THE ELECTRIC
FIELD. Each
electric particle projects into space a field of electric force, and as the
particles move along a wire, the lines of force move with them. It is the
motion of these lines of electric force that sets up a magnetic field
transverse to them. A variable electric field is always accompanied by a
magnetic field; and conversely, a variable magnetic field is accompanied by an
electric field. The joint interplay of electric and magnetic forces is what is
called an electromagnetic field and is considered as having its own objective
existence apart form any electric charges or magnets with which it may be
associated. Examples are the photon, or quantum of light, and the
electromagnetic field radiated by an aerial.
Modern physics defines the electromagnetic
field as a district form of matter possessing definite properties: it is
disturbed continuously in space; in a vacuum it propagates at the speed of
light (300,000 km/sec); it interacts with charges and currents to convert
itself into other forms of energy(chemical, mechanical, etc.)
The theory of the electromagnetic
field was stated by the Scotch physicist James Clerk Maxwell in his
"electricity and Magnetism" published in 1873.
In the case of a stationary charged body the magnetic fields, built up by the
elementary charges constantly moving inside it cancel each other and there is
practically no magnetic field. The same is true of a stationary permanent
magnet which only displays a magnetic field and has no electric field. This
condition enables us to investigate electric and magnetic fields separately.
We shall regard the electric field as
one of the aspects of the electromagnetic field.
A measure of the strength of an
electric field given by the mechanical force per unit charge experienced by a
very small body placed in this field and is denoted by the letter E.
By definition
E=F/q
If the strength of an electric field is the
same both in magnitude and direction at any point in space., the field is
called uniform.
It is relevant to note that quantities
which have both magnitude and direction are called vectors, as distinct from
quantities which have only magnitude and are called scalars. Typical
vectors are force, velocity, acceleration, while typical scalars are
temperature, quantity of matter, energy, power. Vectors are shown graphically
as arrows with their lengths giving magnitude on a chosen scale and the arrows
themselves, direction.
An inertialess charge placed in an electric
field would follow a path called a line of force. The total number of lines of
electric force through a surface placed in an electric field is called the
electric flux and is denoted by the letter N .For a surface S normal to the
vector of a uniform field of strength E, the flux is
N=ES.
For a nonuniform field the flux is determined in a different way.
We have already defined a line of
electric force.
Placing a positive charge at different points in the field set up by a
positively charged spherical body, we obtain a set of such paths, or lines of
electric force.
Obviously, any number of lines of electric force can be imagined in an electric
field.
In order to represent its strength, there is a well- established convention to
draw as many lines of electric force through every square centimeter of area
normal to the lines at a field point ,as will be equal to the field strength at
that point. Consequently, the density of lines of force will give a graphic
idea of the field strength.
We know that like charges repel one another.
Therefore, on any conductor the electric charge will concentrate only on its
surface. The quantity of electricity per unit area is called the surface charge
density. It depends on the quantity of electric charge on a given body and on
the shape of the latter.
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