Coulomb's Law: The Field Around a Point Charge
1 · Predict
A single point charge sits alone in space. If you move a test point twice as far away, how much does the electric field strength drop?
- The field drops to 1/4 — it falls off with the square of distance.
- The field drops to 1/2 — it falls off linearly with distance.
- The field stays the same regardless of distance.
2 · Set Up
- Open the single-charge preset and press Reset. The source charge is fixed at q = 5 nC.
- Enable the field-strength readout at a test point.
- Set the test point's distance from the charge for each trial and record the field strength.
3 · Collect Data
| Distance r (m) | Potential V (V) | Field strength E (N/C) |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 |
Plot field strength E (y-axis) against 1/r² (x-axis) for your three trials. Is the line straight through the origin?
4 · Analyze
- For one trial, compute E = kq/r² using k = 8.99×10⁹ N·m²/C², q = 5 nC. Compare to the table.
- Explain, using the inverse-square law, why doubling the distance quarters the field strength rather than halving it.
5 · Extend
- Your potential column (V = kq/r) falls off more gently than the field (1/r instead of 1/r²). Explain why potential and field have different distance dependence even though both come from the same source charge.
- Coulomb's law E = kq/r² has the exact same mathematical form as Newton's gravitational field g = GM/r². Why might two completely different forces (electric and gravitational) share this inverse-square structure?
The Physics Behind This Experiment
Coulomb's Law
A point charge creates a radial electric field that falls off with the square of distance: E = kq/r², where k = 1/(4πε₀) ≈ 8.99×10⁹ N·m²/C². The force on a test charge is F = qE.