Conduction Definition and 423 Threads

Thermal conduction is the transfer of internal energy by microscopic collisions of particles and movement of electrons within a body. The colliding particles, which include molecules, atoms and electrons, transfer disorganized microscopic kinetic and potential energy, jointly known as internal energy. Conduction takes place in all phases: solid, liquid, and gas.
Heat spontaneously flows from a hotter to a colder body. For example, heat is conducted from the hotplate of an electric stove to the bottom of a saucepan in contact with it. In the absence of an opposing external driving energy source, within a body or between bodies, temperature differences decay over time, and thermal equilibrium is approached, temperature becoming more uniform.
In conduction, the heat flow is within and through the body itself. In contrast, in heat transfer by thermal radiation, the transfer is often between bodies, which may be separated spatially. Also possible is the transfer of heat by a combination of conduction and thermal radiation. In convection, the internal energy is carried between bodies by a moving material carrier. In solids, conduction is mediated by the combination of vibrations and collisions of molecules, of propagation and collisions of phonons, and of diffusion and collisions of free electrons. In gases and liquids, conduction is due to the collisions and diffusion of molecules during their random motion. Photons in this context do not collide with one another, and so heat transport by electromagnetic radiation is conceptually distinct from heat conduction by microscopic diffusion and collisions of material particles and phonons. But the distinction is often not easily observed unless the material is semi-transparent.
In the engineering sciences, heat transfer includes the processes of thermal radiation, convection, and sometimes mass transfer. Usually, more than one of these processes occurs in a given situation.
The conventional symbol for thermal conductivity is k.

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  1. P

    Why Is Iron a Better Conductor of Heat Than Wood?

    We know, heat transfer in a material happens due to vibration or vigorous to and fro movement of particles. Also, the density of wood is much more less than the density of iron i.e, distance between particles is more. Thus, particles in wood has more space to vibrate vigorously than the...
  2. jegues

    Buck/Boost DC-DC Converter Discontinuous Conduction Mode

    EDIT: I have updated the original post to make my confusion more clear! Hello all, Attached below are the pages from my textbook for which I am concerned. On page 245 we can calculate the average inductor current as follows, I_{L} = \frac{1}{2}I_{max}(D+D_{1}) Now what I thought an...
  3. Y

    Delocalizing Electrons: Time and Diffusion Lengths in Conduction Bands

    Recently there was a discussion concerning whether an electron in the conduction band was localized or delocalized. The back and forth arguments seemed inconclusive, difficult to follow (too brief too couched in jargon/unrealistic models) and ended in something close to insults back and forth...
  4. J

    Heat conduction (Fourier complex integral)

    Homework Statement A semi-infinite bar (0 < x < 1) with unit thermal conductivity is fully insulated at x = 0, and is constantly heated at x = 1 over such a narrow interval that the heating may be represented by a delta function: \frac{\partial U}{\partial t}=\frac{\partial^2 U}{\partial...
  5. A

    Thermal conduction of heat by a solid body

    Hello.. The query is regarding thermal conduction of heat from a solid body: Suppose we have a long metallic rod which is insulated on the curved surface and one of the bases. If we supply Q amount of Heat for a small time to the rod from the non-insulated end, initially the temperature of...
  6. C

    Density of carriers in a Conduction band

    Homework Statement Find the density of carriers N in the conduction band of intrinsic silicon at room temperature Homework Equations lower integrand Ec, upper integrand= Ec+1*qe, Eg=1.12eV, Ev=0 Ec=Ev+Eg Ef=Ev+Eg/2 m*n=1.08*(9.11*10^31), T=300K...
  7. G

    Heat transfer via conduction from hollow cylinder to Bitumen

    Hey all, not sure if this is where I'm supposed to post this question, but it is all about heat transfer. So this is part of my design project for my 4th year of chemical engineering. Problem Statement: A hollow cylinder 400 m long has hot combustion gases T = 500 Celsius flowing through...
  8. D

    Need help with heat transfe (conduction) questions

    Homework Statement A small pond has a layer of ice 1cm thick floating on its surface. The air temperature is -10 C. Steps (a-c) should help you fi nd the final answers in (d) and (e) (a) What is the temperature of the liquid water just below the ice? (b) Write the expression for...
  9. D

    Mass Diffusion and Heat Conduction: A Steel Carburization Example

    One can show that mass diffusion without chemical reactions obeys the same basic equation as heat conduction. The one dimensional equation in dimensionless variables is given by $$ D_{AB}\frac{\partial^2 C_A}{\partial x^2} = \frac{\partial C_A}{\partial t} $$ where C_A is the concentration...
  10. B

    Charging a spherical shell by conduction

    Homework Statement An insulated spherical conductor of radius R1 carries a charge Q. A second conducting sphere of radius R2 and initially uncharged is then connected to the first by a long conducting wire. (a) After the connection, what can you say about the electric potential of each...
  11. G

    Fouriers law of heat conduction.

    If I have the equation -k*dT/dr=0 do I get rid of the constant k and integrate, or do i integrate first and keep the constant?
  12. Q

    Understanding Winter Thermal Conduction on Wooden Floors

    in winter, wooden floor feels cold to the barefoot but the rugs and carpets don not feel cold although the floor and carpet are at the same temperature?
  13. S

    Finding power needed to heat an oven through radiation and conduction

    I was given an oven that comprises of several cylinders that fit inside one another. The innermost cylinder is a Molybdenum oven that will hold a sample of metal. A ceramic cylinder fits over this. A tungsten wire heating element is wrapped around this cylinder. Two more ceramic cylinders fit...
  14. B

    Calculator Question: Graphing an Equation for Heat Conduction in a Rod

    Homework Statement For the rod in Problem 10 (already solved this, see below): (a) plot u vs. x for t= 5, 10, 20, 40, 100, and 200 (b) plot u vs. t for x= 10, 20, and 30 (c) how long does it take for the entire rod to cool off to a temp. of no more than 1 degree C? Homework...
  15. fluidistic

    Number of electrons in the conduction band of various metals

    Homework Statement Calculate the number n_0 of electrons per volume unit in the conduction band for the lithium, copper and aluminium. Compare this result with the number of electrons of valence per volume unit of these metals. Homework Equations None given, no ressource either...
  16. B

    How Is the Constant C₂ Determined in a Heat Conduction Problem?

    Homework Statement Folks, I am self studying through a heat conduction problem involving a 2nd order linear homogenous differential equation which has the solution of the form ##\theta (x)=C_1\cosh mx+ C_2\sinh mx## (1) where ##m \equiv \sqrt \frac{c}{a}= \sqrt{\frac{\beta P}{k A}} ##...
  17. S

    Speed of heat conduction through glass

    Is anyone here familiar with the speed of a temperature wave through Si02 or other electrical insulators? Thanks, S.L.
  18. Z

    Something About My Deduction On the Fourier's Law of Heat Conduction

    My hypothesis is to use two different stable heat sources with different tempreature T1 and T2 (T1>T2) transmits the heat . Then I let the distance between this two heat sourse filled with idea gas or ideal metal in a tube. So if the distance is L, the heat capacity is Cv (Constant). So the...
  19. P

    Conduction in reverse biasing too? Is it the case with transistor?

    Conduction in reverse biasing too?? Is it the case with transistor?? I have a mechanical analogy of diode in which a socket is provided with a one way valve. (figure 1st below). The arrow inside the circle shows the direction of motor pump. When there is enough pressure to lift the valve...
  20. K

    Newtons law of cooling vs conduction equation in solids

    I know that Newtons law of cooling is useful when a solid is transferring heat to a fluid. And conduction equation is used in solids. My question is: 1) Can Newtons law of cooling be applied in case of solids (apart from Heat conduction equation)? 2) Can Newtons law of cooling be applied in...
  21. L

    Don't get the conduction band in solid

    I was reading about electronic band structure in wiki and I didn't understand why the conduction bands are necessary for flowing current in a material. If all the atoms in a solid shares valence band and all its eletrons, then these electrons may travel through the solid. I didn't get why the...
  22. E

    Calculating heat loss and conduction through a wire

    I'm creating a project where I am attempting to melt solder paste using the current running through two copper wires, in a kind of lap joint. I'm trying to figure out how much current would actually be going through the wires if I use a 9V battery. I'm not sure where to take into account heat...
  23. P

    Unravelling Heat Transfer: The Mystery of Conduction & Convection

    I am confused as to how heat transfer really works. I've read all about conduction/convection/radiation. Radiation makes sense to me, electrons enter a higher band, come back down and give off energy in the form of EM waves. These waves can then be absorbed by other atoms causing electrons to...
  24. J

    How Does Fourier's Law Apply to Heat Flow Through a Composite Wall?

    Using Fourier’s law and an electrical analogy, derive the expression given below for the heat flow, Q, through a flat composite wall of surface area, A, with three layers of thicknesses x1, x2 and x3 and thermal conductivities k1, k2 and k3, and an overall temperature difference ΔT. State any...
  25. Q

    Application of Fourier law of conduction

    Consider a wire of length l. At time t=0, one end of the wire is at temperature T_0, while the rest of the wire is at temperature T_1, T_1<T_0 (for example, one end of the wire is kept in contact with an external hot body, at temperature T_0). After how much time the temperature near the second...
  26. C

    Conduction - Heat Equation - Units Don't Add Up

    Conduction - Heat Equation - Units Don't Add Up! Hi there I have what I think/hope is a simple question: I've been working on heat inputs and outputs in inertia friction welds and have managed to produce a net power term (W) as a function of time. I now want to use that in the heat...
  27. S

    Heat Transfer: Matlab 2D Conduction

    Homework Statement Having trouble with code as seen by the gaps left where it asks me to add things like the coefficient matrices. Any Numerical Conduction MATLAB wizzes out there? A long square bar with cross-sectional dimensions of 30 mm x 30 mm has a specied temperature on each side, The...
  28. S

    Heat conduction in a beam with variable x-section

    Say we have a laterally insulated beam and some boundary conditions at either end, be it convective or fixed-temperature, but the cross-sectional area is variable. If the cross-section were constant I'd just say it were a 1-D problem, but I'd imagine that having the cross-sectional area be a...
  29. T

    Derivation of Heat Conduction in Spherical Co-Ordinates

    I have two questions. I believe I have solved the first question and would like confirmation of this answer; the second question I'm a little bit lost on so any help there would be greatly appreciated! I am working on a problem set in which I must derive the equation for heat conduction in...
  30. F

    Pn junction, diode and energy level of conduction band during forward biasing

    Hello Forum, I am looking at the energy levels between the n and p regions of a diode. The p-side has a conduction band that starts at a higher energy (same for the valence band). The n-side has the valence and condution bands that are at lower energy. The Fermi energy line is the same...
  31. I

    Transient heat conduction in a slab

    Homework Statement Consider unsteady state heat conduction through an infinitely wide slab of solid material of thickness 2L. There is no internal heat generation and the thermal properties of the material are independent of temperature and position. Starting from an energy balance, show...
  32. C

    What is the difference between valence and conduction bands in metallic bonding?

    hello, i have a question related to metallic bonding. As far as i know, valence band is defined as the highest filled level in the band and conductionband as the 1st unfilled level. So, if I have 100 Li, we will have 100 orbitals which form a band. Since we have 100electrones, 50 pairs, so we...
  33. L

    Heat conduction in gravity field without convection

    I used to think that the heat does rise even in solid metals with no gas/liquid around (No density argument is possible then.) But couldn't find anything describing or even verifying it. I'am pretty sure the gravitationally induced anharonicity in the atomic core potentials should have at...
  34. C

    Understanding Valleys in the Conduction Band

    Hi and thanks for reading, I don't really understand the valleys in the conduction band, in the E vs. k diagram, there is the L-valley, r-valley and X-valley. Each has a different momentum... and are at different energy levels. I understand that at any temperature above absolute zero, a...
  35. L

    I from a physics brain: thermal capacity, conduction, cooling

    I am doing a zainy experiment at home. I need to devise a way of applying a cooling intensity factor (CIF) of around 70 mW/cm2. I need to cool material that is roughly 30 degrees Celsius down to around 2 degrees Celsius. The cooling has to last for a period of one hour or more. The surface...
  36. D

    Convection, Conduction, Radiation

    In my book I'm reading about heat, specifically convection. There is an image of a person with a drink in their hand, suntanning while a fan blows on them. I get that the drink represents conduction, the sunlight is radiation, but how is the fan convection?
  37. M

    How Do You Calculate Temperature at a Depth Based on Surface Conduction Rates?

    Homework Statement The average rate at which energy is conducted outward through the ground surface in North America is 54 mW/m^2, and the average thermal conductivity of the near-surface rocks is 2.5 W/mK. Assuming a surface temperature of 10 C, find the temperature at a depth of 35 km...
  38. L

    Calculating Heat Loss through 1m Glass Pane - 15oC Difference

    Determine the heat loss through a 1m square pane of 6 mm thick glass for a temperature difference of 15oC. Thermal conductivity of glass is k = 0.72 W m^-1 K^-1 Solution Rate of loss of heat = 0.72W m^-1 K^-1 x 1 m^2 x 15oC/(6x10^-3 m ) <--- Ok so I understand everything but I don't...
  39. L

    Conduction and lossless materials

    Ok, I am looking over stuff for an exam. I am a bit confused on something. When conductivity is zero, a medium is lossless. Zero conductivity means infinite resistance. For infinite resistance, no electrons will flow, but an electric field can propagate forever. A wire has high conductivity, so...
  40. M

    What is the Skin Effect in Electrical Conductors?

    I quote from Oliver Heaviside, "Electromagnetic Theory", 1893, p.15: It was discovered by mathematical reasoning that when an electric current is started in a wire it begins entirely upon its skin, in fact upon the outside of its skin; and that, in consequence, suficiently rapidly...
  41. M

    2d steady state conduction matlab help

    Homework Statement Hey guys, I am doing a steady state 2d heat conduction problem which is a 1mx1m cavity with three different wall temperatures on the 4 different walls. I am using the principle of superposition to work this out. What I want to do is plot a heat map on matlab. I...
  42. W

    Is the conduction band of metals empty at absolute zero?

    Hi, I understand for metals the conduction band is usually partially filled. However, at absolute zero, is the conduction band empty (for metals)? I'm asking this because as far as I know, the definition of valence band is "the highest occupied band at absolute zero" However, I have...
  43. S

    Fermi level, band structure of solids, and effect on electrical conduction

    how is the relationship between fermi level and band structure of solids can effect electrical conduction properties of a material?
  44. F

    Conduction mechanism of SnO2 semiconductors

    Hello everybody and welcome to my first thread. I have read several publications regarding this compound, and found that SnO2 behaves as n-type semiconductor (Eg 3.6 eV) when oxygen vacancies are present. Very summarized and simply put, I understood that this lack of oxygen atoms enhance the...
  45. G

    What Happens at the Micro Level During Electrical Conduction Through a Medium?

    air, glass, mica, rubber, water, etc all are insulators because they don't have free charges to carry the current... I don't understand what do we mean by "carry"?? i mean, i don't really understand what happens at the micro level during electrical conduction through a medium...How do the...
  46. A

    Heat conduction in hollow cilinder.

    Homework Statement Given is a hollow cilinder with inner radius R1 and outer radius R2. The heat conductivity of the material is k. The cilinder has length l and an inner temperature of T1 and outer temperature T2. Determine the temperature gradient in the cilinder and the heat flow that leaks...
  47. L

    Phase Shift effect on diode conduction time

    Ok, so let's say I have a simple half-wave rectifier like the one shown in the image below: http://img585.imageshack.us/img585/1904/unledmwc.jpg When it comes to the analysis of the circuit, during the positive cycle of the input voltage, the diode is forward biased and the output voltage...
  48. Z

    How long does it take for a valence electron being excited to conduction band?

    We know in silicon a valence electron can be excited to conduction band if it absorbs one photon(one photon absorption) or two photon(two photon absorption),my question is that it must need a certain amount of time for the electron to 'travel' from the valence band to the conduction band(I...
  49. A

    Analytical Solutions for 1D Transient Heat Conduction in Cylindrical Coordinates

    Hi, I have written a numerical code to solve the 1D heat equation in cyclindrical coordinates: \frac{\partial T}{\partial t}=\kappa\left(\frac{\partial^{2}T}{\partial r^{2}}+\frac{1}{r}\frac{\partial T}{\partial r}\right) The problem I'm considering is a hollow cylinder in an infinite...
  50. E

    What is the Solution to a Heat Conduction Problem in a Rod?

    Homework Statement Find the solution of the heat conduction problem: 100Uxx=Ut, 0<x<1, t>0; U(0,t)=0, U(1,t)=0, t>0; U(x,0)=sin(2\pix)-sin(5\pix), 0\leqx\leq1 Homework Equations U(x,t)=\sumcne(-n2\pi2\alpha2t)/L2sin((n\pix)/L)...
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