Electric Dipole: Superposing Two Opposite Charges
1 · Predict
A positive and an equal negative charge sit a fixed distance apart. At the midpoint between them, do their fields cancel, or do they add up?
- The fields add — both charges push/pull a test charge in the same direction at the midpoint.
- The fields cancel exactly at the midpoint.
- The fields are perpendicular to each other and don't simply add or cancel.
2 · Set Up
- Open the dipole preset and press Reset. The charges are ±3 nC.
- Enable the field-strength readout at the midpoint.
- Set the half-separation distance between each charge and the midpoint for each trial, and record the field strength there.
3 · Collect Data
| Half-separation s (m) | Field strength at midpoint E (N/C) |
|---|---|
| 1 | |
| 1.6 | |
| 2.5 |
Plot field strength E (y-axis) against 1/s² (x-axis) for your three trials.
4 · Analyze
- For one trial, compute E = 2kq/s² using k = 8.99×10⁹ N·m²/C², q = 3 nC. Compare to the table.
- Explain why the factor of 2 appears: at the midpoint, the positive charge's outward field and the negative charge's inward field point the same direction, so their magnitudes add rather than cancel.
5 · Extend
- Far from a dipole (distance much bigger than the separation), the field falls off even faster than 1/r² — closer to 1/r³ — because the two charges' fields almost cancel. Explain why near-perfect cancellation at large distance would make the field weaker than a single point charge's.
- Water molecules behave like tiny electric dipoles (one side slightly positive, one side slightly negative). Explain why this dipole nature helps water dissolve ionic compounds like salt.
The Physics Behind This Experiment
Superposition at a Dipole's Midpoint
The net field from two charges is the vector sum of each one's individual field (superposition). At a dipole's midpoint, the positive charge's field points away from it and the negative charge's field points toward it — both in the same direction — so they add: E = 2kq/s².