How to use it
- Type each charge as (q, x) for charges on a line or (q, x, y) for charges in a plane. Use a minus sign for negative charges.
- Pick the unit of the charges (µC by default) and of the lengths.
- Enter the point x and y where you want the field.
- Read the field strength, its components, its direction and the electric potential.
Formulas
- One charge: E = k × q ÷ r², pointing along the line from the charge to the point (reversed for negative q)
- Several charges: E = Σ k qᵢ (r − rᵢ) ÷ |r − rᵢ|³, added as vectors
- Potential: V = Σ k qᵢ ÷ |r − rᵢ|
- k = 1 ÷ (4π ε₀) = 8.9875517923 × 10⁹ N·m²/C²
Worked example
+1 µC at (0, 0) and −1 µC at (4, 0), field at the point (2, 2) m. Both charges are √8 m away, so each field has size 8,987.55 ÷ 8 = 1,123.44 N/C.
- The positive charge pushes up and to the right, the negative one pulls down and to the right.
- The y parts cancel, the x parts add: Eₓ = 2 × 1,123.44 × 2 ÷ √8 = 1,588.79 N/C, direction 0°.
- The potentials of equal and opposite charges at equal distances cancel: V = 0 V.
Tips
- Mind the unit of the charge. 1 C is a huge charge; static and lab charges are usually nC or µC.
- Distances count squared. Twice as far means a quarter of the field.
- The field inside a charge is not defined here, so the point cannot sit exactly on a charge.