The electric flux through the other faces is zero, since the electric field is perpendicular to the normal vectors of those faces. The electric flux through the top face ( ) is positive, because the electric field and the normal are in the same direction. Electric flux through the bottom face ( ) is negative, because is in the opposite direction to the normal to the surface. Why does the flux cancel out here?įigure 2.1.5 Electric flux through a cube, placed between two charged plates. A calculation of the flux of this field through various faces of the box shows that the net flux through the box is zero. The electric field between the plates is uniform and points from the positive plate toward the negative plate. Now that we have defined the area vector of a surface, we can define the electric flux of a uniform electric field through a flat area as the scalar product of the electric field and the area vector:įigure 2.1.5 shows the electric field of an oppositely charged, parallel-plate system and an imaginary box between the plates. Is chosen to be the outward normal at every point, to be consistent with the sign convention for electric charge. On a closed surface such as that of Figure 2.1.4(b), In that case, the direction of the normal vector at any point on the surface points from the inside to the outside. However, if a surface is closed, then the surface encloses a volume. (c) Only has been given a consistent set of normal vectors that allows us to define the flux through the surface. (b) The outward normal is used to calculate the flux through a closed surface. , demonstrating that electric flux is a measure of the number of field lines crossing a surface.įigure 2.1.4 (a) Two potential normal vectors arise at every point on a surface. Electric flux is a scalar quantity and has an SI unit of newton-meters squared per coulomb ( We represent the electric flux through an open surface like , then we know from the definition of electric field lines ( Electric Charges and Fields) that That is perpendicular to the uniform electric field To quantify this idea, Figure 2.1.2(a) shows a planar surface Again, flux is a general concept we can also use it to describe the amount of sunlight hitting a solar panel or the amount of energy a telescope receives from a distant star, for example. Similarly, the amount of flow through the hoop depends on the strength of the current and the size of the hoop. As you change the angle of the hoop relative to the direction of the current, more or less of the flow will go through the hoop. The numerical value of the electric flux depends on the magnitudes of the electric field and the area, as well as the relative orientation of the area with respect to the direction of the electric field.Ī macroscopic analogy that might help you imagine this is to put a hula hoop in a flowing river. You can change the audio track language in the Settings menu.Figure 2.1.1 The flux of an electric field through the shaded area captures information about the “number” of electric field lines passing through the area.

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We will identify and calculate the electric flux through each of the five sides of the right triangular box, and then sum them to find the net electric flux. We will also use the equation for electric flux to determine the net electric flux through the closed surface of a right triangular box with uniform, horizontal electric field. In this video, we will learn about electric flux and how it is related to the work equation for a constant force.

Change in electric flux equation