When is the force attractive?

Opposite charge signs attract. Equal signs repel.

Why does distance matter so much?

Coulomb force follows an inverse-square relationship, so doubling distance reduces force to one quarter.

What does relative permittivity do?

A medium with higher relative permittivity reduces the force compared with vacuum.

Coulomb's Law Calculator

Calculate electrostatic force, charge, distance, relative permittivity, electric field and potential for two point charges.

Find

First charge (signed)

Second charge (signed)

Distance between charges

Medium and relative permittivity

Ready examples

Repulsion898.7552 mN

0.8988 N (same signs -> repulsion)

Pair potential energy

0.0899 J

Positive energy corresponds to repulsion.

In electronvolts

5.6096 × 10¹⁷ eV

Electronvolts are convenient for atomic-scale problems.

Electric field at the second charge

8.9876 × 10⁵ N/C

Field magnitude from the first charge; direction depends on its sign.

Potential at the second charge

89,875.5179 V

Potential sign follows the first charge sign.

Electric field at the first charge

8.9876 × 10⁵ N/C

Field magnitude from the second charge; direction depends on its sign.

Potential at the first charge

89,875.5179 V

Potential sign follows the second charge sign.

Inverse-square law

Doubling distance reduces force by 4x; tripling distance reduces it by 9x.

Distance change	Distance	Force	% of original
2 times smaller	5 cm	3.595 N	400 %
original	10 cm	898.7552 mN	100 %
2 times larger	20 cm	224.6888 mN	25 %
3 times larger	30 cm	99.8617 mN	11.11 %
5 times larger	50 cm	35.9502 mN	4 %
10 times larger	1 m	8.9876 mN	1 %

Electric force between point charges

Coulomb's law intent is usually a physics solver: enter two charges and a separation distance, then find force magnitude and whether the interaction is attractive or repulsive.

F = \frac{k ∣ q _{1} q _{2} ∣}{ε _{r} r ^{2}}

F is force, q1 and q2 are charges, r distance and epsilon r relative permittivity.

U = \frac{k q _{1} q _{2}}{ε _{r} r}

U is electric potential energy for the pair of charges.

E = \frac{k ∣ Q ∣}{ε _{r} r ^{2}}

E is electric field magnitude from a point charge.

Model limits

Point-charge model

The equation assumes stationary point charges and a uniform medium. Conductors, nearby objects, finite charge size and dynamic fields can change the result.

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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