AP Physics Laboratory
Equipotential Surfaces & Electric Fields
DISCUSSION:
Electric fields are produced around charged objects and are analogous to gravitational fields in several
ways. However, since the electric field can attract or repel, an arbitrary positive test charge is used to
determine the direction of the vector field, E.
Electric field lines extend outward from positively charged surfaces and end at negatively charged
surfaces. The presence of conductors close to field lines can influence the direction of these lines, just
like gravitational forces can be influenced by nearby masses such as planets.
In this lab, you will map an electric field between two point charges. This will be done by first mapping
equipotential surfaces caused by the two point charges.
Equipotential surfaces are analogous to different heights of gravitational potential. At different distances
from the Earth, objects have a given amount of potential energy. Just as work must be done against
gravitational force to move an object to a new height, work must be done against an electric force to
move an object to a new equipotential surface. Therefore, it follows that electric field lines must run
perpendicular to equipotential lines just as gravitational field lines run perpendicular to heights of the
same potential energy.
THE EXPERIMENT:
1. In this part of the experiment we will produce an electric field between to small electrodes.
2. Set up the experiment as shown above with the battery disconnected. A paper grid is placed
under the bottom of the transparent rectangular box. The two copper strip electrodes are bent
over the edge of the box and the alligator clips are used to secure them in position and connect
them to the 6 V battery.
3. Pour the water into the box to a depth of about 1 cm.
4. On the additional graph paper grid provided, indicate the location of the tips of the electrodes.
5. The multimeter should be set to the 200 V DC range even though we are dealing with less than
20 V. This is because we will only want to read the meter to the nearest tenth of a volt.
6. The common is then connected to the negative electrode. The positive voltmeter lead will act as
a probe to locate the equipotential surfaces.
7. Connect the battery.
8. One person should put the tip of the voltmeter's positive probe, holding it vertically, into the
water a short distance from the negative electrode and move it about until a place where the
potential is about 2.0 volts is located. Use the coordinates on the grid to locate this point on the
identical graph paper provided.
9. The probe should then be moved through the solution a short distance to locate another grid
position with a potential of 2.0 volts. Map this equipotential across the entire width of the grid
collecting at least 7 data points to clearly indicate the equipotential line.
10. Connect these points on the graph paper with a smooth line indicating the 2.0 volt equipotential.
If you cannot accurately draw a smooth line indicating equipotential surfaces with 7 data points,
collect more.
11. Repeat this procedure for 2.5 through 5.5 volts in 0.5 volt increments.
ANALYSIS
12. The electric lines of force are always perpendicular to the equipotential surfaces, including the
surface of conductors such as the electrodes.
13. On the plots of the equipotential sketch in about 8 electric lines of force using arrowheads to
indicate the direction.
14. Compare your electric field lines to the field lines indicated by your book for two opposite point
charges. How does it compare?
15. Noting the discussion section above, sketch the field lines and equipotential surfaces for the
following:
a) Two Positively Charged Point Charges
b) Two Flat, Oppositely Charged Conducting Plates