Community RSS Feed https://community.wolfram.com RSS Feed for Wolfram Community showing any discussions in tag Physics sorted by active I'm having problems with third type heat transfer boundary condition https://community.wolfram.com/groups/-/m/t/2371852 Hi! I&#039;m a beginner level user of Mathematica, and still learning it. Basically, I&#039;m trying to monitor the natural cooling of an imaginary plastic slab with the top/bottom side insulated, only the left/right side is dissipating heat to the environment. Thus the model reduced to a 1D heat transfer model. The slab has a thickness of 10mm, and initial uniform temperature of 275C, the environment temperature is 25C. I&#039;m trying to implement a Neumann type boundary condition at both left (x = -5) and right (x = 5) side, i.e., k*dT/dx = h(T-Tenvironment) at boundary, but I keep getting error messages that the boundary and initial conditions are inconsistent and more importantly the results looks unstable, my code looks like this, can anyone give me a little help? Would be really appreciated! h = 13/(1000^2);(*units are converted to mm*) k = 0.3/1000; c = 1030; rou = 1380/(1000^3); heq = rou*c*D[u[t, x], t] == k*D[u[t, x], {x, 2}]; ic = u[0, x] == 275; bcs1 = (D[u[t, x], x] /. x -&gt; 5) + h*(u[t, 5] - 25) == 0; bcs2 = (D[u[t, x], x] /. x -&gt; -5) + h*(u[t, -5] - 25) == 0; sol = NDSolve[{heq, ic, bcs1, bcs2}, u[t, x], {t, 0, 20}, {x, -5, 5}]; F[t_, x_] = u[t, x] /. sol[]; Plot3D[F[t, x], {x, -5, 5}, {t, 0, 15}, PlotRange -&gt; All, AxesLabel -&gt; Automatic] Y L 2021-09-23T04:19:16Z Represent constant as symbol instead of Entity? https://community.wolfram.com/groups/-/m/t/2369384 Good day, I am using the constant &#034;ElectricConstant&#034;. in my formula: ![ElectricConstant in Formula] I would rather express it as the symbolic \epsilon_0. Is this possible? Pi is a constant and that is represented in symbolic form. Could you please advise me how? Kind regards, Connor : https://community.wolfram.com//c/portal/getImageAttachment?filename=Screenshot2021-09-19113305.png&amp;userId=2369033 Connor Fitzgerald 2021-09-19T09:36:27Z [GiF] Bouncing rays inside a grid of cylinders https://community.wolfram.com/groups/-/m/t/2369872 ![enter image description here] &amp;[Wolfram Notebook] : https://community.wolfram.com//c/portal/getImageAttachment?filename=bouncing_rays_hero.gif&amp;userId=20103 : https://www.wolframcloud.com/obj/42fb2fe0-ea47-43f8-972a-7437371431d6 Sander Huisman 2021-09-20T18:25:21Z