Flux counts how much field passes through a surface. Hold a hoop face-on to a stream and a lot goes through; turn it edge-on and nothing does. Same hoop, same stream, different answer — and the only thing that changed is the angle. Tilt the surface here and watch the number fall to zero.
Pick a surface: flat rectangle, flat disc, hemisphere, sphere, cube, cylinder or cone. Some can be open or closed. Set its size, then move or rotate it with the gizmo. Choose which way the field points and how strong it is, and how densely to draw the lines. Four display toggles: field lines, the area vector, the crossing markers, and the projected shadow.
The panel shows the area of the surface, the cosine of the angle between the field and the surface's normal, the flux itself, and the net number of line crossings. Turn the surface edge-on and cos θ goes to zero, and so does the flux. Now switch to a closed surface — a sphere or a cube — and no matter how you turn it, the net crossings come to zero. Everything that goes in comes back out.
The field is uniform everywhere — this is not the field of a point charge. Area is in square metres; the field strength slider carries no unit.
You should be able to say why the flux through any closed surface in a uniform field is zero, without calculating anything.