# PHY C17: Electric Field Strength (A2)

After our basic understanding of electric field strength, we can now take a (slightly) deeper look:

• Formula: E = kQ/r2
• Graphs

What is electric field strength?
Electric field strength at a point is the force per unit charge acting on a positive charge placed at that point.

It is usually denoted by E (annoyingly, since it shares this symbol with energy, but they are NOT THE SAME).

In my AS post, you’ve seen electric field strength as described in a uniform field. Now we will see how it works in a non-uniform field.

E = F/q
Since F = kQq/r2
E = kQ/r2

Similar to the formula for gravitational field strength.
Note that the ‘q’ has disappeared!

• E is INDEPENDENT of the smaller test charge.
• E ONLY describes the effect of the charges creating the electric field (Q) at that point.
• E is the same for 2 small charges at an equal distance from the large charge (Q),
even if they have different charges (qA & qB).

How does E vary with distance from a point charge?
Decreases, following an inverse-square curve:

Remember the type of field!

• In UNIFORM fields, field strength can be considered to be constant at all distances
• In RADIAL fields, field strength decreases with the square of the distance

This applies to both gravitational AND electric fields!