k = 1.38e-23;
a = 0.71e-10;
r1 = 0.035;
r2 = 0.040;
u = 1.66e-27;
cV = 3*k/2;
sigma = 4*pi*a^2;
sigmaSB = 5.67e-8;
beta = (2*cV/(3*sigma))*sqrt(k/(pi*40*u));
T2 = 293;
T1 = [294:1:450];
Qkappa = (4*pi*beta.*(T1.^(1.5)-T2^(1.5)))./(3*log(r2/r1));
Qrad = 2*pi*sigmaSB.*(r1.*T1.^4-r2*T2^4);
Qtot = Qkappa + Qrad;
figure(1);
plot(T1,Qkappa,T1,Qrad,'--',T1,Qkappa+Qrad,':','linewidth',2);
grid on;
xlabel('Temperatur i indre sylinder (K)');
ylabel('Varmestrøm pr lengdeenhet (W/m)');
handle=legend('Varmeledning','Stråling','Totalt');
set(handle, 'Box', 'off');