Engineering
Coulomb's Law Electrostatic Force & Electric Field Strength formulas
Calculate point-charge electrostatic force F = ke * |q1 * q2| / r^2, attractive/repulsive polarity, and electric field E = ke * |q1| / r^2.
Compute inverse-square electrostatic force and field strength without sign bugs!
This example models two stationary point charges in vacuum with a rounded Coulomb constant. Air is only approximately vacuum-like; dielectric media require the appropriate permittivity. Extended charge distributions and nearby conductors generally require more analysis.
Force magnitude is k*ABS(q1*q2)/r^2, with positive separation in meters and signed charges in coulombs. Like nonzero signs repel and unlike signs attract along the line joining the charges. If either charge is zero the two-point-charge force is zero.
The separate field magnitude k*ABS(q1)/r^2 depends on the source q1, not the test charge q2. A zero test charge therefore does not eliminate a nonzero source field. Field direction is outward from a positive source and inward toward a negative source.
| Electrostatic Quantity | Formula / Cell | +4 uC & -5 uC at r = 0.20 m | Result |
|---|---|---|---|
| Charges q1 & q2 | B2 = 4E-6 C, B3 = -5E-6 C | Separation B4 = 0.20 m, ke = 8.98755E9 | Opposite signs (q1*q2 < 0) |
| Force Magnitude F (N) | ke * ABS(q1 * q2) / r^2 | 8.98755E9 * 2E-11 / 0.04 | 4.4938 N |
| Interaction Polarity | IF(B2*B3 < 0, "Attractive", ...) | Opposite charges pull together | "Attractive" |
| Field E1 & Double r = 0.40m | ke * ABS(q1) / r^2 | E1 = 898,755 N/C | F(0.4m) = 1/4 | F drops to 1.1234 N |
Coulomb electrostatic force magnitude F in Newtons
Uses ABS(B2 * B3) so force magnitude is always non-negative regardless of charge signs.
=IF(COUNT(B2:B5)<>4,"Invalid inputs",IF(OR(B4<=0,B5<=0),"Invalid inputs",ROUND((B5 * ABS(B2 * B3)) / (B4 ^ 2), 4)))Classify the two-charge interaction
Classifies charge interaction direction from sign product B2 * B3.
=IF(COUNT(B2:B5)<>4,"Invalid inputs",IF(OR(B4<=0,B5<=0),"Invalid inputs",IF(B2 * B3 < 0, "Attractive", IF(B2 * B3 > 0, "Repulsive", "Zero force"))))Source electric-field magnitude in N/C
This scalar is the field from q1 at the chosen distance. It is independent of q2.
=IF(COUNT(B2:B5)<>4,"Invalid inputs",IF(OR(B4<=0,B5<=0),"Invalid inputs",ROUND((B5 * ABS(B2)) / (B4 ^ 2), 0)))Try the example
Use the original values or change them to test your understanding.