Physics 212 Lab 2 – Electric Potential and Electric Fields Introduction: In this lab we will study the equipotential lines in an electric field in order to study the structure of the electric lines of force. You will plot the position of the equipotential lines of an electric field, and from these lines, determine the shape of the force lines in that field. The goal of this lab is to show that lines of force are perpendicular to lines of equipotential. Theory: There is an electric field in any region where there is a force on an electric charge at rest. It is convenient to represent an electric field by the lines of force. The direction of the lines is the direction of the force on a positive electric charge in the field. The electric field intensity at any point is the force that would be exerted on a unit positive charge, +q, at that point (force per Coulomb). The electric field intensity is proportional to the number of electric lines of force per unit area. An equipotential line (or an equipotential surface) is a set of points that all have the same potential. Since there is no change in potential, no work is required to move a charge on an equipotential line or surface. Therefore, there is no component of electric force along an equipotential line. Electric lines of force are always perpendicular to equipotential lines. Procedure: In this experiment, a series of points of the same potential is found and a line of equipotential is drawn. A set of equipotential lines is determined and the electric field lines are constructed perpendicular to these equipotential lines. An electric field is produced in a conducting fluid by applying a low potential difference between two fixed electrodes. Points of the same potential (measured in volts) are determined by using the leads of a dc voltmeter as probes. Arrange the apparatus as shown in the sketch below. Place the insulating board and a piece of mylar graph paper in the bottom of the pan. Mix about ½ teaspoon of salt and a quart of tap water and fill the metal pan to a depth of about 1 centimeter with the solution. Be sure the tray is level so the solution is at a uniform depth. 1. Place the ends of two electrodes about 8 inches apart and on the centerline of the graph paper. Now connect the electrodes A and B to the power supply and adjust the voltage to approximately 10 volts. 2. Place probe C at a location somewhere between A and B. Connect the voltmeter (or galvanometer) to its lead probes as shown in the sketch. With the power on, explore the fluid with probe D for the equipotential lines, i.e., several points where the potential difference is zero. Record the equipotential points on a sheet of graph paper and connect the lines with a smooth line.
3. Repeat step 2 with C at other positions between A and B until at least six equipotential lines have been constructed. 4. Determine the shape of the electric fields by constructing five or more electric force lines between the ends of electrodes A and B. 5. Compare the electric field intensities by comparing the spacing between the electric force lines 1 cm from electrode A with the spacing between the electric force lines at the center of the plot. 6. Repeat steps 1-5 with electrode A connected to a geometric piece of metal placed in the pan and electrode B as before. Power Supply
Tray w/ conducting solution
A
C
B
Voltmeter D
Hand probe
Name _____________________________
Report Sheet for Lab 2 – Electric Potential and Electric Fields Production of Equipotential and Electric Field Line Graphs Follow the procedure described above and attach sheets as necessary. Questions: 1. Define electric potential, electric lines of force, equipotential lines, and electric field.
2. In this experiment, can a direction be assigned to the electric force lines? Explain.
3. What is the current path between electrodes A and B?
4. Is there a current through the meter when the probes are both on the same equipotential line? Explain.
