Electric field
Also called E field
The electric field at a point is the electric force per unit charge that a positive test charge placed there would feel. It is a vector measured in newtons per coulomb, and it exists at the point whether or not a charge is there to feel it.
Read as an instruction rather than a formula. Put a small positive test charge at the point you care about, measure the electric force on it, divide by the charge. The probe cancels, and what survives describes the point. The unit is newtons per coulomb, which is the same unit as volts per meter.
The direction rule belongs to the source, not the probe: the field points away from isolated positive charge and toward isolated negative charge. The force on a charge sitting in it is , so a negative charge accelerates opposite the field vector.
Fields superpose as vectors. The net field at a point is the vector sum of the fields of every nearby charge, resolved into components. That is where the field parts company with electric potential, which superposes as signed numbers with no angles.
The field line rules all follow from the above:
- The field is tangent to the line through a point, in the direction of the arrowhead.
- Lines leave positive charge and arrive at negative charge.
- Closer spacing means a stronger field.
- Lines never cross, because one point cannot have two field directions.
Field lines carry relative magnitude only. For a number, go to the equation or a vector field map. The electric field and potential guide runs that calculation end to end.