Electromagnetism
One positive source charge
One positive source charge produces radial field lines and an inverse-square electric field.
Map the electric field around a point charge using field lines and test-charge motion. Visualize Coulomb's law and inverse-square field strength.
Coulomb's law
Watch radial field lines and the E-arrow grid diverge from the positive charge.
An isolated point charge produces a radial electric field E = kq/r² that falls off with the inverse square of distance. Field lines radiate outward from + charges and inward toward − charges. Press Play to see the E-arrow grid pulse; move the test charge and read |E| and V in the Properties panel. Coulomb repulsion and atomic structure all start from this pattern.
- F = kq₁q₂/r²
- E = kq/r²
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
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?
Predictions to weigh
- 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.
Variable roles
What you set:
- Distance r (m)
What you measure:
- Potential V (V)
- Field strength E (N/C)
How the investigation runs
- 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.
Governing equation
Coulomb's Law — E = k·q/r²
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.
What the printable worksheet asks students to work out
- 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.
Where this shows up beyond the lab
- 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?
- AP Physics 2 — Unit 10: Electric Force, Field, and Potential
- AP Physics C: Electricity and Magnetism — Unit 8: Electric Charges, Fields, and Gauss's Law
- IB Physics — D.2 Electric and magnetic fields
- General High School Physics — Magnetism & electromagnetism
- NGSS High School Physics — Gravitational and electrostatic forces
- Middle School Physical Science — Electric and magnetic force strength
- Middle School Physical Science — Fields without contact
- Welcome to Single Charge
- Select the charge
- Press Play
- Force between charges
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.