Electric Field

Field Vector Direction

away from positive, toward negative

A field vector is a local direction claim. Positive sources point away; negative sources point toward.

Example

A field vector is a local direction claim. Positive sources point away; negative sources point toward. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Field points away from positive charge

Use a positive 1 coulomb test charge. Near a positive source, the field arrow points away from the source.

+ source⇒E points away+\text{ source} \Rightarrow E \text{ points away}
Field away from positiveA positive source creates a field pointing away from it.+2 Csource+1 Ctest4 N/C

Field points toward negative charge

Use the same positive test charge near a negative source. The field arrow points toward the negative source.

− source⇒E points toward-\text{ source} \Rightarrow E \text{ points toward}
Field toward negativeA negative source creates a field pointing toward it.-2 Csource+1 Ctest4 N/C

Direction changes even when size matches

Both examples use a field size of 4 newtons per coulomb. The size matches, but the source sign changes the direction.

E=4 N/Csource sign sets directionE = 4\ \text{N/C}\qquad \text{source sign sets direction}
Field toward negativeA negative source creates a field pointing toward it.-2 Csource+1 Ctest4 N/C

Sign changes direction, not the local unit

Both rows still measure field in newtons per coulomb. The source sign chooses whether the arrow points away from the source or toward it.

source signfield directionpositive test force+awaywith field−towardwith field\begin{array}{c|c|c}\text{source sign}&\text{field direction}&\text{positive test force}\\+&\text{away}&\text{with field}\\-&\text{toward}&\text{with field}\end{array}
Field toward negativeThe negative-source row is the checked diagram.-2 Csource+1 Ctest4 N/C
electrostatics The source sign sets the field direction before any test charge size is chosen.