Map the "voltage contour lines" in salt water with the IOLab. Follow the diagrams top to bottom.
THE BIG IDEA
You drive 3.3 V across a shallow tray of salt water between two pennies — one on the IOLab's DAC (the +3.3 V source) and one on GND (0 V). A probe clip on A7 reads the voltage at any point in the water.
An equipotential line connects points at the same voltage. Lines never cross (a point can't have two voltages), and electric field lines cross them at 90°. Choose a sensible voltage spacing between lines — like contour lines on a topographic map.
equipotential (equal V) field line (⟂, 90°)
1
Gather your materials
IOLab device + dongle
A styrofoam plate
Tap water + table salt
3 alligator-clip wires
2 pennies
A sharpie + a ruler
From your kit: the alligator clips + pennies are the same ones you used for the ECG heart-rate lab.
2
Salt water in the plate
Pour about ¼ inch of tap water into the styrofoam plate and stir in table salt until it dissolves. The salt makes the water conductive so current can flow — the exact amount doesn't matter, more salt just conducts better.
Keep the plate level so the water depth is even everywhere.
3
Wire the pennies + probe (DAC · GND · A7)
2 pennies on OPPOSITE sides (edge submerged) → DAC & GND. A 3rd bare clip → A7 = your probe.
Clip an alligator wire to each penny and set the pennies on opposite sides of the plate with the penny's edge submerged (not fully under). Wire one penny → DAC, the other penny → GND. Take a third clip with no penny and plug it into A7 (Analog 7) — that's your voltage probe.
Don't let the two pennies touch each other or slide together — that shorts the circuit.
4
Software config = your "calibration"
In the IOLab software: click the gear icon → Chart Display → check Output Configuration → set DAC Output = 3.3 V → turn ON Analog 7. Then click the chart title to collapse the graph into a simple digital meter that shows the exact voltage at your probe.
This is the calibration step for this lab. No reading later? It's almost always this: DAC must be 3.3 V with Output Configuration checked, A7 ON, and the chart clicked into meter mode.
5
Probe for equal-voltage points + mark them
Dip the A7 probe at a point in the water and read the voltage. Now hunt for other points that read the same voltage (say 1.5 V). Each time you find one, mark it with a SHALLOW indentation in the styrofoam with the probe or a pencil.
Shallow only! Don't puncture the plate or the water leaks out.
Collect several points at the same voltage — together they form one equipotential line. Then move toward the other penny and repeat at other voltages (e.g., 2.5 V, 0.8 V) to build more lines.
6
Do TWO plates: symmetric & asymmetric
Purple = equipotential lines · pink dashed = a field line crossing them at 90°.
Map two separate plates: (1) symmetric — pennies straight across from each other; (2) asymmetric — pennies closer together on one side and farther on the other (never touching). The asymmetric one crowds its lines where the pennies are closest.
7
Connect, label, and photograph
When you have enough points, empty the water and dry the plate. With a sharpie, connect each set of same-voltage dents into a smooth line and label every line with its voltage. Then photograph both finished plates (symmetric + asymmetric) — that's your required deliverable for the report.
Tip: pick a clean voltage spacing between lines (e.g., every 0.5 V). Too many lines = cluttered; too few = you lose the shape — just like contour intervals on a map.
✓
Foolproofing — if something goes wrong
Probe reads ~0 or no gradient? → add more salt, and make sure both penny edges are submerged.
No reading at all? → DAC must be 3.3 V (Output Configuration checked), A7 ON, chart clicked into meter mode.
Weird/jumpy numbers? → pennies shifted or touching → keep them fixed and apart; keep the probe clip clean.
Water leaking? → your dents are too deep — keep them shallow.
Values drifting over time? → water is evaporating → work reasonably quickly; keep the plate level.
!
Uncertainty / error sources (for the report)
Evaporation — water level & salinity change during the run.
Probe placement — a dent marks a finite spot, not a perfect point.
Pennies shifting, tilting, or unequally submerged changes the field.
Plate not level → uneven water depth.
Meter reading resolution.
Finite, edge-bounded medium vs the idealized infinite dipole.
NEVER write "human error." The slides say it means "you admitted you screwed up and we'll grade accordingly." Name a specific physical cause (evaporation, probe placement, penny shift, etc.) instead.
4 things that make or break it: ① Enough salt (conductive water). ② DAC = 3.3 V + A7 on + meter mode. ③ Pennies fixed & not touching. ④ Shallow dents · label your lines · photograph both plates.
Equipotential-surfaces method (salt water · DAC/GND pennies · A7 probe) · pairs with your report + data logger · ← back to PHYS 1164