Example 1
Q = 1e-6 C, r = 0.1 m, default k.
Electric Field from Point Charge
Type Q and r. You can leave k blank: the calculator uses 8.99e9. 1e-6 C at 0.1 m is about 8.99×10⁵ N/C. r must be greater than zero.
Coulomb force: F = k q1 q2 / r². Definition: E = F/q.
Result
Constant k (N·m²/C², optional), Charge Q (C) and Distance r (m). The result shows up here.
How it works
The field of a point charge is E = k Q / r². Blank k means the constant 8.99e9 N·m²/C². At 1 µC and 0.1 m you get about 8.99×10⁵ N/C. Typed 9e9 is the school rounding: same Q and r, about 9.00×10⁵ N/C. r is squared, so twice as close is four times E.
Three fields: k optional in N·m²/C², Q in coulombs, r in meters. The result is in N/C, the same as V/m. Type a microcoulomb as 1e-6, not as 1. A comma, a period, and 1e-6 are the same charge.
Q and r need a number. r = 0 does not divide. Blank k is fine and fills in 8.99e9. A negative Q flips the direction of E; the magnitude follows |Q|.
Coulomb force F = k q1 q2 / r² is on the neighbouring page: two charges and newtons. Here the field from one Q. The definition E = F/q waits when you know the force on a test charge.
At Q = 1e-6 C and r = 0.05 m, E is four times the 0.1 m value, about 3.60×10⁶ N/C. At r = 1 m the same Q is about 8.99×10³ N/C. At Q = −2e-6 C and r = 0.5 m, E is smaller and points the other way.
Type 1e-6 and 0.1, leave k blank, click Calculate, and match about 8.99×10⁵ N/C. Then type 9e9 in k and compare the rounding. Charge Q = I t lives on its own page.
How to use
- You can leave k blank; the calculator then uses 8.99e9. Type school 9e9 if the problem asks for that rounding.
- Type Q in coulombs, for example 1e-6, and r in meters, for example 0.1. 1 µC is 1e-6, not 1.
- Click Calculate. The calculator computes E = k Q / r². 1 µC at 0.1 m with default k is about 8.99×10⁵ N/C.
- Q and r need a number. r must be greater than zero. Blank k is fine.
- Want the force between two charges? Open F = k q1 q2 / r². Here it stays E from one Q.
Formula
E = k · Q / r2
k ~ 8.99e9 N·m2/C2 (blank = default). r > 0.
E from a point charge
Field of one charge: E = k·Q/r². A blank k uses 8.99e9. 1e-6 C at 0.1 m is about 8.99×10⁵ N/C.
- E
- Field strength in N/C. kQ/r², not F/q from a measured force.
- k
- The same 8.99e9 constant. A blank field fills the default.
- Q
- The source charge in coulombs. 1e-6 C in the example. One charge, not a q1 q2 pair.
- r
- Distance from the charge in meters. r > 0. Squared in the denominator.
Real-life examples
Example 2
Negative Q reverses E direction.
Example 3
Typical classroom numbers.
Example 4
Q = 1e-6, r = 0.05 m: strong E.
Example 5
Q = 1e-6, r = 1 m: weaker E.
Example 6
k = 9e9, Q = 1e-6, r = 0.1 m.
Example 7
Q = 100e-9, r = 0.01 m.
Example 8
Q = 1e-5, r = 0.3 m.
Example 9
Q = 2e-6, r = 2 m.
Example 10
Double r -> E 4 times smaller.
Ways to use this calculator
- You compute E from 1 µC at 0.1 m: about 8.99×10⁵ N/C.
- You compare with E = F/q when you know force on a test charge.
Frequently asked questions
How much E at Q = 1e-6 C and r = 0.1 m?
With default k = 8.99e9: E ≈ 8.99×10⁵ N/C. With typed 9e9 about 9.00×10⁵ N/C.
Do I have to type k?
No. Blank k is 8.99e9. Typed 9e9 overrides the constant, as in the calculator example.
Which units do I type?
Q in coulombs, r in meters. 1 µC = 1e-6 C. Result in N/C = V/m. k in N·m²/C² if you type it.
What if r = 0?
Zero does not divide. r must be positive. Q and r need a number.
How is this different from F = k q1 q2 / r²?
Here E from one Q, in N/C. There force between two charges, in newtons.
How is this different from E = F/q?
Here the source formula from Q and r. There the definition from a measured force on a test charge.
Does negative Q work?
Yes. It flips the direction of E. Magnitude follows |Q|.
How much at Q = 1e-6 C and r = 0.05 m?
E ≈ 3.60×10⁶ N/C, four times the 0.1 m value. At r = 1 m about 8.99×10³ N/C.
Does a comma in 0.1 work?
Yes. 0,1 and 0.1 mean the same r [m]. 1e-6 parses too for a small charge.
Where is Q = I t?
On the charge page Q = I t. Here Q is already an input to E = k Q / r².
Knowledge sources
The formula is the school one. Units follow SI; NIST SP 330 and BIPM define the measures, not your result.
Page updated in 2026.