Electric field strength

A charged object placed in an electric field will experience a force.The electric field strength at a point in an electric field - E - is the force per unit charge on a small, positive test charge placed in the field at that pointThe unit is NC^-1In symbols:E = F / Qwhere F is the force in newtons on a charge Q in coulombs. Electric field strength is a vector. Its direction is the direction of the force on a positive charge, so a field points away from a positive charge and towards a negative charge.The test charge must be small so that its own electric field is negligible and does not change the field being measured.This is similar to gravitational field strength, g = F / m, the force per unit mass on a small test mass. One difference is that gravitational forces are always attractive, while electric forces can be attractive or repulsive.Worked example: a charge of +2.5 μC experiences a force of 80 mN at a point in a field.E = 80 × 10^-3 / 2.5 × 10^-6 = 3.2 × 10⁴ N C^-1A negative charge placed at the same point would feel a force of the same size in the opposite direction.