What does an electric-field arrow tell me about a charge at that location?

Read the arrow as a description of the electric field at that location—not as a tiny arrow attached to a particular charge. The field vector gives the direction of force per unit positive test charge. The force on a charge is related to the field by F = qE. [S1]

First note the arrow’s direction, then consider the sign of the charge you place there:

1. A positive charge feels a force along the arrow.
2. A negative charge feels a force opposite the arrow.

The field is defined independently of a test charge. The arrow does not say that a charge will travel along it; it indicates the force direction at that point. [S1]

The field magnitude describes force per unit charge, but the arrow’s length alone doesn’t tell you a numerical magnitude unless the diagram provides that information. The supplied field definition does not specify how a particular diagram represents magnitude, so look for a stated field value or other explanation rather than assuming the drawn length is a scale.

Force also depends on the charge. The relationship is F = qE, and the field’s units are newtons per coulomb (N/C). [S1] For force magnitude, use |F| = |q|E: a larger-magnitude charge in the same field has a larger-magnitude force. The charge’s sign determines which way that force points.

Imagine a diagram with a right-pointing field arrow at one spot. A positive charge placed there would feel a rightward force; a negative charge would feel a leftward force. Neither result says the charge must move in that direction: the arrow gives the electric force direction at that spot, not a promised travel route. [S1]

If the field is stated to be 3 N/C and you place a hypothetical +2 C charge there, the force magnitude is (2 C)(3 N/C) = 6 N, pointing right. For a hypothetical −2 C charge in the same field, the magnitude is also 6 N, but the force points left. These are example values used to illustrate F = qE, not measurements from a real setup. [S1]

Use a short sign-and-units check. Mark the location the arrow represents, record the field direction and any stated magnitude, and write down the charge’s sign. Use F = qE for the vector relationship; if you calculate magnitude alone, use |F| = |q|E. Check that the units reduce to newtons: coulombs multiplied by newtons per coulomb gives newtons.

Use a numerical field magnitude only when one is given or explained. The field definition tells you what the arrow’s direction means and how the field relates to force; it does not say that a charge follows the arrow as it moves. [S1]