Establish that equipotentials are perpendicular to field lines
Three steps, the way the exam actually works: work through the lab, write down your own observations, then answer a 6-point free response. What you recorded goes to the grader with your writing, so a conclusion that does not follow from your own evidence will cost you the point — exactly as it would with a real reader.
Predict before you look
- Potential is a signed scalar sum and field is a vector sum. Which of the two can cancel to zero while the other does not?
- Moving a charge along a surface of constant potential does no work. What does that require about the angle between the motion and the field?
Nothing to submit here — these are to think through, so the prediction below is an informed one rather than a guess.
Commit to an answer now. It is not graded and being wrong costs nothing — the point is to have something specific to reconcile against once you have the data.
Answer every prediction to unlock the lab. A sentence is enough.
Run the investigation
Predictions first
The procedure and the simulation unlock once you have committed above. Observing before predicting is how a wrong intuition survives a lab intact.
Record what you found
This is your reading of the source, not ours. The grader sees them, so your conclusions have to follow from what you actually recorded.
| Where the zero-potential boundary lies (describe in words) | |
|---|---|
| Angle between the field arrows and the zero-potential boundary | ° |
| Direction the field arrows point at the midpoint (describe) | |
| Direction the released positive test charge moves (describe) |
Answer the free response
The shading in this lab is potential and the arrows are field, so the relationship between them is directly observable. (a) Describe where the zero-potential boundary lies for a dipole and explain why it must lie there, using the fact that potential is a signed scalar sum. (b) State the angle you measured between the field arrows and that boundary, and explain why that angle is forced rather than coincidental. (c) At the midpoint between the two charges the potential is zero but the field arrows are clearly not. Explain how both can be true at once. (d) A classmate concludes from part (c) that a positive charge released at the midpoint will not move, because the potential there is zero. Correct the reasoning, and state one thing this simulation does not model that would matter for computing an actual field strength in newtons per coulomb.
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