# Heat EQ help

Gold Member
hi pf!

i'm wondering if you can help me with the heat eq for a basic cylinder wire problem. namely, we have a wire with radius ##r_i## and length ##L##and resistance is ##R## and current is ##I##. Thus heat produced $$Q = R I^2 \pi r_i^2 L$$. When using the heat eq, we assume time rate of change is negligable. flux is governed by fouriers law, and the divergence theorem gives us the following: $$\int_V k \nabla^2 T dv + \int_V \frac{Q}{\pi r^2 L}dv = 0$$.is this right though? namely, is ##Q## divided by an arbitrary ##r## or the radius ##r_i##?

thanks so much!

Bystander
Homework Helper
Gold Member
heat produced
Q=RI 2 πr 2 i L
Care to tell us how you came up with this expression?

@Chestermiller , @Orodruin

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Gold Member
Care to tell us how you came up with this expression?​
I knew units for heat generation in 3-D need to be watts per cubic meter. So I simply tracked units. This Q multiplied by dv gives us watts, which is the unit we're after.

bigfooted
Gold Member
But what are the units of $k\nabla^2T$ (k should be the thermal diffusivity)?

[edit: I mean k=thermal conductivity of course]

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Gold Member
But what are the units of $k\nabla^2T$ (k should be the thermal diffusivity)?
Watts per cubic meter, right?

bigfooted
Gold Member
and of the term $\frac{Q}{\pi r^2 L} = I^2 R \frac{\pi r_i^2 L}{\pi r^2 L}$ (with $I^2R$ the electric power)?

Gold Member
and of the term $\frac{Q}{\pi r^2 L} = I^2 R \frac{\pi r_i^2 L}{\pi r^2 L}$ (with $I^2R$ the electric power)?
watts? am i missing something here? seems like you are eluding to something.

Gold Member
oh shoooot! i should have defined ##Q : Q = I^2 R / \pi r_i ^2 L## right? but is it ##r_i## or ##r##?

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Chestermiller
Mentor
oh shoooot! i should have defined ##Q : Q = I^2 R / \pi r_i ^2 L## right? but is it ##r_i## or ##r##?
ri. The rate of heat generation per unit volume in the wire is constant. ri should also be what appears in the equation with the integrals.

Chet

Gold Member
Hi chet!

ok, so what i should have modeled from the start is $$\int_v k \nabla ^2T dv = \int_v Q dv : Q = I^2 R / \pi r_i^2 L$$ do you all agree? if so, solving would be (using 1-D radial flow in polar coordinates) $$-k\frac{1}{r}\frac{d}{dr} ( r T') = Q \implies \\ -k d(r T') = rQdr \implies \\ \int_?^{??} -k d(r T') = \int_0^{r_i}rQdr$$
but what are my bounds for integration? any ideas?

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Chestermiller
Mentor
Q is a constant, so it comes out of the integral. You integrate both sides from 0 to ri.

Chet

Gold Member
Are you sure? I'm thinking if we had a differential equation over some interval of time ##[0,T]##, say, $$\frac{dy}{dt} = k \implies \\ \int_0^T \frac{dy}{dt} dt = \int_0^T k dt \implies \\ \int_{y(0)}^{y(T)} dy = \int_0^T k dt$$ but notice we do not have ##[0,T]## on both sides.

Chestermiller
Mentor
Are you sure? I'm thinking if we had a differential equation over some interval of time ##[0,T]##, say, $$\frac{dy}{dt} = k \implies \\ \int_0^T \frac{dy}{dt} dt = \int_0^T k dt \implies \\ \int_{y(0)}^{y(T)} dy = \int_0^T k dt$$ but notice we do not have ##[0,T]## on both sides.
OK. 0 to rT' evaluated at ri.

Chet

Gold Member
OK. 0 to rT' evaluated at ri.

Chet
But in this case we wouldn't have a function of ##r##. Would we instead just integrate from ##0,r## generally so we can still have a profile rather than a number?

Chestermiller
Mentor
But in this case we wouldn't have a function of ##r##. Would we instead just integrate from ##0,r## generally so we can still have a profile rather than a number?
That's fine, but you seemed to be applying the equation over the entire volume. Integrating out to R is just fine.

Chet

Gold Member
So if I'm understanding this correctly we would have $$-k\int_{0*T'(0)}^{r*T'(r)} d(r T') = Q\int_0^r r dr \implies \\ -k r T'(r) = Qr^2/2 \implies \\ -kT'(r) = Qr/2 \implies \\ -kT'(r) = \frac{R I^2}{2 \pi r_i^2 L r}$$ is this right so far?

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Gold Member
But then if I made a flux balance we could write ##q = R I^2 / (2 \pi r L)## watts/sq. meter. Fourier's law implies also ##q = -k T'(r)## (1-D radial flow). Thus, $$-kT'(r) = \frac{R I^2} { 2 \pi r L}$$ which doesn't agree with the above. Can you help me with what I'm doing wrong?

Chestermiller
Mentor
So if I'm understanding this correctly we would have $$-k\int_{0*T'(0)}^{r*T'(r)} d(r T') = Q\int_0^r r dr \implies \\ -k r T'(r) = Qr^2/2 \implies \\ -kT'(r) = Qr/2 \implies \\ -kT'(r) = \frac{R I^2}{2 \pi r_i^2 L r}$$ is this right so far?

Gold Member
Sorry, we would have $$-kT'(r) = \frac{RI^2 r}{2 \pi r_i^2 L}$$ right? But this still doesn't agree with the flux balance.

Chestermiller
Mentor
Sorry, we would have $$-kT'(r) = \frac{RI^2 r}{2 \pi r_i^2 L}$$ right? But this still doesn't agree with the flux balance.
Who says? Multiply both sides by 2πrL and see what you get.

Chet

Gold Member
Who says? Multiply both sides by 2πrL and see what you get.

Chet
I must be missing something. The flux balance states ##-kT'(r) = R I^2 / (2 \pi r L)## yet the heat eq method states ##-kT'(r) = R I^2 r / (2 \pi r_i^2 L)##. These two are different. I must have made a mistake but I'm not seeing it.

Gold Member
If the ##r_i^2## was simply ##r^2## then we would have agreeing equations.

Chestermiller
Mentor
I must be missing something. The flux balance states ##-kT'(r) = R I^2 / (2 \pi r L)## yet the heat eq method states ##-kT'(r) = R I^2 r / (2 \pi r_i^2 L)##. These two are different. I must have made a mistake but I'm not seeing it.

The first equation is correct only at r = ri. The second equation is correct at all radial locations.

Gold Member
The first equation is correct only at r = ri. The second equation is correct at all radial locations.
Can you elaborate here. I'm wondering what I have done wrong in the flux balance. It really looked right to me.

Chestermiller
Mentor
Can you elaborate here. I'm wondering what I have done wrong in the flux balance. It really looked right to me.
If the rate of heat generation within the wire is spatially uniform, what fraction of the heat is generated between r = 0 and arbitrary radial position r? What is the rate of heat generation within the wire per unit volume? What is the rate of heat generation between r = 0 and arbitrary radial position r?

Chet

Gold Member
If the rate of heat generation within the wire is spatially uniform, what fraction of the heat is generated between r = 0 and arbitrary radial position r? Chet
##\pi r^2 L / (\pi r_i^2 L) = (r/r_i)^2##
What is the rate of heat generation within the wire per unit volume?
Chet
##R I^2 / (\pi r_i^2 L)##
What is the rate of heat generation between r = 0 and arbitrary radial position r?
Chet
$$\int_0^L \int_0^{2 \pi} \int_0^r \frac{R I^2}{ \pi r_i^2 L} (r dr d \theta d z) = R I^2 \left(\frac{r}{ r_i}\right)^2$$

Gold Member
Am I missing something though? How does this relate to flux (if we are doing the flux balance)?

Chestermiller
Mentor
Am I missing something though? How does this relate to flux (if we are doing the flux balance)?
OK. Now go back to that equation I indicated and multiply both sides by 2πrL. Show us what you get. Then see if you can interpret what each side of the equation represents physically.

Chet

Gold Member
The r.h.s. is (obviously) the heat generation at some arbitrary distance r, as you've already said. and i agree that the left hand side has the same units, but it's difficult for me to see this without the r.h.s (i understand that it is heat generation, but i don't think it's obvious that it's total heat generation from 0 to r).

but the relation is obvious now! thanks! although what did i do wrong in trying to make the flux balance with an arbitrary r? as you've said, it's only correct when ##r=r_i##.

Chestermiller
Mentor
The r.h.s. is (obviously) the heat generation at some arbitrary distance r, as you've already said. and i agree that the left hand side has the same units, but it's difficult for me to see this without the r.h.s (i understand that it is heat generation, but i don't think it's obvious that it's total heat generation from 0 to r).

but the relation is obvious now! thanks! although what did i do wrong in trying to make the flux balance with an arbitrary r? as you've said, it's only correct when ##r=r_i##.
The flux balance is wrong because it implicitly assumes that all the heat generation takes place between r = 0 and radial location r, and none of the heat is generated between r and ri.

Chet

Gold Member
The flux balance is wrong because it implicitly assumes that all the heat generation takes place between r = 0 and radial location r, and none of the heat is generated between r and ri.

Chet
ahh yes, this makes sense! so, if we were to look at the flux at some ##r > r_i## would we be able to use the flux argument?

Chestermiller
Mentor
ahh yes, this makes sense! so, if we were to look at the flux at some ##r > r_i## would we be able to use the flux argument?
r > r1 is outside the wire. We don't know what's happening out there, do we?

Chet

Gold Member
r > r1 is outside the wire. We don't know what's happening out there, do we?

Chet
sorry, i'm speaking in hypotheticals. and yea, if it was the same material but no heat generation.

Chestermiller
Mentor
sorry, i'm speaking in hypotheticals. and yea, if it was the same material but no heat generation.
Then it would be OK.

Chet

joshmccraney
Gold Member
Then it would be OK.

Chet
Thanks chet!