed as a mathematical analog of electrical capacitance, although it also includes thermal analogs of electrical resistance as well.

The lumped capacitance model is a common approximation in transient conduction, which may be used whenever heat conduction within an object is much faster than heat transfer across the boundary of the object. The method of approximation then suitably reduces one aspect of the transient conduction system (spatial temperature variation within the object) to a more mathematically tractable form (that is, it is assumed that the temperature within the object is completely uniform in space, although this spatially uniform temperature value changes over time). The rising uniform temperature within the object or part of a system, can then be treated like a capacitative reservoir which absorbs heat until it reaches a steady thermal state in time (after which temperature does not change within it).

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 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.

If the Biot number is less than 0.1 for a solid object, then the entire material will be nearly the same temperature with the dominant temperature difference will be at the surface. It may be regarded as being "thermally thin". The Biot number must generally be less than 0.1 for usefully accurate approximation and heat transfer analysis. The mathematical solution to the lumped system approximation gives Newton's law of cooling.

A Biot number greater than 0.1 (a "thermally thick" substance) indicates that one cannot make this assumption, and more complicated heat transfer equations for "transient heat conduction" will be required to describe the time-varying and non-spatially-uniform temperature field within the material body.

The single capacitance approach can be expanded to involve many resistive and capacitive elements, with Bi < 0.1 for each lump. As the Biot number is calculated based upon a characteristic length of the system, the system can often be broken into a sufficient number of sections, or lumps, so that the Biot number is acceptably small.

Some characteristic lengths of thermal systems are:

For arbitrary shapes, it may be useful to consider the characteristic length to be volume / surface area.

#### Thermal purely resistive circuits

A useful concept used in heat transfer applications once the condition of steady state heat conduction has been reached, is the representation of thermal transfer by what is known as thermal circuits. A thermal circuit is the representation of the resistance to heat flow in each element of a circuit, as though it were an electrical resistor. The heat transferred is analogous to the electric current and the thermal resistance is analogous to the electrical resistor. The values of the thermal resistance for the different modes of heat transfer are then calculated as the denominators of the developed equations. The thermal resistances of the different modes of heat transfer are used in analyzing combined modes of heat transfer. The lack of "capacitative" elements in the following purely resistive example, means that no section of the circuit is absorbing energy or changing in distribution of temperature. This is equivalent to demanding that a state of steady state heat conduction (or transfer, as in radiation) has already been established.

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:

Equations for different heat transfer modes and their thermal resistances.
Transfer ModeRate of Heat TransferThermal Resistance
Conduction${\displaystyle {\dot {Q}}={\frac {T_{1}-T_{2}}{\left({\frac {L}{kA}}\right)}}}$${\displaystyle {\frac {L}{kA}}}$
Convection${\displaystyle {\dot {Q}}={\frac {T_{\rm {surf}}-T_{\rm {envr}}}{\left({\frac {1}{h_{\rm {conv}}A_{\rm {surf}}}}\right)}}}$${\displaystyle {\frac {1}{h_{\rm {conv}}A_{\rm {surf}}}}}$
Radiation${\displaystyle {\dot {Q}}={\frac {T_{\rm {surf}}-T_{\rm {surr}}}{\left({\frac {1}{h_{r}A_{\rm {surf}}}}\right)}}}$${\displaystyle {\frac {1}{h_{r}A}}}$, where
${\displaystyle h_{r}=\epsilon \sigma (T_{\rm {surf}}^{2}+T_{\rm {surr}}^{2})(T_{\rm {surf}}+T_{\rm {surr}})}$

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.

As an example, consider a composite wall of cross-sectional area ${\displaystyle A}$. The composite is made of an ${\displaystyle L_{1}}$ long cement plaster with a thermal coefficient ${\displaystyle k_{1}}$ and ${\displaystyle L_{2}}$ long paper faced fiber glass, with thermal coefficient ${\displaystyle k_{2}}$. The left surface of the wall is at ${\displaystyle T_{i}}$ and exposed to air with a convective coefficient of ${\displaystyle h_{i}}$. The right surface of the wall is at ${\displaystyle T_{o}}$ and exposed to air with convective coefficient ${\displaystyle h_{o}}$.

Using the thermal resistance concept, heat flow through the composite is as follows:

${\displaystyle {\dot {Q}}={\frac {T_{i}-T_{o}}{R_{i}+R_{1}+R_{2}+R_{o}}}={\frac {T_{i}-T_{1}}{R_{i}}}={\frac {T_{i}-T_{2}}{R_{i}+R_{1}}}={\frac {T_{i}-T_{3}}{R_{i}+R_{1}+R_{2}}}={\frac {T_{1}-T_{2}}{R_{1}}}={\frac {T_{3}-T_{o}}{R_{0}}}}$

where

${\displaystyle R_{i}={\frac {1}{h_{i}A}}}$, ${\displaystyle R_{o}={\frac {1}{h_{o}A}}}$, ${\displaystyle R_{1}={\frac {L_{1}}{k_{1}A}}}$, and ${\displaystyle R_{2}={\frac {L_{2}}{k_{2}A}}}$

#### Newton's law of cooling

Newton's law of cooling is an empirical relationship attributed to English physicist Sir Isaac Newton (1642 - 1727). This law stated in non-mathematical form is the following:

The rate of heat loss of a body is proportional to the temperature difference between the body and its surroundings.

Or, using symbols:

${\displaystyle {\text{Rate of cooling}}\sim \!\,\Delta T}$

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 - how many degrees' change in temperature per unit of time.

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. On a cold day, a warm home will leak heat to the outside at a greater rate when there is a large difference between the inside and outside temperatures. Keeping the inside of a home at high temperature on a cold day is thus more costly than keeping it at a lower temperature. If the temperature difference is kept small, the rate of cooling will be correspondingly low.

As Newton's law of cooling states, the rate of cooling of an object - whether by conduction, convection, or radiation - is approximately proportional to the temperature difference ΔT. Frozen food will warm up faster in a warm room than in a cold room. Note that the rate of cooling experienced on a cold day can be increased by the added convection effect of the wind. This is referred to as wind chill. For example, a wind chill of -20 °C means that heat is being lost at the same rate as if the temperature were -20 °C without wind.

#### 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.

In such situations, heat can be transferred from the exterior to the interior of a body, across the insulating boundary, by convection, conduction, or diffusion, so long as the boundary serves as a relatively poor conductor with regard to the object's interior. The presence of a physical insulator is not required, so long as the process which serves to pass heat across the boundary is "slow" in comparison to the conductive transfer of heat inside the body (or inside the region of interest—the "lump" described above).

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:

${\displaystyle {\frac {dQ}{dt}}=-h\cdot A(T(t)-T_{\text{env}})=-h\cdot A\Delta T(t)\quad }$

where

Q is thermal energy in joules
h is the heat transfer coefficient between the surface and the fluid
A is the surface area of the heat being transferred
T is the temperature of the object's surface and interior (since these are the same in this approximation)
Tenv is the temperature of the environment
ΔT(t) = T(t) - Tenv is the time-dependent thermal gradient between environment and object

Putting heat transfers into this form is sometimes not a very good approximation, depending on ratios of heat conductances in the system. If the differences are not large, an accurate formulation of heat transfers in the system may require analysis of heat flow based on the (transient) heat transfer equation in nonhomogeneous or poorly conductive media.

#### Solution in terms of object heat capacity

If the entire body is treated as lumped capacitance heat reservoir, with total heat content which is proportional to simple total heat capacity ${\displaystyle C}$, and ${\displaystyle T}$, the temperature of the body, or ${\displaystyle Q=CT}$. It is expected that the system will experience exponential decay with time in the temperature of a body.

From the definition of heat capacity ${\displaystyle C}$ comes the relation ${\displaystyle C=dQ/dT}$. Differentiating this equation with regard to time gives the identity (valid so long as temperatures in the object are uniform at any given time): ${\displaystyle dQ/dt=C(dT/dt)}$. This expression may be used to replace ${\displaystyle dQ/dt}$ in the first equation which begins this section, above. Then, if ${\displaystyle T(t)}$ is the temperature of such a body at time ${\displaystyle t}$, and ${\displaystyle T_{env}}$ is the temperature of the environment around the body:

${\displaystyle {\frac {dT(t)}{dt}}=-r(T(t)-T_{\mathrm {env} })=-r\Delta T(t)\quad }$

where

${\displaystyle r=hA/C}$ is a positive constant characteristic of the system, which must be in units of ${\displaystyle s^{-1}}$, and is therefore sometimes expressed in terms of a characteristic time constant ${\displaystyle t_{0}}$ given by: ${\displaystyle r=1/t_{0}=\Delta T/(dT(t)/dt)}$. Thus, in thermal systems, ${\displaystyle t_{0}=C/hA}$. (The total heat capacity ${\displaystyle C}$ of a system may be further represented by its mass-specific heat capacity ${\displaystyle c_{p}}$ multiplied by its mass ${\displaystyle m}$, so that the time constant ${\displaystyle t_{0}}$ is also given by ${\displaystyle mc_{p}/hA}$).

The solution of this differential equation, by standard methods of integration and substitution of boundary conditions, gives:

${\displaystyle T(t)=T_{\mathrm {env} }+(T(0)-T_{\mathrm {env} })\ e^{-rt}.\quad }$

If:

${\displaystyle \Delta T(t)\quad }$ is defined as : ${\displaystyle T(t)-T_{\mathrm {env} }\ ,\quad }$ where ${\displaystyle \Delta T(0)\quad }$ is the initial temperature difference at time 0,

then the Newtonian solution is written as:

${\displaystyle \Delta T(t)=\Delta T(0)\ e^{-rt}=\Delta T(0)\ e^{-t/t_{0}}.\quad }$

This same solution is almost immediately apparent if the initial differential equation is written in terms of ${\displaystyle \Delta T(t)}$, as the single function to be solved for. '

${\displaystyle {\frac {dT(t)}{dt}}={\frac {d\Delta T(t)}{dt}}=-{\frac {1}{t_{0}}}\Delta T(t)\quad }$

### Applications

This mode of analysis has been applied to forensic sciences 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. [3]

## Mechanical systems

The simplifying assumptions in this domain are:

## Acoustics

In this context, the lumped component model extends the distributed concepts of Acoustic theory subject to approximation. In the acoustical lumped component model, certain physical components with acoustical properties may be approximated as behaving similarly to standard electronic components or simple combinations of components.

• A rigid-walled cavity containing air (or similar compressible fluid) may be approximated as a capacitor whose value is proportional to the volume of the cavity. The validity of this approximation relies on the shortest wavelength of interest being significantly (much) larger than the longest dimension of the cavity.
• A reflex port may be approximated as an inductor whose value is proportional to the effective length of the port divided by its cross-sectional area. The effective length is the actual length plus an end correction. This approximation relies on the shortest wavelength of interest being significantly larger than the longest dimension of the port.
• Certain types of damping material can be approximated as a resistor. The value depends on the properties and dimensions of the material. The approximation relies in the wavelengths being long enough and on the properties of the material itself.
• A loudspeaker drive unit (typically a woofer or subwoofer drive unit) may be approximated as a series connection of a zero-impedance voltage source, a resistor, a capacitor and an inductor. The values depend on the specifications of the unit and the wavelength of interest.

## Heat transfer for buildings

The simplifying assumption in this domain are:

• 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 LEMs 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. Ramallo-González's method (Dominant Layer Method) is the most accurate and simple so far. [4] 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. In his thesis, [5] Ramallo-González shows the whole process of obtaining the LEM of a complete building.

LEMs of buildings have also been used to evaluate the efficiency of domestic energy systems [6] In this case the LEMs are allowed to run many simulations under different future weather scenarios.

## Related Research Articles

A thermistor is a type of resistor whose resistance is dependent on temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.

Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship:

In fluid dynamics, the Nusselt number (Nu) is the ratio of convective to conductive heat transfer at a boundary in a fluid. Convection includes both advection and diffusion (conduction). The conductive component is measured under the same conditions as the convective but for a hypothetically motionless fluid. It is a dimensionless number, closely related to the fluid's Rayleigh number.

The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by , , or .

The electrical resistance of an object is a measure of its opposition to the flow of electric current. The inverse quantity is electrical conductance, and is the ease with which an electric current passes. Electrical resistance shares some conceptual parallels with the notion of mechanical friction. The SI unit of electrical resistance is the ohm (Ω), while electrical conductance is measured in siemens (S).

Thermal insulation is the reduction of heat transfer between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with specially engineered methods or processes, as well as with suitable object shapes and materials.

Thermal conduction is the transfer of heat internal energy by microscopic collisions of particles and movement of electrons within a body. The microscopically colliding particles, that include molecules, atoms and electrons, transfer disorganized microscopic kinetically and potential energy, jointly known as internal energy. Conduction takes place in all phases of including solids, liquids, gases and waves. The rate at which energy is conducted as heat between two bodies is a function of the temperature difference temperature gradient between the two bodies and the properties of the conductive through which the heat is transferred.

Newton's law of cooling states that the rate of heat loss of a body is directly proportional to the difference in the temperatures between the body and its surroundings. The law is frequently qualified to include provisos that the temperature difference is small and the nature of heat transfer mechanism remains the same. As such, it is equivalent to a statement that the heat transfer coefficient, which mediates between heat losses and temperature differences, is a constant. This condition is generally met in thermal conduction as the thermal conductivity of most materials is only weakly dependent on temperature, but it is often met only approximately in conditions of convective heat transfer, where several physical processes make effective heat transfer coefficients somewhat dependent on temperature differences. Finally, in the case of heat transfer by thermal radiation, Newton's law of cooling holds only for rather small temperature changes.

In physics and mathematics, the heat equation is a partial differential equation that describes how the distribution of some quantity evolves over time in a solid medium, as it spontaneously flows from places where it is higher towards places where it is lower. It is a special case of the diffusion equation.

The Biot number (Bi) is a dimensionless quantity used in heat transfer calculations. It is named after the eighteenth century French physicist Jean-Baptiste Biot (1774–1862), and gives a simple index of the ratio of the heat transfer resistances inside of a body and at the surface of a body. This ratio determines whether or not the temperatures inside a body will vary significantly in space, while the body heats or cools over time, from a thermal gradient applied to its surface.

Johnson–Nyquist noise is the electronic noise generated by the thermal agitation of the charge carriers inside an electrical conductor at equilibrium, which happens regardless of any applied voltage. Thermal noise is present in all electrical circuits, and in sensitive electronic equipment such as radio receivers can drown out weak signals, and can be the limiting factor on sensitivity of an electrical measuring instrument. Thermal noise increases with temperature. Some sensitive electronic equipment such as radio telescope receivers are cooled to cryogenic temperatures to reduce thermal noise in their circuits. The generic, statistical physical derivation of this noise is called the fluctuation-dissipation theorem, where generalized impedance or generalized susceptibility is used to characterize the medium.

Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes. Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heat transfer. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system.

A heat sink is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, thereby allowing regulation of the device's temperature at optimal levels. In computers, heat sinks are used to cool CPUs, GPUs, and some chipsets and RAM modules. Heat sinks are used with high-power semiconductor devices such as power transistors and optoelectronics such as lasers and light emitting diodes (LEDs), where the heat dissipation ability of the component itself is insufficient to moderate its temperature.

In physics, the dissipation factor (DF) is a measure of loss-rate of energy of a mode of oscillation in a dissipative system. It is the reciprocal of quality factor, which represents the "quality" or durability of oscillation.

A temperature coefficient describes the relative change of a physical property that is associated with a given change in temperature. For a property R that changes by dR when the temperature changes by dT, the temperature coefficient α is defined by the following equation:

The heat transfer coefficient or film coefficient, or film effectiveness, in thermodynamics and in mechanics is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat :

In physics and engineering, the Fourier number (Fo) or Fourier modulus, named after Joseph Fourier, is a dimensionless number that characterizes transient heat conduction. Conceptually, it is the ratio of diffusive or conductive transport rate to the quantity storage rate, where the quantity may be either heat or matter (particles). The number derives from non-dimensionalization of the heat equation or Fick's second law and is used along with the Biot number to analyze time dependent transport phenomena.

In physics and engineering, the time constant, usually denoted by the Greek letter τ (tau), is the parameter characterizing the response to a step input of a first-order, linear time-invariant (LTI) system. The time constant is the main characteristic unit of a first-order LTI system.

Miniaturizing components has always been a primary goal in the semiconductor industry because it cuts production cost and lets companies build smaller computers and other devices. Miniaturization, however, has increased dissipated power per unit area and made it a key limiting factor in integrated circuit performance. Temperature increase becomes relevant for relatively small-cross-sections wires, where it may affect normal semiconductor behavior. Besides, since the generation of heat is proportional to the frequency of operation for switching circuits, fast computers have larger heat generation than slow ones, an undesired effect for chips manufacturers. This article summaries physical concepts that describe the generation and conduction of heat in an integrated circuit, and presents numerical methods that model heat transfer from a macroscopic point of view.

Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow. Thermal resistance is the reciprocal of thermal conductance.

## References

1. Anant Agarwal and Jeffrey Lang, course materials for 6.002 Circuits and Electronics, Spring 2007. MIT OpenCourseWare (PDF), Massachusetts Institute of Technology.
2. Incropera; DeWitt; Bergman; Lavine (2007). Fundamentals of Heat and Mass Transfer (6th ed.). John Wiley & Sons. pp. 260–261. ISBN   978-0-471-45728-2.
3. Heat Transfer - A Practical Approach by Yunus A Cengel
4. 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.
5. Ramallo-González, A.P. 2013. Modelling Simulation and Optimisation of Low-energy Buildings. PhD. University of Exeter.
6. 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.