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The edges of the rectangular parallelepiped which is a cube are of length change in x along x, change in y along y and change in z along z. A vector field E (x, y, z) exists in the region of space. The coordinate points labeled are (x, y plus change in y, z) along the y axis, (x plus change in x, y, z) on the x axis and (x, y, z plus change in z) on the z axis. The four faces represent four unit vectors labeled n sub 1 caret, n sub 2 caret, n sub 3 caret and n sub 4 caret. The 3 flux lines labeled E pass through the rectangular parallelepiped.