Electric field strength (E) is a concept arising from a "thought experiment." It is defined as the force (F) acting on a test object divided by the size of the test object (q), particularly when the test object is a positive test charge. The formula is given by

E = \(\frac Fq\)​

Here

• E is the electric field strength,
• F is the force acting on the test charge, and
• q is the magnitude of the test charge.

Practical measurement of electric field strength is difficult because the test object, which carries its own field, will distort and alter the field in which it is placed. However, theoretically, we can use the definition to understand the concept.

The unit of electric field strength is Newton per Coulomb (NC^-1). This means that a field with a strength of 1 NC^-1 would exert a force of 1 Newton on a test charge of 1 Coulomb.

At a specific point in space, the electric field strength is the force per unit charge experienced by a small positive point charge placed at that point.