Heat Conduction!

So the point of this post is to develop a mathematical model for the conduction of heat through a wire.  This post will be deriving "The Heat Equation" in the one dimensional sense.

Imagine you have a wire.  If you can, imagine a small volume of the wire between two cross sections.  Here is what I mean..

Now we are going to make a simplifying assumption and assume that the temperature in the wire is uniform with respect to the cross section.  In other words, the cross sectional area A or B has the same temperature throughout its area.  The temperature of the wire will be denoted u(x,t).  Notice that we are constructing a model with an expression of two variables.  The temperature depends on both WHERE it is in the wire (in the x axis only, remember our simplifying assumption!) and WHEN you are looking at is.  In these types of models, when looking for a solution, you often want to look for one that has what is known as a "steady state." Meaning, as time approaches infinity, the temperature function would not depend on time anymore, but only on x.  I might do an example of this in the future.

So using our function notation, the temperature in the cross section A will be u(x,t) and the temperature in B will be u(x+Δx,t).  (The distance between B and A is Δx).  Let's get a few physics principles out of the way to help us in the derivation.


1.  Heat Conduction: The rate of heat flow, is proportional to the partial derivative of the temperature function with respect to x.  This is essentially saying that the rate of heat flow is proportional to the temperature gradient.  If you think about it, it makes sense.  If one side of the wire is extremely hot and the other is ice cold, the rate of heat flow will be larger than if both sides were hot but at different temperatures.  Since this relationship is directly proportional, we must use a proportionality constant k to equate them.  This is called the thermal conductivity of the material and can actually vary from point to point.  Therefore k=k(x).


2.  Direction of Flow:  The flow of heat is always from higher temperature to lower.  Again this makes sense conceptually.  You will never be able to witness a cup of hot coffee get hotter while it sits on the table! (I can get into a WHOLE thing dealing with thermodynamics, and I intend to eventually!.... But at another time..)


3.  Specific Heat Capacity:  Specific heat capacity (The name is most definitely a misnomer as it does not make sense to the physical definition of heat! You physicists know what I am talking about! Again, I will rant about that later on another post.) is defined as the amount of heat necessary to raise the temperature of an object of mass m by an amount Δu (remember u is temperature) is equal to cmΔu.  The variable c is yet another proportionality constant and is the specific heat capacity of the substance.  This value can also vary from point to point, therefore c=c(x).


Now we introduce the variable Q to denote the amount of heat transferring through the surface A in a time Δt.

So what does this mess mean?  Well take a look, we combined a lot of what we just said above!  Remember what we just said, that the amount of heat passed through a cross section A in a time Δt proportional to the the temperature gradient (which is the partial derivative expression), and in order to equate them, you need a proportionality constant k(x).  This value is them multiplied over the area of A and a time Δt.  The negative sign is used to indicate direction.  If the temperature gradient is positive from left to right, then it flows the opposite direction.

So now we can apply this formula for the heat flowing through the cross section B!

Often in physics, we need to apply some sort of conservation law.  Conservation laws in mathematics are extremely powerful in developing models and figuring out problems.  In this case we will be saying that the amount of heat entering the volume V (The volume between cross section A and B) equals the amount of heat exiting.  Now, in reality this is not always true, therefore we need to account for that.  What could happen is, heat could be generated inside the wire, therefore adding more heat coming out of cross section B.  We model that term in much the same way as we have modeled the previous heat flow terms.

So using this idea of a conservation law, we obtain.. (E represents the amount of energy)

Now notice that we have equated A and B.  We can do this because they are indeed the same area. Now the change in heat (or energy) is also equal to the change in heat capacity of the substance.  So..

Where Δu is the change in temperature, m is the mass of the wire element, and c is the specific heat capacity.  We can use the definition of density to change the mass term.  Plugging things in we obtain..

Notice that the density and specific heat capacity can change from point to point.  We are allowed to use the expression for Δu there because we are assuming the change in temperature of the volume is the change in temperature of x.  Strictly in one dimension.  Equating the two expressions for ΔE...

We will now divide both sides by AΔxΔt, then take the limit as Δx and Δt approach zero.

We will now let all the physical parameters be constant throughout the wire.  This is usually a good approximation.

And that is the heat equation! Notice that we have defined beta as k/cp.  This is called the diffusivity of the material.  We have also simplified things a bit by defining P(x,t) as (1/cp)G(x,t).

At this point, we need to be able to impose boundary conditions on the equation.  What is often done is to either say that each end of the wire is at zero degrees, or that each end of the wire is kept at a constant temperature.  In other words..

(Why does the second one represent constant temperature at each end of the wire? Think about it!)  We will also need an initial distribution of heat at t=0.  In other words..

So summing everything up, the heat equation with boundary conditions are..

So what if we want two or three dimensions? The heat equation would then become..

So basically we have used the Laplacian operator to make the problem into three dimensions!

And THAT my friends, is the heat equation.  In order so solve this equation you need to use the method of separation of variables and solve the various eigenvalue problems that arise. (Depending on the dimension you choose.)  After that you will need to compute the Fourier series of the initial heat distribution.  I think I will do a post on this later on.  

Hope all you math nerds enjoyed!  If you see anything wrong or have questions, please comment!