Conduction is the transfer of heat by direct contact of particles of matter. The transfer of energy could be primarily by elastic impact as in fluids or by free electron diffusion as predominant in metals or phonon vibration as predominant in insulators. In other words, heat is transferred by conduction when adjacent atoms vibrate against one another, or as electrons move from one atom to another. Conduction is greater in solids, where a network of relatively fixed spacial relationships between atoms helps to transfer energy between them by vibration.

Heat conduction is directly analogous to diffusion of particles into a fluid, in the situation where there are no fluid currents. This type of heat diffusion differs from mass diffusion in behavior, only in as much as it can occur in solids, whereas mass diffusion is mostly limited to fluids.

Metals (e.g. copper, platinum, gold, iron, etc.) are usually the best conductors of thermal energy. This is due to the way that metals are chemically bonded: metallic bonds (as opposed to covalent or ionic bonds) have free-moving electrons which are able to transfer thermal energy rapidly through the metal.

As density decreases so does conduction. Therefore, fluids (and especially gases) are less conductive. This is due to the large distance between atoms in a gas: fewer collisions between atoms means less conduction. Conductivity of gases increases with temperature. Conductivity increases with increasing pressure from vacuum up to a critical point that the density of the gas is such that molecules of the gas may be expected to collide with each other before they transfer heat from one surface to another. After this point in density, conductivity increases only slightly with increasing pressure and density.

Convection is the transfer of thermal energy by the movement of molecules from one part of the material to another. As the fluid motion increases, so does the convective heat transfer. The presence of bulk motion of the fluid enhances the heat transfer between the solid surface and the fluid.

Radiation is the transfer of heat energy through empty space. All objects with a temperature above absolute zero radiate energy at a rate equal to their emissivity multiplied by the rate at which energy would radiate from them if they were a black body. No medium is necessary for radiation to occur, for it is transferred through electromagnetic waves; radiation works even in and through a perfect vacuum. The energy from the Sun travels through the vacuum of space before warming the earth.

By Jiamin.

MOre About Waves

Types of wave
There are various ways of classifying wave types. One of these is based on the way the wave travels. In a transverse wave, the displacement of the medium is perpendicular to the direction in which the wave travels. An example of this type of wave is a mechanical wave projected along a tight string. The string moves at right angles to the wave motion. Electromagnetic waves are another example of transverse waves. The directions of the electric and magnetic fields are perpendicular to the wave motion. In a longitudinal wave the disturbance takes place parallel to the wave motion. A longitudinal wave consists of a series of compressions and rarefactions (states of maximum and minimum density and pressure, respectively). Such waves are always mechanical in nature and thus require a medium through which to travel. Sound waves are an example of longitudinal waves. Waves that result from a stone being dropped into water appear as a series of circles. These are called circular waves and can be generated in a ripple tank for study. Waves on water that appear as a series of parallel lines are called plane waves


Characteristics of waves
All waves have a wavelength. This is measured as the distance between successive crests (or successive troughs) of the wave. It is given the Greek symbol λ. The frequency of a wave is the number of vibrations per second. It is expressed in hertz, symbol Hz (1 Hz = 1 cycle per second). The reciprocal of this is the wave period. This is the time taken for one complete cycle of the wave oscillation. The speed of the wave is measured by multiplying wave frequency by the wavelength

Properties of waves
When a wave moves from one medium to another (for example a light wave moving from air to glass) it moves with a different speed in the second medium. This change in speed causes it to change direction. This property is called refraction. The angle of refraction depends on whether the wave is speeding up or slowing down as it changes medium. Reflection occurs whenever a wave hits a barrier. The wave is sent back, or reflected, into the medium. The angle of incidence (the angle between the ray and a perpendicular line drawn to the surface) is equal to the angle of reflection (the angle between the reflected ray and a perpendicular to the surface). See also total internal reflection. An echo is the repetition of a sound wave by reflection from a surface. All waves spread slightly as they travel. This is called diffraction and it occurs chiefly when a wave interacts with a solid object. The degree of diffraction depends on the relationship between the wavelength and the size of the object (or gap through which the wave travels). If the two are similar in size, diffraction occurs and the wave can be seen to spread out. Large objects cast shadows because the difference between their size and the wavelength is so large that light waves are not diffracted around the object. A dark shadow results. When two or more waves meet at a point, they interact and combine to produce a resultant wave of larger or smaller amplitude (depending on whether the combining waves are in or out of phase with each other). This is called interference. Transverse waves can exhibit polarization. If the oscillations of the wave take place in many different directions (all at right angles to the directions of the wave) the wave is unpolarized. If the oscillations occur in one plane only, the wave is polarized. Light, which consists of transverse waves, can be polarized


By:Cody-Gene

Water Waves

By:Cody-Gene

Waves

Longitudinal wave - A longitudinal wave is a wave in which the motion of the medium is parallel to the motion of the wave.

For example: the motion of the medium.... is left to right...
the motion of the waves...Is to the right or to the left...

Sound waves are longitudinal. The air vibrates back and forth along the same direction as the wave is traveling.




Transverse wave - A transverse wave is a wave in which the motion of the medium is perpendicular to the motion of the wave.

For example: the motion of the medium is up to down....
the motion of the waves...Is to the right or to the left...
Water waves are mostly transverse. The water moves up and down while the wave travels over the surface of the water......


By:Cody-Gene

Heat is energy or more precisely transfer of thermal energy. As energy, heat is measured in watts (W) whilst temperature is measured in degrees Celsius (°C) or Kelvin (K). The words “hot” and “cold” only make sense on a relative basis. Thermal energy travels from hot material to cold material. Hot material heats up cold material, and cold material cools down hot material. It is really that simple. When you feel heat, what you are sensing is a transfer of thermal energy from something that's hot to something that is cold.

The discipline of heat transfer is concerned with only two things: temperature, and the flow of heat. Temperature represents the amount of thermal energy available, whereas heat flow represents the movement of thermal energy from place to place. On a microscopic scale, thermal energy is related to the kinetic energy of molecules. The greater a material’s temperature, the greater the thermal agitation of its constituent molecules (manifested both in linear motion and vibrational modes).
By :Felicia Khoo

In general all waves mechanical and electromagnetic (non-mechanical) waves are classified into two types. These are

• longitudinal waves

• transverse waves.

Longitudinal waves

Propagation of sound waves in air

A wave motion in which the particles of the medium oscillate about their mean positions in the direction of propagation of the wave, is called longitudinal wave.

Sound waves are classified as longitudinal waves. Let us now see how sound waves propagate. Take a tuning fork, vibrate it and concentrate on the motion of one of its prongs, say prong A. The normal position of the tuning fork and the initial condition of air particles is shown in the fig (a). As the prong A moves towards right, it compresses air particles near it, forming a compression as shown in fig (b). Due to vibrating air layers, this compression moves forward as a disturbance. As the prong A moves back to its original position, the pressure on its right decreases, thereby forming a rarefaction. This rarefaction moves forward like compression as a disturbance. As the tuning fork goes on vibrating, waves consisting of alternate compressions and rarefactions spread in air as shown in fig (d). The direction of motion of the sound waves is same as that of air particles, hence they are classified as longitudinal waves. The longitudinal waves travel in the form of compressions and rarefactions.

Transverse waves

A wave motion, in which the particles of the medium oscillate about their mean positions at right angles to the direction of propagation of the wave, is called transverse wave.

These waves can propagate through solids and liquids but not through gases, because gases do not possess elastic properties. Examples of these waves are: vibrations in strings, ripples on water surface and electromagnetic waves.

In a transverse wave the particles of the medium oscillate in a direction perpendicular to the direction of propagation as shown in the figure.

Particles of the medium oscillate in a direction perpendicular to the direction of propagation

Thus, during their oscillations, the particles may move upwards or downwards from the plane passing through their mean positions. The uppermost point of the wave, i.e., the position of maximum positive displacement is crest and the lowest point, i.e. the position of maximum displacement is called trough. Thus in a transverse wave crests and troughs appear alternatively.

From : xinlei:)

Longitudinal Waves

Longitudinal waves are waves that have the same directions of oscillation or vibrations along their directions of travel,which means that the medium (particle) is in the same directions or opposite direction as the motion of the waves.Mechanical longitudinal waves have been also referred to as compressional waves.Check out youtube,Physics with Mr Noon:Longitudinal waves(My video upload failed).

By:Muhd Zakir

Heat transfer is the transition of thermal energy from a hotter mass to a cooler mass. When an object is at a different temperature from its surroundings or another object, transfer of thermal energy, also known as heat flow, or heat exchange, occurs in such a way that the body and the surroundings reach thermal equilibrium; this means that they are at the same temperature. Heat transfer always occurs from a higher-temperature object to a cooler-temperature one as described by the second law of thermodynamics or the Clausius statement. Where there is a temperature difference between objects in proximity, heat transfer between them can never be stopped; it can only be slowed.
From : Xi Li

Waves.

Longitudinal Waves

In a longitudinal wave the particle displacement is parallel to the direction of wave propagation. The animation below shows a one-dimensional longitudinal plane wave propagating down a tube. The particles do not move down the tube with the wave; they simply oscillate back and forth about their individual equilibrium positions. Pick a single particle and watch its motion. The wave is seen as the motion of the compressed region (ie, it is a pressure wave), which moves from left to right.

Transverse Waves

In a transverse wave the particle displacement is perpendicular to the direction of wave propagation. The animation below shows a one-dimensional transverse plane wave propagating from left to right. The particles do not move along with the wave; they simply oscillate up and down about their individual equilibrium positions as the wave passes by. Pick a single particle and watch its motion.

Water Waves

Water waves are an example of waves that involve a combination of both longitudinal and transverse motions. As a wave travels through the waver, the particles travel in clockwise circles. The radius of the circles decreases as the depth into the water increases. The movie below shows a water wave travelling from left to right in a region where the depth of the water is greater than the wavelength of the waves. I have identified two particles in blue to show that each particle indeed travels in a clockwise circle as the wave passes.

By,

Amy from 3n2 :)

Transverse and Longitudinal Waves

Most kinds of waves are transverse waves. In a transverse wave, as the wave is moving in one direction, it is creating a disturbance in a different direction. The most familiar example of this is waves on the surface of water. As the wave travels in one direction - say south - it is creating an up-and-down (not north-and-south) motion on the water's surface. This kind of wave is very easy to draw; a line going from left-to-right has up-and-down wiggles. So most diagrams of waves - even of sound waves - are pictures of transverse waves.


But sound waves are not transverse. Sound waves are longitudinal waves. If sound waves are moving south, the disturbance that they are creating is making the air molecules vibrate north-and-south (not east-and-west, or up-and-down. This is very difficult to show clearly in a diagram, so most diagrams, even diagrams of sound waves, show transverse waves.

Lancelot Ambrosio 3n2

Water waves

Dispersion(Water waves)
In fluid dynamics, dispersion of water waves generally refers to frequency dispersion, which means that waves of different wavelengths travel at different phase speeds. Water waves, in this context, are waves propagating on the water surface, and forced by gravity and surface tension. As a result, water with a free surface is generally considered to be a dispersive medium.
Surface gravity waves, moving under the forcing by gravity, propagate faster for increasing wavelength. For a certain wavelength, gravity waves in deeper water have a larger phase speed than in shallower water. In contrast with this, capillary waves only forced by surface tension, propagate faster for shorter wavelengths.
Besides frequency dispersion, water waves also exhibit amplitude dispersion. This is a nonlinear effect, by which waves of larger amplitude have a different phase speed from small-amplitude waves.

chinann

Longitudinal waves are waves that have the same direction of oscillation or vibration along their direction of travel, which means that the oscillation of the medium (particle) is in the same direction or opposite direction as the motion of the wave. Mechanical longitudinal waves have been also referred to as compressional waves or compression waves.

A transverse wave is a moving wave that consists of oscillations occurring perpendicular to the direction of energy transfer. If a transverse wave is moving in the positive x-direction, its oscillations are in up and down directions that lie in the y-z plane.

Joanna =)

Heat transfer is the transition of thermal energy from a hotter mass to a cooler mass. When an object is at a different temperature than its surroundings or another object, transfer of thermal energy, also known as heat flow, or heat exchange, occurs in such a way that the body and the surroundings reach thermal equilibrium; this means that they are at the same temperature. Heat transfer always occurs from a higher-temperature object to a cooler-temperature one as described by the second law of thermodynamics or the Clausius statement. Where there is a temperature difference between objects in proximity, heat transfer between them can never be stopped; it can only be slowed.

Caijun;

A transverse wave is a moving wave that consists of oscillations occurring perpendicular to the direction of energy transfer .

KAIXIAN . (:

A transverse wave is created by a disturbance that is perpendicular to the direction the wave will travel.

ADELINE . ;D

E- learning (Chem Question)

Paint manufactures such as Nippon does not make all the different colours of paints in fact, they a few selected colours, and the paint shops mix different paints to get the paints colour needed.

A seller, Mark, mixed wrong paints of bucket together..

" What a mess! " He thought.

" I have to pay to pay for this, and they are not cheap! "

Then he remembered what he learn in secondary school Chemistry.

" Paints are just dyes, I can use paper chromatography to separate them! Then, i don't have to pay for the waste paint! "


Is chromatography a feasible method to separate the bucket of mixed paint? What are the advantages and disadvantages of chromatography? Lastly, look around the internet and see the different type of chromatography there are.

Thermal Energy

Hi.Zakir here,


Energy is the capacity of a physical system to perform work. Energy exists in several forms such as heat, kinetic or mechanical energy, light, potential energy, electrical, or other forms.

Changes

Changes of State =)

Ice become water = Solid to Liquid

Water becomes steam = Liquid to Gas

Steam becomes water = Gas to Liquid

Water becomes ice = Liquid to Solid


Melting and Freezing =)

MELTING :
- A process in which a substance changes its state
from SOLID to LIQUID
- For a pure substance , melting occurs at a definite
(canstant) temperature ~~~ MELTING POINT

FREEZING :
- A process in which a substance changes its state
from LIQUID to SOLID
- For a pure substance , freezing occurs at a definite
(constant) temperature ~~~ FREEZING

Done by : Fazira n Nadrah =)

The ultimate source of thermal energy available to mankind is the sun, the huge thermo-nuclear furnace that supplies the earth with the heat and light that are essential to life. The nuclear fusion in the sun increases the sun's thermal energy. Once the thermal energy leaves the sun (in the form of radiation) it is called heat. Heat is thermal energy in transfer. Thermal energy is part of the overall internal energy of a system.
At a more basic level, thermal energy comes form the movement of atoms and molecules in matter. It is a form of kinetic energy produced from the random movements of those molecules. Thermal energy of a system can be increased or decreased.
When you put your hand over a hot stove you can feel the heat. You are feeling thermal energy in transfer. The atoms and molecules in the metal of the burner are moving very rapidly because the electrical energy from the wall outlet has increased the thermal energy in the burner. We all know what happens when we rub our hands together. Our mechanical energy increases the thermal energy content of the atoms in our hands and skin. We then feel the consequence of this - heat. Laws of Thermodynamics .
-Rosabella

thermal conductivity

In physics, thermal conductivity, k, is the property of a material that indicates its ability to conduct heat. It appears primarily in Fourier's Law for heat conduction. Thermal conductivity is measured in watts per kelvin per metre (W·K−1·m−1). Multiplied by a temperature difference (in kelvins, K) and an area (in square metres, m2), and divided by a thickness (in metres, m) the thermal conductivity predicts the power loss (in watts, W) through a piece of material.

The reciprocal of thermal conductivity is thermal resistivity.

chinann

Helllo Beautiful Readers,

This is something about PHYSICS=)

Thermal Conductivity

In this section, we introduce thermal conductivity with examples of products exploiting the conductivity of aluminium.
Definition
The thermal conductivity of a material is the measure of a material’s ability to conduct heat. High conductivity = good thermal conductor; low conductivity = good thermal insulator.

The thermal conductivity is important in engineering for conditions of steady-state heat flow - i.e. when the imposed temperatures are stable, and the designer needs to know how much heat is being conducted down a thermal gradient. In a heat exchanger we wish to transfer as much heat as possible between two fluids, one hot, the other cold; in a window frame, we aim to lose as little heat as possible from a heated home to the environment.

The mathematical formula defining thermal conductivity λ (or sometimes k) is:

where q is the heat conducted per second (or power), per unit area, and dT/dx is the temperature gradient (K m−1) in the direction of heat conduction.
Units and values

Thermal conductivity is measured in W m−1 K−1. Aluminium alloys have values in the range 75-235 W m−1 K−1, at the top end of the range for metallic alloys (5-390 W m−1 K−1). Only copper has a higher thermal conductivity, while steel has values in the range 10-55 W m−1 K−1.
Authors/Contributors

Lovee,Khairunnisa'=)

Thermal properties

Thermal conductivity was determined using the Colora Thermoconductor.
The sample was placed between ground silver plates, kept at the given
boiling points of two liquids by a constant supply of heat to the liquid with the
higher boiling point. When steady equilibrium was attained, the liquid having
the lower boiling point vaporized at a constant rate. It was condensed and
then collected in a measuring vessel. The time required to distill a specified
volume was measured. From a previously obtained calibration curve for
similar sized discs of known thermal conductivity, the thermal resistivity and
conductivity of the test sample was derived.
For determination of specific heat, a sample was analyzed using a quantitative
Adiabatic Calorimeter. This sample was weighed, tightly enclosed in an
electrically heated gold-plated copper container, and suspended inside a
massive nickel-plated and polished guard. The entire assembly was evacuated,
backfilled with N2, then cooled to a uniform temperature below the lowest
mean temperature required for data. When steady conditions were obtained,
a controlled rate, continuous power input was supplied to the heater on the
sample container. By utilizing the output of a multi-junction differential
thermopile, the power to the guard heater was automatically controlled so
that the temperature of the guard was equal to the temperature of the sample.
This allowed negligible heat transfer from the sample to its surroundings. From
observations of the power input to the heater, a continuous record of sample
temperature variation with time, a record of the sample and container enthalpy
change with temperature was obtained.

Fateen

When impurities are added to a substance, it would lower its freezing point.
Impurities are other foreign substances.
Example:
I have 2 cups of water, A and B.
I add salt to B. When I try to freeze the water in both cups, A would freeze at 0oC but B would freeze maybe at –10oC.
This means B would stay as a liquid even at 0oC or -2oC or lower until the temperature reaches –10oC.
When A becomes solid ice at 0oC, B is still liquid.

Melting point of ice is lowered by an increase in pressure and is increased by a drop in pressure.

At higher pressure, an ice cube would melt at temperatures lower than 0oC.

At lower pressure (on the mountain), ice would only melt at temperatures higher than 0oC. Which is why there can be snow on the mountain top even when surrounding temperatures are above 0oC.

Boiling is a change of state from liquid to gas.
Boiling occurs at a fixed temperature. During boiling, temperature remains constant.
This temperature is known as the boiling point of the substance.
Heat is taken in / absorbed during this process.
Liquid will remain at boiling point until all the liquid has changed to gas.
(phenomena is pretty similar to melting)

The reverse of boiling is condensation.

It is the process of change from gas to liquid.
It occurs at a fixed temperature which is the boiling point of the substance.
Heat is given off during this process.

The thermal energy that is absorbed or released; causing a change in state is known as latent heat.

For melting or freezing, it is known as latent heat of fusion, Lf.
For boiling or condensation, it is known as latent heat of vaporisation, Lv

Unit for both Lf, Lv is joule or J

Adding impurities will raise the boiling point of an object.
That means to say the liquid will now boil at a higher temperature.

Lower pressure would lower the boiling point of water.
Water will boil very quickly on the mountain top but the temperature reached is lower than 100oC.

Increasing pressure would raise the boiling point of water.
Water will boil at a higher temperature above 100oC

Evaporation is the change of state from liquid to gas at any temperature.

cora.

Thermal properties

Water also has some physical properties relating to heat that add to its usefulness as a biological solvent.
Heat Capacity
For one thing, it has an unusually high specific heat, or heat capacity. Heat capacity, you should remember, is the amount of heat or the number of calories required to raise the temperature of one gram of that substance by one degree Celsius. Water's high heat capacity means that is can absorb or lose a relatively large amount of heat without undergoing a large change in temperature. Since many biological functions will not take place outside a narrow range of temperatures, water's ability to absorb and lose heat without a large temperature change provides an organism with a protection against the ill effects of a large external temperature change. Another way to say this is that large amount of water in our bodies stabilizes the temperature of our bodies.

Done by ; MIRAAA ;D

changes of state

* Melting

-A process in which a substance changes its state from SOLID to LIQUID

* Freezing

-A process in which a substance changes its state from LIQUID to SOLID

*
Ice becomes water = Melting

Water becomes steam = Boiling

Steam becomes water = Condense

Water becomes ice = Freezing


lancelot ^^, 3n2

Thermal Energy

The thermal energy of an object consists of the total kinetic energy of all its atoms and molecules.

It is a form of energy related to heat and temperature. Thermal energy can be created internally with chemical, nuclear and electrical reactions. It can also be created or increased from external effects, such as mechanical motion, radiation and thermal conduction.

Syazwan.

Thermal Conductivity

In this section, we introduce thermal conductivity with examples of products exploiting the conductivity of aluminium.
Definition
The thermal conductivity of a material is the measure of a material’s ability to conduct heat. High conductivity = good thermal conductor; low conductivity = good thermal insulator.

The thermal conductivity is important in engineering for conditions of steady-state heat flow - i.e. when the imposed temperatures are stable, and the designer needs to know how much heat is being conducted down a thermal gradient. In a heat exchanger we wish to transfer as much heat as possible between two fluids, one hot, the other cold; in a window frame, we aim to lose as little heat as possible from a heated home to the environment.

The mathematical formula defining thermal conductivity λ (or sometimes k) is:

where q is the heat conducted per second (or power), per unit area, and dT/dx is the temperature gradient (K m−1) in the direction of heat conduction.

Units and values

Thermal conductivity is measured in W m−1 K−1. Aluminium alloys have values in the range 75-235 W m−1 K−1, at the top end of the range for metallic alloys (5-390 W m−1 K−1). Only copper has a higher thermal conductivity, while steel has values in the range 10-55 W m−1 K−1.


Nadia.

•When impurities are added to a substance, it would lower its freezing point.

•Impurities are other foreign substances.

•Example:

–I have 2 cups of water, A and B.

–I add salt to B. When I try to freeze the water in both cups, A would freeze at 0oC but B would freeze maybe at –10oC.

–This means B would stay as a liquid even at 0oC or -2oC or lower until the temperature reaches –10oC.

–When A becomes solid ice at 0oC, B is still liquid.

In physics, thermal conductivity, k, is the property of a material that indicates its ability to conduct heat. It appears primarily in Fourier's Law for heat conduction. Thermal conductivity is measured in watts per kelvin per metre (W·K⁻¹·m⁻¹). Multiplied by a temperature difference (in kelvins, K) and an area (in square metres, m²), and divided by a thickness (in metres, m) the thermal conductivity predicts the power loss (in watts, W) through a piece of material.

The reciprocal of thermal conductivity is thermal resistivity.

THERMAL PROPERTIES(i)

The transfer of heat is a very important factor to understand and to be able to control during the processing and storage of many food and agricultural products. There are a number of thermal properties that must be understood for the successful process design of all procedures that involve heating and cooling of foods.

Specific Heat

The most basic thermal property is specific heat (c_p). The p subscript indicates that the value is the specific heat at constant pressure. The specific heat of any material is the amount of heat required to raise one unit of mass of the material by one degree. The most commonly used units for specific heat are kJ/(kg-K), Btu/(lb-F), and cal/(g-K). An important equation relating specific heat, mass of the sample (M), the amount of heat that must be added (Q), and the initial and final temperatures of the sample (T1 and T2) is seen below.



There are several equations that can be used to predict unknown specific heats for food and agricultural products. The first two equations are based on the wet basis moisture content (M) of the product. Wet basis moisture content, introduced here, will be covered more extensively in a later section. For the sake of these equations use wet basis moisture content in decimal (rather than percentage) form. These equations predict specific heat in kJ/(kg-K).

Above freezing: c_p = 0.837 + 3.348M
Below freezing: c_p = 0.837 + 1.256M

Another equation commonly used to estimate the specific heat for foods in kJ/(kg-K) takes into account the mass fraction (X) of all of the solids that compose the product. As seen as subscripts in the following equation, w is water, p protein, f fat, c carbohydrate, and a ash.

c_p = 4.180X_w + 1.711X_p + 1.928X_f + 1.547X_c + 0.908X_a

Thermal Conductivity

Thermal conductivity (k) is another important thermal property. Normally expressed in the units W/(m-K) or Btu/(h-ft-F), it is a property that tells how well a material conducts heat. Heat conduction is the transfer of energy between neighboring molecules within a material. The following equation relates the thermal conductivity to the amount of heat that flows through the material per unit of time (dQ/dt), the cross sectional area of the material through which the heat flows (A), and the temperature difference per unit of length of the conducting material (dT/dx).

Thermal conductivity can be greatly influenced by a number of factors such as the water content, porosity, and even fiber orientation of the material. However, there are a couple of equations that allow for the estimation of k when experimental data are not available. The following equation yields the best results when used at higher moisture contents. It relates the weight fraction of water X_w, thermal conductivity of water k_w, and the thermal conductivity of the solids portion of the material k_s, which is assumed to be 0.259 W/(m-K).

k = k_wX_w + k_s(1 – X_w)

Below is another equation for estimating thermal conductivity. It is only considered useful for materials containing greater than 50% water.

k = 0.056 + 0.57 X_w

Thermal Diffusivity

Thermal Diffusivity (a) is a thermal property to describe a homogeneous, isotropic material where k, r, and c_p are constant throughout the material for the entire temperature range being considered. This quantity reveals the material’s ability to conduct heat relative to its ability to store heat. Thermal diffusivity is equal to k/(r*c_p) and is usually expressed in the units of m²/s or ft²/s. Its primary use is in the following partial differential equation, Fourier’s general law of heat conduction. This equation expresses the temperature (T) variation within a three dimensional object (x,y,z)

Thermal Diffusivity can also be estimated based on the weight fraction of its water, fat, protein, and carbohydrate components using the following equation.

a = 0.146*10^-6X_w + 0.100*10^-6X_f + 0.075*10^-6X_p + 0.082*10^-6X_c

Latent Heat

Latent heat (L) is the heat that is exchanged with a material during a phase change, when the heat exchanged does not result in a change in the temperature of the material. The units for latent heat are kJ/kg or Btu/lb. Latent heat is usually subdivided into latent heat of freezing and latent heat of vaporization. An example of latent heat of freezing is the 335 kJ that 1 kg of water releases while maintaining its temperature at 0 C when changing from the liquid to the solid state. Latent heat of vaporization is represented by the 2257 kJ that 1 kg of water must absorb while temperature remains constant at 100 C to evaporate from liquid into vapor. Latent heat can represent a huge expenditure of energy in food processing when freezing or evaporation is involved. Latent heat is best determined through experimentation, but it also can be estimated based on the mass fraction of water in the product.

kaixian

In physics, thermal conductivity, k, is the property of a material that indicates its ability to conduct heat. It appears primarily in Fourier's Law for heat conduction. Thermal conductivity is measured in watts per kelvin per metre (W·K⁻¹·m⁻¹). Multiplied by a temperature difference (in kelvins, K) and an area (in square metres, m²), and divided by a thickness (in metres, m) the thermal conductivity predicts the power loss (in watts, W) through a piece of material.The reciprocal of thermal conductivity is thermal resistivity.

Adeline

Matter comes in many forms. At this point, we will consider three: solid, liquid and gas. A solid has a definite shape and it takes up a fixed volume. On the other hand, liquid has a fixed volume, but no definite shape. A liquid will change its shape to fit its container. A gas has no definite shape and no fixed volume. Like a liquid, a gas will change its shape to fit its container, but it will also expand to fill the entire volume of the container.

Solid (s) – rigid, fixed volume, fixed shape.
Liquid (l) – definite volume, but no definite shape.
Gas (g) – no fixed shape, no fixed volume।

Yi Hui

Effect of Impurities on Freezing Point

When impurities are added to a substance, it would lower its freezing point.
Impurities are other foreign substances
Example:
I have 2 cups of water, A and B.
I add salt to B. When I try to freeze the water in both cups, A would freeze at 0oC but B would freeze maybe at –10oC.
This means B would stay as a liquid even at 0oC or -2oC or lower until the temperature reaches –10oC.
When A becomes solid ice at 0oC, B is still liquid.



From:Xi Li (:

Matter is anything that has mass and takes up space.

Anything around us and in the entire universe can be classified as either matter on energy.

The Particle Theory of Matter:

* 1. Matter is made up of tiny particles (Atoms & Molecules)
* 2. Particles of Matter are in constant motion.
* 3. Particles of Matter are held together by very strong electric forces
* 4. There are empty spaces between the particles of matter that are very large compared to the particles themselves.
* 5. Each substance has unique particles that are different from the particles of other substances
* 6. Temperature affects the speed of the particles. The higher the temperature, the faster the speed of the particles.

Change in temperature

When a material reaches the temperature at which a change in state occurs, the temperature will remain the same until all the energy is used to change the state.

Melting

When a solid is heated, its temperature rises until it reaches its melting point. Any additional heat added to the material will not raise the temperature until all of the material is melted.

Thus, if you heat some ice, its temperature will rise until it reaches 0° C (32° F). Then the ice will stay at that temperature until all the ice is melted. The heat energy is used to melt the ice and not to raise the temperature. After the ice is melted, the temperature of the water will continue to rise as more heat is applied.

Boiling

When a liquid is heated, its temperature rises until it reaches its boiling point. The temperature will then remain at that point until all of the liquid is boiled away.

For example, the temperature of a pot of water will increase until it reaches 100° C (212° F). It will stay there until all the water is boiled away. The temperature of the steam can then be increased.

Cooling

Likewise, when a gas is cooled, its temperature will drop until it reaches the condensation point. Any additional cooling or heat loss will not lower the temperature until all of the gas is condensed into the liquid state.

Then the temperature of the liquid will continue to drop as more cooling is applied। Once the liquid reaches the freezing point, the temperature will remain at that point until all of the liquid is solidified. Then the temperature of the solid can continue to decrease.

-Caijun

Conduction

Conduction is the transfer of heat by direct contact of particles of matter. The transfer of energy could be primarily by elastic impact as in fluids or by free electron diffusion as predominant in metals or phonon vibration as predominant in insulators. In other words, heat is transferred by conduction when adjacent atoms vibrate against one another, or as electrons move from one atom to another. Conduction is greater in solids, where a network of relatively fixed spacial relationships between atoms helps to transfer energy between them by vibration.

Heat conduction is directly analogous to diffusion of particles into a fluid, in the situation where there are no fluid currents. This type of heat diffusion differs from mass diffusion in behavior, only in as much as it can occur in solids, whereas mass diffusion is mostly limited to fluids.

Metals (e.g. copper, platinum, gold, iron, etc.) are usually the best conductors of thermal energy. This is due to the way that metals are chemically bonded: metallic bonds (as opposed to covalent or ionic bonds) have free-moving electrons which are able to transfer thermal energy rapidly through the metal.

As density decreases so does conduction. Therefore, fluids (and especially gases) are less conductive. This is due to the large distance between atoms in a gas: fewer collisions between atoms means less conduction. Conductivity of gases increases with temperature. Conductivity increases with increasing pressure from vacuum up to a critical point that the density of the gas is such that molecules of the gas may be expected to collide with each other before they transfer heat from one surface to another. After this point in density, conductivity increases only slightly with increasing pressure and density.

To quantify the ease with which a particular medium conducts, engineers employ the thermal conductivity, also known as the conductivity constant or conduction coefficient, k. In thermal conductivity k is defined as "the quantity of heat, Q, transmitted in time (t) through a thickness (L), in a direction normal to a surface of area (A), due to a temperature difference (ΔT) [...]." Thermal conductivity is a material property that is primarily dependent on the medium's phase, temperature, density, and molecular bonding.

A heat pipe is a passive device that is constructed in such a way that it acts as though it has extremely high thermal कांदुक्टिविटी

ATIQAH

n science, change in the physical state (solid, liquid, or gas) of a material. For instance, melting, boiling, and evaporation and their opposites, solidification and condensation, are changes of state. The former set of changes are brought about by heating or decreased pressure (except for the melting of ice, which is favoured by pressure); the latter by cooling or increased pressure.

These changes involve the absorption or release of heat energy, called latent heat, even though the temperature of the material does not change during the transition between states. See also states of matter. Changes of state can be explained by the kinetic theory of matter. In the unusual change of state called sublimation, a solid changes directly to a gas without passing through the liquid state। For example, solid carbon dioxide (dry ice) sublimes to carbon dioxide gas.
Joanna