PHY 201L Lab 2

Physics201L Equipotential Lines and Electric Field Mapping Objective: To examine certain electric fields; in particular to map the equipotential lines of an electric field and hence determine the electric lines of force. Equipment: DC power supply; wooden board with spring contacts; voltmeter; connecting wires; conducting papers with different configurations. Theory: An electric field, commonly called field of force, is a region in which forces act on electric charges if present. If a force F acts on a charge Q at a point in the field, the field strength E, by definition the force per unit charge, is : E= F/Q that is the magnitude of electric field strength is the force per charge. Force is a vector quantity having direction as well as magnitude. The direction of an electric field at any point is the direction of the force on a positive test charge placed at any point in the field. An electric field can be found by analyzing the map of the electric field lines. The electric field lines, also called the lines of forces, originate on and are directed away from positive charges, and end on and are directed from positive charges towards negative charges. The electric field lines enable one to find the direction of the vector E; the vector E is always tangential to the lines of forces. Points of an electric field having the same potential are called equipotential points, and a line through these points is an equipotential line. No work is done electrically if a test charge is moved along such lines, from where it follows that there can be no component of the electric field or lines of force must be everywhere perpendicular to the equipotential lines. Often it is easier to determine equipotential lines than lines of force. And since the two are perpendicular, the ones can be constructed from the others. Procedure: A conducting plate with silver metallic terminal is fastened beneath the field- mapping board. An electric field is produced by connecting these terminals to a source of potential. Points of equal potential in this field are discovered by a movable- exploring probe connected to a voltmeter. The series of such points define an equipotential line. The entire field is explored by moving the probe from one point to another after determining the set of equal potential points corresponding to the previous point. The student is asked first to establish by means of a DC power supply a potential difference between the silvered electrodes. Where as a result, an electric field pattern (depending on the electrodes configuration) is set up over the conducting paper’s surface. The following step is to build the detecting system for equal potential points in the field. 1. Place the conducting paper on the wooden board, with the spring contacts touching the silver electrodes. 2. Fasten a conducting paper on the wooden board in a way that the silver electrodes get in contact with the spring contacts. 3. Connect the voltmeter in parallel to the DC power supply. 4. Notice that a coordinate system is marked on the conducting paper. So the student has to record the coordinates of those points as to reproduce the pattern in a graph. 5. The equipotential surfaces are plotted by connecting one lead of the voltmeter to one of the electrodes. This electrode becomes the reference. The other voltmeter lead (the probe) is used to measure the potential at any point on the paper simply by touching the probe to the paper at that point. 6. To map an equipotential, move the probe to the point at which the voltmeter is indicating the desired potential. Record the coordinate of this point Move the probe to a new position which maintains the voltmeter at the same reading. Record this point. Continue in order to find a series of points at the same potential across the paper. 7. Connect the points with a smooth line on the graph and write the potential difference. This is the equipotential line. 8. Repeat the measurements for different potentials between the probe and the reference electrode. 9. Construct a number of lines of force perpendicular to the equipotential lines. 10. Repeat the same procedure for the other electrodes configurations. Date:_____________ Physics201L Equipotential Lines and Electric Field Mapping Names: _________________________ Section:________________ Instructor: _____________________ Parallel plates configuration: Fill in the (x,y) coordinates in the tables below for the specified voltages: V=10V V1= V2 = V3 = V=8V V1= V2 = V3 = V=6V V1= V2 = V3 = Using EXCEL, draw a graph for each table, showing the three equipotential lines on each graph. Grade: Dipole configuration: V=10V V1= V2 = V3 = V=8V V1= V2 = V3 = V=6V V1= V2 = V3 = Do the same for this configuration. Attach the graphs to your report. Answer the following questions. 1. Is it possible for two different equipotential lines or two lines of force to cross? Explain. 2. Sketch the equipotential lines for an isolated negatively charged particle, spacing the lines to show equal difference of potential between lines. 3. What conclusion can you draw about the field strength concerning the dipole configuration? 4. Sketch the field pattern and equipotential lines of two positively charged small spheres placed a short distance from each other. 5. Is the electric field inside the capacitor constant? Outside it? 6. How do the edges of the plates affect the electric field?