18.1 Electric Fields and Field Lines

1. What Is an Electric Field?

Fields of Force

A field of force is a region of space where an object experiences a force without anything physically touching it. You have already met one example: gravity. The gravitational field around the Earth pulls any mass towards it, even from far away, with no direct contact.

An electric field works in the same way, but instead of acting on masses, it acts on charges. Whenever a charged object is placed in a region where another charge is present, it feels a push or a pull — even through empty space. That region of influence is called an electric field.

Defining Electric Field Strength

To measure how strong an electric field is at any point, physicists use a tiny imaginary object called a positive test charge. This test charge is placed at the point of interest, and the force acting on it is measured.

Electric field strength (E) at a point is defined as:

In equation form:

$E = \frac{F}{q}$

Where:

• E = electric field strength (unit: N C⁻¹, which means newtons per coulomb)
• F = the force experienced by the charge (unit: newtons, N)
• q = the magnitude of the positive test charge (unit: coulombs, C)

Why must the test charge be positive? Because the direction of the electric field is defined as the direction of force on a positive charge. If we used a negative charge, the force would point the opposite way, which would give us the wrong direction for the field.

Electric field strength is a vector quantity — this means it has both a size (magnitude) and a direction. The direction of E at any point is the same as the direction of the force that a positive charge would feel at that point.