Physics 220 (Section 1)
Lab #2 -- Equipotentials and
Electric Fields in Two Dimensions
CID: ____________
Introduction
In general, electric fields exist in three dimensions where the regions of constant potential are two-dimensional surfaces, the equipotential surfaces. Sets of equipotential surfaces separated by fixed voltage differences (that is, surfaces at -10V, 0 V, +10V, +20 V, etc.) form a set of field contours. However, if we are only interested in a two-dimensional slice of space such as a plane, the regions of constant potential are lines. These lines are the intersection of the field contours with the plane. The equipotential lines lines, then, are much the same as contours on a topographic map, and we can visualize the electric potential as a terrain of mountains (where the potential is large) and valleys (where the potential is small). If you release a rock from rest along a slope, it will begin traveling in the direction where the slope is steepest. If you release a charge at rest in three dimensions, it will begin traveling in the direction that the field contours are decreasing most rapidly. This is the direction of the electric field and it is always perpendicular to the field contour. (Of course, once a charge or boulder is moving, it isn’t required to continue to follow the field lines, but the force will still be in the direction of the field lines.)
Equipotentials and Fields
In this lab you are given a sheet of grid-paper made of a special material that is very slightly conductive. The disc in the upper right hand side of the grid is connected to a battery that has a potential of +10 Volts relative to the bar at the bottom of the grid. We will map the resulting equipotential curves onto the graph below. Use the voltmeter to find at least 10 grid locations that are all at +1 Volt. Connect these points with a line and label the line “1 Volt”. Repeat for 1.5, 2, 3, 4, 6, and 8 Volts. Next, use these equipotential curves to draw several electric field lines, also indicating their direction. Remember that the electric field always points perpendicular to these curves and toward lower potential. Be sure to draw a number of electric field lines that go to the surface of the conducting ring. Use your knowledge of the behavior of field lines on the surface and inside of conductors to aid you in this.