D.2 · IB Physics
Field lines and equipotentials of two charges
Set the sign and size of two point charges, drag them around, and see how the field lines and equipotentials change. Move the probe to read the field strength and potential at any point.
Drag a charge or the probe (or tap to place the probe). Arrow keys move the probe by 0.01 m (Shift: 0.05 m). Width of view = 1.00 m.
E = kQ / r² (vectors add)
V = kQ / r (scalars add)
E = −ΔV / Δr
k = 8.99 × 10⁹ N m² C⁻²
V = kQ / r (scalars add)
E = −ΔV / Δr
k = 8.99 × 10⁹ N m² C⁻²
- |E|
- —
- Direction of E
- —
- V
- —
- r₁ (to q₁)
- —
- r₂ (to q₂)
- —
Equipotentials are drawn for |V| ≤ 1500 V. Lines per charge ∝ |q| (2 per nC). Direction measured anticlockwise from +x.
Try this
- With the dipole preset, move the probe along the perpendicular bisector: V reads 0 V everywhere there, yet |E| is not zero. Which way does E point?
- Choose like charges and find the point between them where |E| = 0. Is the potential there also zero?
- With +8 nC and −2 nC, place the probe between two neighbouring equipotentials and check that |E| ≈ ΔV ÷ (gap between them in metres).
Common error: assuming that zero field means zero potential (or the reverse) — E is the potential gradient, so it depends on how fast V changes, and field lines always cross equipotentials at right angles.