Thermal stratification of a lake in summer

Why does ice form on the top of a lake?

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Learn in this article why ice form always on top of a lake in winter. The negative thermal expansion of water (density anomaly) has an existential importance for life on earth. More...
Forestville (Canada) and Karlsruhe (Germany) at 49° North

Gulf Stream & global ocean conveyor belt

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The Gulf Stream is an ocean current in the Atlantic Ocean which, as part of the earth's global conveyor belt, has a decisive influence on the climate in Northern and Western...
Heat flow through a wall by thermal convection and thermal conduction

Difference between thermal conductivity, diffusivity, transmittance, resistance and heat transfer coefficient

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Learn more in this article about the differences and importance of thermal conductivity, thermal diffusivity, heat transfer coefficient, thermal transmittance and thermal resistance, etc. Thermal conductivity Thermal conductivity (lambda) describes the heat transfer...
Convection current for radiators placed below a window

Why are radiators usually located under a window?

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Central heating systems use the principle of thermal convection. The water is heated by a central heater and then transferred to the radiators by means of pumps. It is thus a...
Momentum transfer as the cause of viscosity in ideal gases

Viscosity of an ideal gas

The viscosity of ideal gases is mainly based on the momentum transfer due to diffusion between the fluid layers. Learn more about it in this article. This article provides answers to the...
Hydrodynamische Grenzschicht, thermische Grenzschicht und Konzentrationsgrenzschicht

Dimensionless numbers of the boundary layers (Prandtl, Schmidt and Lewis number)

To describe the heat and mass transport, dimensionless numbers are introduced to describe the processes within the boundary layers. Learn more about this in this article. This article provides answers to the...
Definition of the thermal boundary layer (temperature boundary layer)

Thermal and concentration boundary layer

In addition to the hydrodynamic boundary layer, the thermal boundary layer and the concentration boundary layer also have a decisive influence on the entire heat and mass transport in a flow....
Convective heat transfer between a heated pipe and a fluid flowing through it

Prandtl number

The Prandtl number is a dimensionless similarity parameter to describe the transport of heat and momentum. Learn more about it in this article. This article provides answers to the following questions, among...
Lewis number as a measure for the ratio of the thickness of the thermal boundary layer and the concentration boundary layer

Lewis number

The Lewis number is a dimensionless similarity parameter to describe heat and mass transport. Learn more about it in this article. This article provides answers to the following questions, among others What does...
Convective heat transfer at a flat plate with laminar-turbulent flow

Calculation of the Nusselt numbers for forced flows over plates and in pipes

In this article you will find formulas for calculating the local and average Nusselt numbers for forced flows over plates and in pipes with circular cross sections. This article provides answers to...
Definition of the heat transfer coefficient for convective heat transport

Heat transfer coefficient for thermal convection

The heat transfer coefficient describes the convective heat transfer from a solid to a flowing fluid and vice versa! Learn more about it in this article. This article provides answers to the...
Velocity gradients in a laminar and turbulent boundary layer

Hydrodynamic boundary layer

The hydrodynamic boundary layer of a flow has a decisive influence on heat and mass transport. Learn more about it in this article. This article provides answers to the following questions, among...
Influence of the hydrodynamic and thermal boundary layer on convective heat transfer

Nusselt number to describe convective heat transfer

The Nusselt number is a dimensionless similarity parameter to describe convective heat transfer, independent of the size of the system. Learn more about it in this article. This article provides answers to...
Temperature profile through the wall of a cylindrical pipe

Temperature profiles and heat flows through different geometries

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In this article we discuss temperature curves and heat flows through a plane wall, through a cylindrical pipe and through a hollow sphere. This article provides answers to the following questions, among...
Principle of thermal conduction in solids by oscillation of the atoms

Thermal conduction in solids and ideal gases

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The thermal conductivity in crystalline, non-metallic solids first increases and then decreases again with increasing temperature. Learn more about this behavior in this article. This article provides answers to the following questions,...
Illustration des Photonengases in einem Schwarzen Hohlkörper

Thermodynamic derivation of the Stefan-Boltzmann Law

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In this article the Stefan-Boltzmann-Law is to be derived using the laws of thermodynamics. This article provides answers to the following questions, among others: How can the Stefan-Boltzmann law be derived from the...
Radiation of a black body into half-space

Different forms of Planck’s law

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Planck's law of radiation can be expressed in different forms. The most important ones are discussed in this article. This article provides answers to the following questions, among others: What is meant by...
Spectral distribution of the intensity of the radiation of a blackbody (Planck spectrum)

Planck’s law and Wien’s displacement law

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Planck's law describes the radiation emitted by black bodies and Wien's displacement law the maximum of the spectral intensity of this radiation. Learn more about it in this article. This article provides...
Design of a Laser-Flash-Analyzer (LFA) for the determination of thermal diffusivity conductivity

Laser-Flash method for determining thermal conductivity (LFA)

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With the Laser-Flash method (Laser Flash Analyser, LFA), the thermal conductivity is determined by the temperature rise in a test sample that is heated by a short laser pulse from one...
Determination of thermal conductivity with the Transient Hot Wire method (HTW)

Transient-Hot-Wire method method for determining thermal conductivity (THW)

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With the Transient-Hot-Wire method (THW), the thermal conductivity is determined by the change in temperature over time at a certain distance from a heating wire. Learn more about the experimental determination...
Design of a Heat-Flow-Meter for determining thermal conductivity (HFM method)

Heat-Flow-Meter method for determining thermal conductivity (HFM)

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With the Heat-Flow-Meter method (HFM) the thermal conductivity is determined by comparative measurement of the heat flow using a reference sample. Learn more about this absolute measurement method in this article. This...
Design of a measuring device for the determination of thermal conductivity according to the Guarded-Hot-Plate Method (GHP)

Guarded-Hot-Plate method for determining thermal conductivity (GHP)

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With the Guarded-Hot-Plate method (GHP) the thermal conductivity is determined by the electrical power output of a hot plate with guided heat conduction. Learn more about this measurement method in this...
Temperature distribution along a heated thin rod

Derivation of heat equation (diffusion equation)

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The heat equation describes the temporal and spatial behavior of temperature for heat transport by thermal conduction. Read more about it in this article. This article provides answers to the following questions,...
Mean free path in a thin gas with low pressure and a dense gas with high pressure

Thermal conductivity of gases

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The thermal conductivity of ideal gases is not dependent on pressure for gases that are not too strongly diluted. This is no longer the case for gases with low pressure. Learn...
Experimental setup for the measurement of thermal conductivity

Experimental setup for determining thermal conductivity

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In this article you can learn more about the experimental determination of the thermal conductivity of materials using steam and ice. This article provides answers to the following questions, among others What...
Assembly of a building wall to calculate the U-value

Thermal transmittance (U-value)

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The thermal transmittance (U-value or U-factor) describes the heat transfer through a solid object, which is located between two fluids (gas or liquid) with different temperatures. Learn more about it in...
Design and principle of a thermos (vacuum flask), how does work

How does a thermos work? Design of a vacuum flask!

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The reason why hot tea or coffee stays warm for so long in a thermos is the special design of the flask. This design limits the different mechanisms of heat transfer...
Sun as a blackbody with a surface temperature of 5778 K

Heat transfer by thermal radiation

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With thermal radiation, heat is transferred by electromagnetic waves without the presence of a substance! You can read more about this mechanism in this article. The mechanisms of thermal convection and thermal...
Heat transfer using the example of a central heating system

Heat transfer by thermal convection

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With heat transfer by thermal convection, heat is transported with a flowing substance. Convection only occurs in fluids, i.e. gases and liquids. You will find more information in this article. This article...
Experiment to demonstrate heat transport by thermal conduction

Heat transfer by thermal conduction

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Heat transfer by thermal conduction means that heat is conducted through a material. Heat energy is transferred from molecule to molecule at the atomic level. More information on this topic can...
Heat flow from a hot object to a cooler one

Rate of heat flow: Definition and direction

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The rate of heat flow refers to the heat energy transferred per unit of time ("heat output"). The drive for the heat flow is a temperature difference. Learn more about the...
Thermal conduction through a house wall

Thermal conductivity (Fourier’s law)

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Thermal conductivity is a measure of how well or poorly a material conducts heat energy (measure of the strength of heat conduction)! You can read more about this in this article. This...
Mixing temperature when pouring cold milk into a cup of hot coffee

Heat and thermodynamic equilibrium

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In thermodynamics, heat is the transport of energy due to a temperature difference. Heat in this respect is never "contained" in an object! Learn more about the thermodynamic concept of heat...
Thermal image of a cup with hot tea and a building

Heat transfer (heat transport)

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Heat transfer is the transport of thermal energy from a warmer object to a cooler object. A distinction is made between thermal convection, thermal conduction and thermal radiation. You can read...
Operating principle of a heat pump (conversion of work into heat)

The process quantities: Heat and work

Work and heat are process quantities that describe the process of a supply of energy ("energy in transit")! Learn more about it in this article. Work and heat transferred to a substance Energy...
Energy transfer done on a gas by heat and work (compression)

Internal energy & first law of thermodynamics

Internal energy is the sum of the different forms of energy on a microscopic level inside a substance. Learn more about it in this article. Energy transfer as heat and as work In...
Turbofan engine of an airplane

Thermodynamic systems

A thermodynamic system is a confined space of matter (e.g. gas) within which thermodynamic processes take place. The system boundary separates the system from the environment. Learn more about it in...
Visible radiation of a light bulb

Stefan-Boltzmann law & Kirchhoff’s law of thermal radiation

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The Stefan-Boltzmann law states that the radiant power of an object in thermal equilibrium is proportional to the fourth power of temperature and directly proportional to its surface! Learn more about...
Degrees of freedom of a molecule

Internal energy & heat capacity of ideal gases

This article provides answers to the following questions, among others: What is the internal energy of a gas?What are the characteristics of the thermal energy of a gas?What is meant...
Mean free path

Mean free path & collision frequency

The mean free path is the average distance a particle travels without colliding with other particles! Learn more about the mean free path in this article! This article provides answers to the...
Balls on a vibrating plate

Derivation of the Maxwell-Boltzmann distribution function

The Maxwell-Boltzmann distribution function of the molecular speed of ideal gases can be derived from the barometric formula. This article shows the derivation. Introduction For ideal gases, the distribution function (f(v)) of the...
Pressure distribution during compression of a gas

Equipartition theorem

This article provides answers to the following questions, among others: What statement does the equipartition theorem provide?What is a degree of freedom in the thermodynamic sense?Why does a direction of oscillation have...
Cylinder with a movable piston

Ideal gas law (explained and derived)

The ideal gas law describes the relationship between pressure, volume, mass and temperature of ideal gases. Learn more about it in this article. Parameters influencing the gas pressure The figure below shows a...
Thermographic image of a car

How does a thermal imaging camera work?

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This article provides answers to the following questions, among others: How do thermal imaging cameras work?Why is the thermal image displayed in false colors? Thermal imaging cameras are based on the same principle...
Infrared thermometer (Pyrometer)

How does a infrared thermometer (pyrometer) work?

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This article provides answers to the following questions, among others: How do IR thermometers (pyrometers) work?What are the advantages of pyrometers? The thermometers presented in the previous articles must directly touch the object...
Thermocouple

How does a thermocouple work?

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This article provides answers to the following questions, among others: How does a thermocouple work?What is the Seebeck effect?What are the differences between the types of thermocouples?What is cold junction compensation?What are...
Resistance thermometer (Pt100)

How does a resistance temperature detector (resistance thermometer) work?

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This article provides answers to the following questions, among others: How do resistance thermometers work?Why should the measurement current for an RTD be as low as possible?How can the temperature be determined...
Cut through a vapour filled thermometer (vapour-in-metal thermometer)

How does a vapour pressure thermometer (vapour-in-metal) work?

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This article provides answers to the following questions, among others: How do vapor pressure thermometers work?Why do vapor pressure thermometers have a non-linear scale? The less popular vapour pressure thermometers work according to...
Cut through a gas filled thermometer (gas-in-metal thermometer)

How does a gas filled thermometer (gas-in-metal) work?

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This article provides answers to the following questions, among others: How do gas filled thermometers work?What are the advantages of gas-in-metal thermometers compared to liquid-in-metal thermometers?How can the influence of the...
Cut through a liquid filled thermometer (liquid-in-metal thermometer)

How does a liquid filled thermometer (liquid-in-metal) work?

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This article provides answers to the following questions, among others: How do a liquid-in-metal thermometers work?Why should the inner diameter of the coiled tube be kept as small as possible?How can large...
Bimetal thermometer (spiral type)

How does a bimetallic strip thermometer work?

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This article provides answers to the following questions, among others: How do bimetal thermometers work?Why is the bimetal wrapped into a spiral or a helix?Why are some bimetal thermometers equipped with switch...
Principle of temperature measurement with a liquid-in-glass-thermometer

How does a liquid-in-glass thermometer work?

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This article provides answers to the following questions, among others: How do liquid thermometers work?What properties must thermometric liquids have?Why is mercury hardly used as a thermometer fluid nowadays?Which liquids are predominantly...
Design of a velocity selector to determine the speed distribution in gases

Determination of the speed distribution in a gas

This article provides answers to the following questions, among others: How can the speeds of the particles in a gas be determined experimentally?What is a histogram?What is a relative frequency distribution?How does...
Randomly distributed velocities of the particles of a gas

Pressure and temperature

This article provides answers to the following questions, among others: What is the kinetic theory of gases?What are the assumptions on which the kinetic theory of gases is based?How can the pressure...
White glow of a blackbody at 6000 K

Blackbody radiation

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Blackbody radiation (cavity radiation) is the thermal radiation of a blackbody, i.e. a body that absorbs all incident radiation. Find out more about blackbody radiation in this article. This article provides answers...
Density anomaly of water in the temperature range between 4 °C and 0 °C (negative thermal expansion)

Density anomaly of water (negative thermal expansion)

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Density anomaly refers to the paradoxical behavior of a substance to expand suddenly when cooling down instead of contracting further (anomalous decrease in density). Learn more about it in this article. This...
Speed distribution of an ideal gas for different temperatures

Maxwell–Boltzmann distribution

The Maxwell-Boltzmann distribution describes the molecular speed distribution of ideal gas molecules. Find out more in this article. This article provides answers to the following questions, among others: What is the Maxwell-Boltzmann distribution?For...
Schematic illustration of an evaporation process

Why do liquids evaporate?

This article provides answers to the following questions, among others: How can the evaporation of liquids be explained?Why does a cooling effect occur during evaporation?Why does one freeze faster with wet skin...
Thermal response with cooled and uncooled hand

Temperature scales

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This article provides answers to the following questions, among others: What is temperature in the physical sense a measure for?Which reference points are used as the basis for the Celsius scale and...
Why does metal feel colder or warmer than wood depending on the temperature?

Why does metal feel colder than wood (human thermal response)

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Find out in this article why metal feels colder than wood of the same temperature, while at higher temperatures the metal suddenly feels warmer than wood. The property of an object to...
Illustration of Brownian particle motion with balls

Temperature and particle motion

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This article provides answers to the following questions, among others: What is the general relationship between the temperature and the particle movement of a substance?Why does the volume of a substance usually...
Particle model of matter (oxygen, hydrogen, water)

Particle model of matter

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This article provides answers to the following questions, among others: What is the particle model of matter?How can the solid, liquid and gaseous states of matter be explained with the particle...