Electric Field

The electric field measures the force per unit charge. Unlike gravity which can only be attractive the electric field can be attractive or repulsive depending on the sign of the charge. For positive charges, the field line always flow away from the charge, negative charges the field lines pull a positive charge toward it.

· They never start or stop in empty space – they stop or start either on a charge or “at infinity”.

· They never cross – if they did, a small positive charge placed there would feel forces in different directions, which could be resolved into the one true direction of the field line there.

· The density of field lines on a diagram is indicative of the strength of the field.

Electric Field of Point Charge Distributions

Positive Charge

Figure 1. Electric field from a positive charge

Negative Charge

Figure 2. Electric field from a negative charge

Dipole