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Electric field vectors are also a useful representation when students are learning about flux. One of our favorite visualization activities uses Maple to verify Gauss’s law for a point charge somewhere inside a unit cube. Using code that students can easily examine and alter, this worksheet first plots the electric field vectors due to a point charge as discussed above. Then the value of the integrand of the flux through the top of the cube is plotted. This intermediate step allows for a class discussion about where one might expect the integrand to be large or small - both the distance of the charge from the area in question and the angle of the electric field vector with respect to the directed area are relevant. And finally, the integral of the flux through each face of the cube and the total flux through all the faces of the cube are calculated and shown to be proportional to the value of the charge enclosed by the cube. The fun of this activity comes when the students interactively move the charge with respect to the cube. If the charge is outside the cube, then the total flux is zero, even though the fluxes through the individual faces are nonzero. If the charge is at the center of the cube or in a few other special places, then Maple calculates the flux analytically, in closed form. At all other points, the calculation is automatically numerical and students see this effect - the answer is quoted to a certain number of digits of accuracy. Students rapidly seek the special cases that they think might test the bounds of the code by placing the charge on a face, edge, or vertex of the cube. Here they discover, for example, that a charge on the face contributes only ½ the flux of an interior charge. They can then sketch field lines for this case and see which ones actually pass through the faces of the cube.