We have studied many forces up until this point which are contact forces.
Gravitation and the electric force are not contact forces. Objects do not have
to touch for these forces to act. They can act over a distance.
The idea of a field was devised by
Michael Faraday to help explain how these forces can act over a distance. The
field refers to the area surrounding an object. In the case of an electric
field, the field extends outward from every charge and permeates all of space.
When a second charge is placed in this field, it feels a force because of the
electric field at that point.
Electric field
The altered area of
space around a charged object. This field exerts a force on any charged
object in its vicinity. The closer the charged object is brought to the charged
object creating the field, the greater the force exerted on it.
Test charge
a positive charge of very small magnitude. The test charge is used to determine the direction of the electric field. The electric field is defined as the force on a test charge with the test charge being so small that it approaches zero. Defining the electric field in this manner means that the electric field only describes the effect of the charges creating the electric field at that point.
Electric field lines
The electric field can be
represented with electric field lines. Their density is a measure of the
strength of the electric field at that point. Their direction is one that a
positive test charge would take in the field. Field lines are always directed
from the positive charge and toward a negative charge.
·
The field lines indicate the direction of the
electric field; the field points in the direction tangent to the field line at
any point.
·
Electric field lines start on a positive charge
and end on a negative charge.
·
The number of lines starting on a positive
charge, or ending on a negative charge, is proportional to the magnitude of the
charge.
·
The closer the lines are drawn together, the
stronger the electric field is in that region.
·
The field lines between two parallel plates are
parallel and equally spaced, except near the edges. Thus the electric field is
uniform between two parallel plates (except at the edges).
·
The field lines indicate the direction of the
electric field.
·
Electric field lines never cross. To do so would
imply that a positive test charge would have two directions simultaneously at
that point.
·
Electric field lines are drawn perpendicular to
a surface outside of a conductor.
Electric field strength (or
intensity)
symbol is E and SI unit is N/C;
E = F/q’
Where q’ represents the test charge are any other charge located in the field
Electric field strength of a point charge:
E = kq/d2
Where q represents the charge that creates
the electric field and d is the distance from this charge.
Electric field (E) is a vector quantity; it has
both magnitude and direction. Its direction is the same as the direction of the
electric force (F) on a test charge. The resultant electric field due to
several point charges can be determined using the same method as was used in
Coulomb's Law problems. Calculate the strength of the electric field due to
each point charge at point P. Determine the direction of the electric
field by determining the direction that a test charge placed at point P
would take. Use vectors to determine the magnitude and direction of the
resultant electric field at point P.
Superposition
principle for electric fields If the field is due to more than
one charge, the resultant field at a point is found by adding the individual
fields due to each charge at that point vectorially.
Electric Fields and
Conductors
· Because of their mutual repulsion, any excess charge on a conductor is distributed equally on the outer surface of the conductor.
· The density of the charges is greatest near edges and sharp points.
· Where the charge density is greatest is where electrical leakage and electrical discharges (“sparks / zaps”) are most likely to occur.
·
It is for this reason that an antenna with a
broken tip makes a radio have a lot of static. It is also the concept behind
· Because of mutual repulsion and the fact that no charge can exist inside a conductor, the electric field inside a conductor is zero when the charges are at rest.
· This means that conductor may act as an electrical shield against unwanted electrical charges.
· This is why your car is a safe place to be during a lightning storm. Any excess electrical charges remain on the outside metal surface of the car and the electric field inside the car is zero.
·
This also explains why your car radio will not
work in the tunnel.
· The electric field is always perpendicular to the surface outside of a conductor.
Click here for more notes from the web. There is very good explanation of why the charge density is greater at tips and sharp points.