Lecture Notes (Electric Fields) - Dickey Physics

Lecture Notes
(Electric Fields)
Intro:
- the gravitational force and the electrostatic force are both
capable of acting through space, producing an effect even
when there isn’t any physical contact between the objects
involved
- field forces can be discussed in a variety of ways, but an
approach developed by Michael Faraday (1791–1867) is the
most practical
- in this approach, an electric field is said to exist in the region of
space around a charged object; the electric field exerts an
electric force on any other charged object within the field.
Electric Field:
- an electric field cannot exert a force on the charge which
generates it, in just the same way as you cannot pick yourself
up with your own shoelaces

- electric fields are real; there is an energy associated with an
electric field which fills space; it is actually possible to covert
this energy into heat or work, and vice versa
- an electric field is a vector quantity; it has both magnitude and
direction; the strength is the force per unit charge
- if a body with charge, q, experiences a force, F, at some point in
space, then the electric field, E, at that point is:
- the units of electric field strength are N/C (newtons per coulomb)
- the electric field is depicted with vector arrows; the direction of
the field is shown by the vectors and is defined to be the
direction in which a small, positive test charge at rest would
be moved
- the test charge is required to be very small, so it doesn’t cause
any significant rearrangement of the charge creating the
electric field
- the direction of the force and that of the electric field at any point
are the same

- if we start with an example of a negatively charged ball, all the
vectors would point to the center of the ball
- any charge placed in an electric field experiences a force on it
resulting from the electric field at that point
Picturing the Electric Field:
- a more useful way to describe an electric field is with electrical
lines of force (electric field lines)
- the electric field lines represent a small number of the infinitely
numerous possible lines that indicate direction of field
- the electric field is tangent to the field lines at all points

- for a single point charge the lines extend to infinity; they point
away from a positive charge and toward the negative point
charge
- for two or more opposite charges, we represent the lines as
emanating from the positive charge and terminating on the
negative charge

- the electric field concept helps us not only understand the forces
between isolated, stationary bodies, but also what happens
when charges move
- the electric field is a storehouse of energy which may be
transported over long distances; this energy may be directed
through metal wires or teamed up with a magnetic field to
move through empty space
Van de Graaf Machine:
- in 1929, Robert J. Van de
Graaf designed and
built an electrostatic
generator that bears his
name
- everyday objects are made
up of equal amounts of
positive electric charge
and negative electric charge in almost perfect balance; we
might say that ordinary matter is made up of cancelled-out
electricity
- a VDG machine takes the mixed-up
positive and negative charges of
matter, sorts them out, then pulls
them far apart from each other
- a VDG is a charge uncanceller or
charge separator; the machine
moves charge continuously, so it
also acts as a mechanical charge
pump
- the machine operates by use of a motor
driven pulley

- the needles attract negative charges from the belt, causing one
side of the belt to be positively charged
- the charged belt attracts electrons from a second comb of needles
found at the top of the device; this enhances the positive
charge of the metal dome
- a discharge occurs when the excess positive charge builds up on
the dome; this is due to a strong electric field
- when this field builds up enough strength, it will ionize the air
around the dome; charges on the dome now have a way to
leak off into the air which will produce lightning discharges
- VDG machines were originally used as power supplies for the
early particle accelerators used in research into radioactivity
- big VDGs are also used to power high energy X-ray machines;
if you want to treat cancer with radiation, make X-ray photos
of locomotive engines, or sterilize food with gamma rays,
you'll want to buy a Van de Graaff powered X-ray generator