- #1
Tsunoyukami
- 215
- 11
I am attempting to complete a problem for a problem set and am having difficulty simplifying an expression; any help would be greatly appreciated!
The question is a physics question which attempts to derive an equation for the temperature within a planet as a function of depth assuming spherical co-ordinates. "Conside a thin spherical shell whose inner boundary has radius r and outer boundary had radius r + ∂r. The heat flux into the shell from the inner boundary is q[itex]_{r}[/itex](r) and the heat flux out of the shell at the top boundary is q[itex]_{r}[/itex](r + ∂r). The energy produced by radioactivity in the shell is [itex]\rho\rho[/itex]HV where V is the volume of the spherical shell, H is the heat generation rate per unit mass and [itex]\rho[/itex] is the density."
To determine the volume of the shell V I have used:
V = V(r + ∂r) - V(r)
Using the fact that ∂r [itex]\rightarrow[/itex] 0 I have found:
V = 4[itex]\pi[/itex]r[itex]^{2}[/itex]∂r + 4[itex]\pi[/itex]r ∂r[itex]^{2}[/itex]
Is this a reasonable time to use the fact that ∂r [itex]\rightarrow[/itex] 0? Is the second term in my expression for V negligible?
The primary question I have is whether or not there is a way to simplify q[itex]_{r}[/itex](r + ∂r); I feel like there is - and I feel like it might have to do with a Taylor expansion, but I haven't been able to figure it out - all the links I find when I do a google search are either too simple or too complex. Any help would be greatly appreciated!
(After I do this I have to write an expression that says the sum of the heat going in, the heat leaving and the heat produced inside the shell equals 0 [by assuming in is either positive or negative and out is the opposite sign of in).
Again, any help would be greatly appreciated!
The question is a physics question which attempts to derive an equation for the temperature within a planet as a function of depth assuming spherical co-ordinates. "Conside a thin spherical shell whose inner boundary has radius r and outer boundary had radius r + ∂r. The heat flux into the shell from the inner boundary is q[itex]_{r}[/itex](r) and the heat flux out of the shell at the top boundary is q[itex]_{r}[/itex](r + ∂r). The energy produced by radioactivity in the shell is [itex]\rho\rho[/itex]HV where V is the volume of the spherical shell, H is the heat generation rate per unit mass and [itex]\rho[/itex] is the density."
To determine the volume of the shell V I have used:
V = V(r + ∂r) - V(r)
Using the fact that ∂r [itex]\rightarrow[/itex] 0 I have found:
V = 4[itex]\pi[/itex]r[itex]^{2}[/itex]∂r + 4[itex]\pi[/itex]r ∂r[itex]^{2}[/itex]
Is this a reasonable time to use the fact that ∂r [itex]\rightarrow[/itex] 0? Is the second term in my expression for V negligible?
The primary question I have is whether or not there is a way to simplify q[itex]_{r}[/itex](r + ∂r); I feel like there is - and I feel like it might have to do with a Taylor expansion, but I haven't been able to figure it out - all the links I find when I do a google search are either too simple or too complex. Any help would be greatly appreciated!
(After I do this I have to write an expression that says the sum of the heat going in, the heat leaving and the heat produced inside the shell equals 0 [by assuming in is either positive or negative and out is the opposite sign of in).
Again, any help would be greatly appreciated!