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 field lines point away from positive charges and towards negative charges. A negative charge placed in the same field feels a force in the opposite direction to the field.The test charge must be small so that its own electric field does not change the field being measured.Rearranging gives F = EQ, the force on any charge placed at that point. For example, a +2.5 μC charge that feels a force of 80 mN is in a field of strength E = 0.080 ÷ (2.5 × 10^-6) = 3.2 × 10⁴ N C^-1. An electron (charge −1.6 × 10^-19 C) at the same point would feel a force of 5.1 × 10^-15 N in the opposite direction to the field.Electric field strength is the electrical version of gravitational field strength g = F/m, the force per unit mass. The main difference is that gravitational forces are always attractive, while electric forces can be attractive or repulsive.