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ELIZABETHTOWN COMMUNITY AND TECHNICAL COLLEGE
BEX 100 BASIC ELECTRICITY FOR NON MAJORS
LECTURE GUIDE – UNIT 3
Instructor: Jerry Brown
Reference: Delmar’s Standard Textbook of Electricity, Third Edition
STATIC ELECTRICITY
OBJECTIVES:
At the completion of the lesson, the student should be able to:
1. Understand the basic nature of static electricity
2. Be able to explain the principle of the Electroscope
3. Know some of the principles of lightening protection
4. Be able to list some nuisance charges of static electricity
5. Be able to list some useful charges of static electricity
STATIC ELECTRICITY
Static electricity is a charge of electrons on a material or an object
that is sitting still and not moving
Electrostatic charges are charges that are built up on insulators (or on
materials that are electrically insulated from surrounding objects)
because insulators are the only materials that can hold electrons and keep
them from flowing.
Static charges can be positive (electrons removed) or negative (electrons
added)
Charging an object
Charge depends on the materials used
Examples –
Rubber rod rubbed on wool cloth – electrons move
from the wool to the rod
Glass rod rubbed on wool cloth – electrons move
from the rod to the wool
Electroscope
Transparent box with metal ball attached to a rod through the top
with metal leaves attached to the other end inside the box
Charge with a known charge by touching ball
If charge is negative, object with unknown charge is brought close
(but not touching) the ball, if leaves go farther apart, object is
charged negative. If leaves get closer together, object has
positive charge.
STATIC ELECTRICITY IN NATURE
Lightening
Static charges build up when a large amount of moisture is confined in a small
volume (clouds)
THEORY – Friction from the movement of a moisture cloud through relatively dry
air results in a static charge built up on the surface of moisture droplets.
Large drops build up a positive charge and small drops build up a negative
charge.
Most lightening discharges within the cloud or between clouds
Whether the lightening bolts travel from cloud to ground or from ground to
cloud depends on the charge.
If a cloud is negative and the ground more positive, lightening travels
from cloud to ground
If the cloud is positive and the ground more negative, lightening travels
from the ground to the cloud
Lightening Rods
Lightening rods provide an easy path to ground – were first invented by
Benjamin Franklin
Lightening arrestors are similar to lightening rods except they are mounted
close to the protected object with a small air gap between the rod and the
object
Power lines are usually protected with lightening arrestors with a high
resistance to ground at transmission voltage. The extremely high voltage from
lightening causes the resistance to lower and the lightening charge bleeds to
ground.
Nuisance static charges
Automobiles and other vehicles, as they move through dry air. This can
create a hazard for vehicles transporting large amounts of flammables.
They use a drag chain to bleed off static charge to ground as it is
created to prevent static build up
-
A person’s body walking across carpet, especially in dry air
-
Accumulated on clothes in a dryer (dry air)
Useful static charges
-
Spray painting. Object to be painted is negatively charged and paint is
atomized and passes through positively charged electrodes that transfer
the charge to the paint droplets. Paint is thus attracted directly to
the object. The result is excellent coverage with very little over
spray that results in significant savings in paint costs as well as in
the costs of environmental controls
-
Dry copy machine. Has an aluminum drum coated with a semiconductor
material (SELENIUM) that changes its conductivity with a change in
light intensity. When in darkness, it has very low conductivity, but in
light, its conductivity is high.
o
A high voltage wire near the rotating drum in darkness causes the
selenium to have a positive charge. A system of lenses and mirrors
causes the lighted image to be copied to be reflected onto the drum
o
Reflected light striking the drum increases the selenium’s
conductivity and electrons from the aluminum neutralize the charge
in the light areas of the drum. The dark areas remain positively
charged.
o
Negatively charged ink powder is applied to the drum and is
attracted to the positively charged areas. Excess powder falls away
o
A positively charged paper is passed under the drum. The ink is
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attracted off the drum to the paper that is then passed under a
heating element which melts the ink onto the paper, creating a
permanent copy.
SUMMARY
- The word static means not moving
- An object can be positively charged by removing electrons from it
- An object can be negatively charged by adding electrons to it
- An electroscope is a device used to determine the polarity (charge) of an
object
- Static charges accumulate on insulator materials
- Lightening is an example of a natural static charge
REVIEW QUESTIONS
1. Why is static electricity considered to be a charge and not a current?
2. If electrons are removed from an object, is the object positively or
negatively charged?
3. Why do static charges accumulate on insulator materials only?
4. What is an electroscope?
5. An electroscope has been charged with a negative charge. An object with
an unknown charge is brought close to the electroscope. The leaves of
the electroscope come closer together. Does the object have a positive
or a negative charge?
6. Can one thundercloud contain both positive and negative charges?
7. A thundercloud has a negative charge, and an object on the ground has a
positive charge. Will the lightening discharge be from the cloud to the
ground or from the ground to the cloud?
8. Name two devices used for lightening protection:
9. What type of material is used to coat the aluminum drum of a copy
machine?
10. What special property does this material have that makes it useful
in a copy machine?
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