Counting field lines through a surface. Electric flux, Gauss's law, and its use to find the field of a charged sphere, a long wire and an infinite sheet, and inside and outside a conductor.
3 lessons, about 2 h 5 min4 simulations11 problems8 practice questionsPython in every lesson
Created by StemPy LabElectric flux counts the field lines that pass through a surface. For a flat surface in a uniform field it is , with the angle between the field and the normal to the surface. It is largest when the surface faces the field and zero when the surface lies along it. For a closed surface we take the normal pointing outward, so lines that leave count as positive and lines that enter count as negative.
Gauss's law says that the net flux through any closed surface is the net charge inside it divided by , . The shape and the size of the surface do not matter. A charge outside the surface sends as many lines in as out, so it adds nothing to the net flux. The law follows from the inverse square law of Coulomb.
When the charge has a simple symmetry, a well-chosen Gaussian surface gives the field in a line or two. A long wire gives , a large sheet gives , and a thin spherical shell gives outside and zero inside. In a conductor the field is zero inside, the charge sits on the surface, and the field just outside is .
Log in to join the discussion.
Loading…