Electromagnetism
Electric dipole
Drag the test charge — field lines run from + to − and force follows F = qE.
Visualize electric field lines and forces around an electric dipole. Place test charges and explore how field strength varies with distance and orientation.
Electric dipole
Drag the test charge — field lines run from + to − and force follows F = qE.
Two opposite charges (+ and −) create a dipole field: lines leave the positive charge and terminate on the negative one. Drag the test charge and watch F = qE point along the local field direction. On the axis, the field falls faster than for a single charge; off-axis, it curves. Molecular polarity and antenna radiation patterns are dipole physics.
- E from + and −
- F = qE
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 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?
Predictions to weigh
- The fields cancel exactly at the midpoint.
- The fields are perpendicular to each other and don't simply add or cancel.
- The fields add — both charges push/pull a test charge in the same direction at the midpoint.
Variable roles
What you set:
- Half-separation s (m)
What you measure:
- Field strength at midpoint E (N/C)
How the investigation runs
- 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.
Governing equation
Superposition at a Dipole's Midpoint — E = k·q/r²
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.
What the printable worksheet asks students to work out
- 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.
Where this shows up beyond the lab
- 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.
- 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 — Fields without contact
- Welcome to Dipole
- Select the test charge
- Press Play
- Field of a dipole
- 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.