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# Electric Field

"Discovery consists of seeing what everybody has seen and thinking what nobody has thought"
Albert von Szent-Gyorgyi

- When two charges exert a force on each other, through what means is the force transmitted ?

The simplest assumption is "action-at-a-distance". They just "know" about each other's presence. If one of the charges moves the other charge is aware of this immediately. This sounds like "magic" with today's scientific understanding.
The mechanism to remove the magic was proposed by Michael Faraday - the electric field. Every charge creates its own electric field in the space around it (actually the space around it means all space); other charges then interact with this field. When a charge moves it creates a disturbance in its electric field which is propagated away from the charge at the speed of light.
In developing his theory of gravitation Newton was aware of the same "action-at-a-distance" problem. To solve the problem, in a similar manner, we introduce the concept of the gravitational field.
Note that the concept of the electric field is a convenient construct to describe electromagnetic phenomena, but its true existence is neither proven nor essential.

- The electric field vector, E, is defined in the following way. If a charge, q, feels a force F, then the electric field, E, at the location of the charge is given by

- Units of electric field are Newtons/Coulomb (N/C) or (as we shall see later) Volts/metre (V/m).
- If the electric field felt by the charge q is due to a point charge Q, located a distance R from q, then its magnitude is given by

- The electric field due to many point charges is given by the vector sum of the fields due to the individual charges

E = E1 + E 2 + E 3 + .....

In performing this sum the direction of each E is the same as the force felt by a positive charge.

- The electric field in a region of space can be represented by electric field lines otherwise known as "lines of force".

The direction of the electric field line is the same as that of the force felt by a positive charge. The density of the field lines provides a measure of the magnitude of the field. FIeld lines alway begin on positive charges and end on negative charges; they cannot be left 'hanging' in empty space.

The diagrams above display the electric field lines in the vicinity of two equal point charges.

"Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."
Albert Einstein

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu