Electric Field Calculator
The electric field E at a point in space due to a point charge q is E = k * q / r^2, where k = 8.9875 * 10^9 N m^2/C^2 is Coulomb's constant and r is the distance from the charge. The field magnitude is expressed in newtons per coulomb (N/C), equivalent to volts per meter (V/m). A positive charge creates a field pointing outward; a negative charge creates a field pointing inward. This calculator computes the magnitude of the field. Enter charge in coulombs and distance in meters; for microcoulombs, multiply by 10^-6 before entering.
Electric field formula
E = k * q / r^2
k = 8.9875 * 10^9 N m^2/C^2
E is in N/C (= V/m), q is in coulombs (C), r is in meters (m). Example: q = 5 uC = 5e-6 C at r = 0.5 m: E = 8.9875e9 * 5e-6 / 0.25 = 179,750 N/C.
Electric fields in practice
- The electric field inside a conductor in electrostatic equilibrium is zero; charges rearrange on the surface to cancel any internal field (Faraday cage principle).
- Parallel-plate capacitors create a nearly uniform electric field E = V/d between the plates, where V is voltage and d is plate separation.
- Dielectric breakdown of air occurs at about 3 * 10^6 V/m (3 MV/m); beyond this field strength, air becomes conductive (lightning).
- Electric field lines always point from high to low potential and are perpendicular to equipotential surfaces.
Frequently asked questions
What is the electric field of a point charge?
The electric field E at distance r from a point charge q is E = k * q / r^2, where k = 8.9875 * 10^9 N m^2/C^2 is Coulomb's constant. E is measured in N/C (equivalently V/m). The field points away from positive charges and toward negative charges.
What is the direction of the electric field?
The electric field is a vector pointing radially outward from a positive point charge and radially inward toward a negative point charge. The magnitude is given by the formula; the direction must be determined by the sign of q and the geometry.
How does electric field differ from electric force?
Electric field E is a property of space created by charge q. Electric force on a test charge q_test placed at that point is F = q_test * E. The field exists whether or not a test charge is present.
What does it mean for E to vary as 1/r^2?
The inverse-square law means the field decreases rapidly with distance. Doubling the distance reduces the field to one-quarter; tripling the distance reduces it to one-ninth. This is the same distance dependence as gravitational fields.
What units are used for electric field?
Electric field is measured in newtons per coulomb (N/C), which is identical to volts per meter (V/m). Both are correct; V/m is more common in engineering contexts while N/C is used more often in physics derivations.
Official sources
- NIST CODATA: NIST Value of Permittivity of Free Space.
- OpenStax University Physics: Electric Field, Vol. 2 Ch. 5.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.