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I have an integral that I would like to evaluate over a continuous range of lower limits and plot the result. I can't figure out how to tell Mathematica to do this.

$$Q_c(u) = \frac{2\pi}{c^2} \left(\frac{k_B T_c}{h}\right)^3 \int_a^\infty \frac{u^2}{e^u-1}du$$

Here's my Mathematica code so far.

Q[x_] := x^2/(Exp[x] - 1);
Kc = 2*\[Pi]/c^2*(kb*300/h)^3;
a[vg_] := vg/(kb*300);
z = 1.0;
Integrate[Kc*Q[x], {x, a[z], \[Infinity]}]

2.85817*10^9

It's simple enough to evaluate it for any specific value of $a$ but when I try to make a new function containing the integration Mathematica won't plot the results.

qloop[z_, Q[x_]] := Integrate[Kc*Q[x], {x, a[z], \[Infinity]}];
Plot[qloop[z, Q[x]], {z, .5, 2.5}]
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2 Answers 2

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You can just plot your integral (whatever Q is, its unit is 1/(m^2 s)) like

c = 3*10^8; kb = 1.38/10^23; T = 300; h = 6.626/10^34; 
Plot[((2*Pi)/c^2)*((kb*T)/h)^3*
NIntegrate[u^2/(Exp[u] - 1), {u, a, Infinity}], {a, 0, 10}, 
AxesLabel -> {"a", "Q"}]

enter image description here

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Instead of passing Q[x], pass just z to qloop. Inside qloop, x is set by the integrate itself and passed to Q[x] then. Also change the function to use ?NumericQ.

You did not have values for c and h (I assumes these are speed of light and Planck constant).

ClearAll[x,a,qloop];
Q[x_?NumericQ] := x^2/(Exp[x] - 1);
kb = 1;
c = QuantityMagnitude@UnitConvert[Quantity["SpeedOfLight"]];
h = QuantityMagnitude@UnitConvert[Quantity["PlanckConstant"]];
Kc = 2*π/c^2*(kb*300/h)^3;
a[vg_] := vg/(kb*300);

qloop[z_?NumericQ] := Integrate[Kc*Q[x], {x, a[z], ∞}];
Plot[qloop[z], {z, .5, 2.5}]

Mathematica graphics

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