Download Static Electricity - HSphysics

STATIC ELECTRICITY
Name: ________________
Class: _________________
Index: ________________
Objectives
-- state that there are positive and negative charges and that charge is
measured in coulombs
-- state that unlike charges attract and that like charges repel
--describe an electric field as a region in which an electric charge
experiences a force
--draw the field of an isolated point charge and show understanding that
the direction of the field lines gives the direction of the force acting on a
positive test charge
-- draw the electric field pattern between 2 isolated point charges
-- show understanding that electrostatic charging by rubbing involves a
transfer of electrons
--describe experiments to show electrostatic charging by induction
-- describe examples where electrostatic charging may be a potential
hazard
-- describe an example of the use electrostatic charging e.g. photocopier,
spraying of paint, electrostatic precipitator and laser printer
Static Electricity
• Rub a plastic ruler on your sleeve/hair and then hold it near
some tiny pieces of paper/tissue.
• What happens ?
• Nucleus: contains positively-charged protons
and non-charged neutrons
• Surrounding: negatively-charged electrons
In an uncharged atom,
the number of protons
= the number of
electrons
• Charging is the process of adding or removing
electrons into or from a conductor (or insulator)
which results in an imbalance of number of
electrons in the charged conductor (or insulator).
Positively-charged object (excess positive charges):
• More protons than electrons
Negatively-charged object (excess negative
charges):
• More electrons than protons
• Unit: Coulomb <C>
• Electric charge is quantized as a multiple of
the electron (-1.602 x 10-19 C) or proton
(+1.602 x 10-19 C) charge.
Q) How many electrons are there in 1 coulomb
of charge?
• 6.242 x 1018
The figs. below show the law of force for charges:
(a) Opposite charges attract
(b) Like charges repel
• Unlike charges
attract

Like charges repel
• Coulomb’s law F=(k qa qb )/ r2 ;
– where k= 8.99 x 109 (Nm2/C2)
• (i.e. qn=qh=1C & r=1m; F = [8.99 x 109 x 1 x 1]/ (1)2 =
8.99 x 109 N)
• Materials that do not allow electrons to move freely
inside them are called electrical insulators.
• electrons are all in fixed positions
• The addition or removal of electrons at any one part
of the insulator does not result in the electrons in
other parts of the same insulator to move.
• charge is localised (or confined) to the region.
• Examples of insulators are wood, plastics, ebonite, glass,
fur, silk.
• The method of charging by friction will only work
when two insulators are rubbed against each other.
• When an insulator is charged by the friction method the
charge remains on the surface of the material.
• This is because the charge cannot move through the
insulator.
• Some materials allow electrons to move about easily
inside them.
• These are called electrical conductors.
• outer electrons (valence electrons) are loosely bound,
relatively free from individual atoms
• We say that these electrons are delocalised.
• When electrons are gained/lost by the conductors, the
other electrons will flow automatically so that electron
re-distribution in the conductors occur.
• Examples are all metals like copper, iron, steel
• Charged by induction
Electric Field Lines
Electric field lines are patterns of lines that point in the direction
that a positive test charge would accelerate if placed upon the line.
As such, the lines are directed away from positively charged source
charges and toward negatively charged source charges.
To communicate information about the direction of the field, each
line must include an arrowhead that points in the appropriate
direction. An electric field line pattern could include an infinite
number of lines. Because drawing such large quantities of lines tends
to decrease the readability of the patterns, the number of lines is
usually limited.
The presence of a few lines around a charge is typically sufficient to
convey the nature of the electric field in the space surrounding the
lines.
Rules for drawing electric field lines
The rules for drawing electric field lines for any static configuration of
charges are:
1) The lines begin on positive charges and terminate on negative charges.
2) The number of lines drawn emerging from or terminating on a charge
is proportional to the magnitude of the charge.
3) No two field-lines ever cross in a charge-free region. (Because the
tangent to the field line represents the direction of the resultant force,
only one line can be at every point.)
4) The line approaches the conducting surface perpendicularly.
Electric Field of a single positive and negative
charge
+
-
Electric Field between 2 unlike point charges
Comment on the distances
between the electric field
lines A, B and C
A
B
+
C
-
Electric Field between 2 like charges
The point X in the diagram
is a null point.
There is no electric field at point X.
+
X
+
Electric Field between 2 like charges
The point X in the diagram
is a null point.
There is no electric field at point X.
-
X
-
Electric Field between 2 charged parallel plates
+
+
+
+
+
+
+
+
- - - - - - - -
Charging by Friction
The Triboelectric Series lists materials according to how
likely they are to let go of electrons or to take on
electrons from other materials. It is somewhat parallel to
Chemistry's Periodic Table.
Give up negative charges
The following materials will
give up electrons when
brought in contact with
materials, especially those that
attract electrons. They are in
the order of most apt to give
electrons to those that barely
give up electrons.
Dry human skin
Leather
Rabbit fur
Glass
Human hair
Nylon
Wool
Lead
Silk
Aluminum
Paper
Cotton ( + )
( ++++++ )
Collects negative charges
The following list of
materials will attract
electrons when brought in
contact with other
materials, especially those
that give up electrons.
They are in the order of
least apt to attract electrons
to those that readily attract
electrons.
Wood ( - )
Amber
Hard rubber
Nickel, Copper
Brass, Silver
Gold, Platinum
Polyester
Styrene (Styrofoam)
Saran Wrap
Polyurethane
Polyethylene (like Scotch Tape)
Polypropylene
Vinyl (PVC)
Silicon
Teflon ( - - - - - - )
The polythene is negatively
charged when it rubs with
dry cloth, because some of
the outer electrons are
transferred from the wool
surface onto the polythene.
Note:
Only electrons can move
-- the protons remain fixed.
The perspex rod is positively
charged when it rubs with
dry cloth, because some of the
outer electrons are scraped
off from the rod and move
on to the cloth.
Note:
Only electrons can move
-- the protons remain fixed.
• Any charged object - whether positively
charged or negatively charged - will have
an attractive interaction with a neutral
object.
Charging by Induction
Induction charging is a method used to charge an object without
actually touching the object to any other charged object.
The overall charge on the system of two objects is the same after the
charging process as it was before the charging process. Charge is
neither created nor destroyed during this charging process; it is
simply transferred from one object to the other object in the form of
electrons.
Charging 2 spheres with equal but opposite charges
Step 1
Place 2 conductors (e.g. metallic spheres)
on insulated stands.
B
A
Make sure that the 2 conductors are
initially uncharged.
insulating stand
Step 2
A
B
Next, put the 2 conductors in contact.
insulating stand
Step 3 (part i)
Bring a negatively-charged rod near the 2
conductors.
Take care that the negatively-charged rod
does not touch any of the 2 conductors.
-----
A
B
Step 3 (part ii)
insulating stand
Notice that the negative charges in the rod
-repel the electrons in A to move further
-away.
Since A touches B, some electrons in A will --be repelled to the further end of B.
As A lost some electrons, A becomes
positively-charged.
As B gained some electrons, B becomes
negatively-charged.
Same number of positive charges
& negative charges are formed.
+
+
++ A
++
++
--B -
insulating stand
Step 3 (part iii)
-----
Take special note of the charge
distribution in both the conductors A & B.
+
+
++ A
++
++
insulating stand
Step 4
With the negatively-charged rod still
in place, separate the 2 conductors
apart.
Note how the charge distribution
in both the conductors remains
unaffected due to the presence of the
charged rod.
--B --
-----
+
+
++ A
++
++
--B -
insulating stand
Step 5 (part i)
With the 2 charged conductors still
separated, remove the negatively-charged
rod.
--B -
+
+
++ A
++
++
insulating stand
Step 5 (part ii)
With the 2 charged conductors still
separated,
remove the negatively-charged rod away.
Now that the charged rod is removed,
the charges in A & B re-distribute
themselves immediately.
+ +
+ A +
+
+
+ +
-
- B
- -
-
insulating stand
Step 5 (part iii)
We started with 2 conductors which are
neutral. Now, we have 2 conductors that are
oppositely-charged.
Also, the number of positive charges in one
conductor is the equal to the number of
negative charges in the other conductor.
The same experiment can also be
repeated with a positively-charged rod.
+ +
+ A +
+
+
+ +
-
- B
- -
-
insulating stand
A
B
insulating stand
Charging a single sphere using earthing method
Step 1
Place a negatively-charged rod near the single
conductor (e.g. metallic sphere) that is sitting on
an insulated stand. Make sure that the single
conductor is initially uncharged.
Step 2 (part i)
Note that the moment the negatively-charged
rod is placed near the conductor, the charge
distribution in the conductor is disturbed
immediately.
-----
---
insulating stand
-+
+
+
++ - +-
insulating stand
Step 2 (part ii)
This happens because the negative charges repel
the conductor’s electrons to its further end.
Since the left-end of the conductor has “lost” some
electrons to the right-end, some positive charges
are noted to appear on the left-end too.
Step 3 (part i)
Next, touch the conductor for a short while. As
the human body is regarded as a good
conductor of electricity, the excess charges in
the conductor (electrons in this case) will flow
through the body down to earth. Touching the
charged conductor in this case is called earthing
---
---
-+
+
+
++ - +-
insulating stand
-+
+
+
++ - +-
insulating stand
Step 3 (part ii)
Finally, all excess charges are discharged.
Step 4
Once the excess electrons are discharged,
remove the finger from the conductor while
keeping the negatively-charged rod
stationary.
---
---
++
+
++
+
insulating stand
++
+
++
+
insulating stand
Step 5 (part i)
Finally, the negatively-charged rod is removed.
++
+
++
+
insulating stand
Step 5 (part ii)
The positive charges remaining in the single
conductor will re-distribute themselves
uniformly.
+ +
+
+
+ +
insulating stand
Discharging
Discharging is the process in which a charged body is removed of
excess charges from it. When a charged body is discharged, it is
said to be neutralised.
Discharging a Negatively-Charged Conductor
(by earthing)
To discharge a negatively-charged conductor, you
only need to touch it. Touching the conductor to
discharge it is known as Earthing.
--- --- -- -
When the conductor is earthed, the free electrons will
flow out of the conductor, through the body, to the
earth.
insulating stand
Discharging a Positively-Charged Conductor (by
earthing)
To discharge a positively-charged conductor,
you only need to touch it too. Touching the
conductor to discharge it is also known as
Earthing.
When the conductor is earthed, the positive
charges in the conductor attract electrons from the
earth to flow towards them. The electrons will
neutralise the positive charges in the conductor.
++
+
+
++
insulating stand
Question
X
---
----
Y
+
+ +
+
A small conducting sphere Y which
is positively charged is suspended in
air with an insulating thread. Y is
held stationary by a force F.
It is brought near to a large
conducting sphere X which is
negatively charged. X is resting on
an insulating stand.
Describe & explain what will happen
when force F is removed.
The Gold Leaf Electroscope
The gold leaf electroscope can be used to find out if an
object is charged and what type of charge it carries.
When the electroscope is charged at A, B becomes charged the
same as the metal plate C. Like charges repel, so the leaf rises at B.
Dangers of Static Electricity
Lightning Conductors
Large amounts of electric charge are built up in clouds because
of friction from winds. When the charge on a cloud is sufficiently
large, the air ionizes and becomes conducting. The charge flows
throughout the air to earth as lightning and strikes the nearest or
sharpest object.
(continue on
next slide)
Dangers of Static Electricity
Fires and Explosions
Charge can build up on many objects such as planes and petrol
tankers. If not discharged carefully, a spark (similar to that
produced by lightning) can start a fire or cause an explosion.
A “anti static strip" will periodically touches the road discharging
any built up static electricity.
Uses of Static Electricity
The Van de Graff Generator
+
+
+
+
To
power +
supply
-
+
+
+
+
+
+
The Van de Graff generator is a
machine for charging things up.
When it is switched on, charge
builds up on its dome. Charge is
+
deposited on the bottom of the
belt and is carried up to the dome
belt
by the belt. If too much charge
builds up on the dome, the dome
discharges itself by letting sparks
insulator fly to any nearby object.
Hollow
+ metal
sphere
Photocopier
Step 1:
Inside the photocopier, a
light-sensitive plate ( or
drum) is given a
negative charge.
Step 2:
An image from the
original document is
projected onto the plate.
The bright areas lose
their charge but the
dark areas keep it.
Step 3:
Toner is attracted to the
charged areas ( the dark
areas).
Step 4:
A blank sheet of paper is
pressed against the plate
and picks up toner.
Step 5:
The paper is heated so
that the toner melts and
sticks to it. The result is
a copy of the original
document.
Spray Painting
Many mass produced objects such as cars are spray painted.
To increase efficiency and reduce paint usage the paint particles and the
car body are given opposite charges. Thus, the paint will be attracted to
parts of the car not yet covered by paint.
Electrostatic Precipitator
Flue ash is a mixture of dust and smoke produced by many factories and
power stations. Charged metal plates in the chimney attract these
particles and remove them from the exhaust gases.
Laser Printer
Inside a laser printer there is a drum which holds an electric charge. Next
to the drum is a transfer corona roller, which can negatively or positively
charge the drum as needed, as well as a toner unit. In most laser printers,
the drum starts out positively charged, although this process can also
work in reverse. The controller manipulates a small laser to “write” on
the drum with a negative charge, creating an electrostatic image.
Then, the drum is rolled through the toner, which is positively charged
so that it will cling to the areas of negative charge on the printer drum.
The printer feeds a piece of paper, which is given an even stronger
negative charge by the transfer corona wire before being rolled past the
drum. The electrostatic image on the drum will transfer to the paper,
which is then discharged to prevent it from clinging to the drum. Then it
is fed through a fuser which heats the toner and causes it to bind with
the fibers in the paper.
Meanwhile, the drum passes a discharge lamp, which will expose the
entire surface of the drum and erase the electrostatic image. The transfer
corona wire applies another positive charge, and the printer is ready for
the next page or job.
References
http://www.revisionworld.com/files/ionic%20bonding.jpg
http://www.dummies.com/how-to/content/physics-drawing-electric-field-lines.html
http://www.physicsclassroom.com/class/estatics/u8l4c.cfm
http://www.sciences.univnantes.fr/physique/perso/maussion/statelec/PagesEngl/06Induc.html
http://www.photocopier-online.com/
http://fiat500usa.blogspot.com/2010/08/inside-fiat-500-factory.html
http://www.school-for-champions.com/science/static_uses.htm