EE101 – Electric Field

Any object can take an electric or electrostatic charge. A positively charged object has a deficit of electrons, and a negatively charged object has an excess of electrons. The electron has the smallest known charge, -1.602 x 10C.

When objects are charged, an electric field is produced. The strength of electric field E (V/m) at a distance d(m) from an isolated point charge Q(C) in air or a vacuum is given by

E=Q/{4{pi}{varepsilon_o}{d^2}}

where:
permittivity of free space {varepsilon_o}=8.854*10^{-12} F/m

Permittivity is defined as the ability of a substance to store electrical energy in an electric field.

If the charge is inside an insulating material with relative permittivity {varepsilon_r}, the electric field strength becomes

E=Q/{4{pi}{varepsilon_r}{d^2}}

Any charged object or particle experiences a force when inside an electric field. The force F (N) experienced by a charge Q (C) in an electric field strength E (V/m) is given by

F=QE

Electric field strength is a vector quantity. The direction of the force on one charge due to the electric field of another is repulsive or attractive. Charges with the same polarity repel; charges with opposite polarities attract.

Work must be done to move charges of the same polarity together. The effort required is described by a voltage or electrostatic potential. The voltage at a point is defined as the work required to move a unit charge from infinity or from earth. It is normally assumed that the earth is at zero potential. Positively charged objects have a positive potential relative to the earth.

If a positively charged object is held some distance above the ground, then the voltage at points between the earth and the object rises with distance from the ground, so that there is a potential gradient between the earth and the charged object. There is also an electric field pointing away from the object towards the ground.

Symbols, quantities and units
Symbol Quantity Unit Unit Symbol
C capacitance farad F
E electric field strength volt per meter V/m
I electric current ampere A
Q electric charge coulomb C
V voltage or electric potential volt V
varepsilon permittivity farad per meter F/m

References:
1. Newnes Electrical Engineer’s Handbook – D.F. Warne


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