Electric field – two point charges

What it shows

Each point charge produces an electric field of strength E = k|q|/r², directed away from a positive charge and towards a negative one. The field of two charges is found by vector addition, E = E₁ + E₂, so both magnitude and direction matter. Field lines show this combined field: they start on positive charges, end on negative ones and never cross, and the field vector at any point is tangent to the line through it. The two charges also exert equal and opposite Coulomb forces on each other, attractive for opposite signs and repulsive for like signs.

How to use

Start with opposite charges and Show field lines ticked, then drag the green test point around and compare the direction of E with the nearby field lines. Press Flip q₂ sign to make the charges alike and search for the point where E becomes zero. Change Distance d, or drag a charge, and read how the Coulomb force changes when the separation is doubled. Unequal values of q₁ and q₂ show how the field pattern becomes lopsided.

Parameters you can change

• Charge q₁ -10–10 nC
• Charge q₂ -10–10 nC
• Distance d 5–40 cm
• Show field lines

Questions to explore

1. When q₁ and q₂ are equal and have the same sign, where is the point at which the electric field is zero?
2. By what factor does the Coulomb force between the charges change when the distance d is doubled?
3. What does the test point show about why two electric field lines can never cross each other?