Electric Charge Behavior and Interactions Model:
Mapping Electric Fields Lab
In the lab you will look at the electric force on an "electrostatic compass" and use your
observations to identify the key properties of the electric field. You will specifically look at the electric field created by a single "-" charge, a single "+" charge, two unlike charges, two like charges, and a single "-" charge and a
conducting sheet.
For each part of the lab, the setup will be involve variations of the setup shown in the diagram on the right:
[] two 10” PVC pipes [] one 10” Lucite pipe [] plastic bag
[] wool
[] Wood base with holes big enough to insert the pipes. (optional)
[] roll of magic tape
[] thread tied to straightened out paper clip [] 4-5 pieces of 8.5" x 11" paper.
Making an electrostatic compass:
We have seen that pulling tape off of a surface leaves the tape with a net charge. Using
oppositely charged tapes, we can make a "compass" where one tape is attracted and the other is repelled from a charged object.
A. Put a 6 cm long base tape on the table with a folded under handle.
B. Place two more tapes on the first. With a pen, label the handle of the middle tape "-" and the handle of the tape on top "+".
C. Slowly remove the top two tapes together, and then make sure they are not charged by firmly rubbing the slick side of the tape.
D. Briskly separate the two tapes, and sticky side up, overlap the edges of the tapes as shown. (Avoid excessive contact with the tape since this will discharge it.) E. Draw an arrow across the tapes from the "-" side to the "+" side as shown. F. Attach the center of the tapes to the straightened out paper clip so the string extends upward from the handles. (See the first picture in this lab.)
Part 1. The electric field due to a negatively charged object
1. Center a sheet of paper on the wood platform.
2. Charge a PVC pipe by rubbing it with wool and poke the pipe through the paper into a hole in the platform.
3. On the paper, label the pipe’s charge "-".
4. Make sure your electric compass is charged and operational, if not, follow the steps to recharge it.
5. Start with the electric compass very close to the PVC pipe. Draw an arrow on the paper directly below the arrow on the tape. 6. Now move the electric compass farther from the pipe so that the new arrow you draw meets head-to-tail with the last arrow you drew. Continue until your arrows go off the edge of the page. 7. From three other starting points near the PVC pipe, map the electric field.
8. Save your electric field map and sketch your observed electric field in the box on the right.
Part 2. The electric field due to a positively charged object
1. Flip over the sheet of paper on the wood platform.
2. Charge a Lucite rod by rubbing it with a plastic bag and poke the pipe through the paper into a hole in the platform.
3. On the paper, label the pipe’s charge "+".
4. Now follow steps 4-7 above to map the electric field. (Be sure to label the pipe’s charge "+".)
5. Save your electric field map and sketch your observed electric field in the box on the right.
Questions for parts 1 and 2:
1. Where do the electric field lines begin?
2. Where do the electric field lines end?
Part 3. The electric field due to objects with unlike charges.
1. Place two pieces of 8.5" x 11" paper on the platform and tape their edges together.
2. Charge a PVC pipe and a Lucite rod and poke each through the paper into holes in the platform, approximately 20 cm apart.
3. Recharge your electrostatic compass, if needed.
4. Map the electric field as before, starting from the Lucite rod. Draw enough field lines until you think you could predict where every field line would go from any starting point around the rod.
5. Check with your teacher before you continue.
6. Save your electric field map and sketch your observed electric field in the box on the right.
Part 4. The electric field due to objects with like charges
1. Flip the paper over.
2. Charge either two PVC pipes or two Lucite rods and poke two matching rods through the paper into holes in the platform, approximately 20 cm apart.
3. Recharge your electrostatic compass, if needed.
4. Map the electric field as before, always starting from a rod. Draw enough field lines until you think you could predict where every field line would go from any starting point around the rod.
5. Check with the teacher before you continue.
6. Save your electric field map and sketch your observed electric field in the box on the right.
Questions for parts 3 and 4:
1. Where do the electric field lines begin?
2. Where do the electric field lines end?
3. How many other electric field lines could you have drawn in each part?
5. Predict what the electric field due to two identical charges opposite to those you
experimentally mapped above by drawing a number of electric field lines below. Put arrows on your electric field lines.
6. In the diagram below, two rods of opposite charge create an electric field. What is the
direction of the electric field at the dot? Show how the direction of the field at the location of the dot is related to the forces it experiences from each of the charged rods.
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-Part 5. The electric field due to a negatively charged object near a conducting sheet
1. Attach an aluminum foil sheet to a book stood upright or another suitable fixture. Once you have positioned the foil, be careful not to touch it so that you know the net charge on the foil is zero.
2. Place the PVC rod about 15 cm from the foil sheet and charge the rod.
3. Map the field between the rod and the aluminum sheet. Draw enough field lines until you think you could predict where every field line would go from any starting point around the rod. 4. Save your electric field map and sketch your observed electric field in the box below.
Questions for part 5:
1. At what angle do electric field lines meet the conducting surface? What would happen to electric charges at the surface of the metal if the field lines did not meet the surface at this angle?
2. Based on your understanding of the motion of electric charges in materials, draw a likely charge distribution on the aluminum sheet that produced the field you observed.
3. Where must electric field lines start and end?
4. Carefully draw some electric field lines on your diagram above taking into account your answers for questions 1 and 3.
5. The field lines you have drawn for part 5 look almost exactly like the drawing you made in what other part of the lab?
a. How are they the same?
