Electron Movement in Objects
CONDUCTORS VS. INSULATORS
What is true of atoms in terms of being neutral, negatively charged, and positively charged is also
true of objects. Objects can work like ions by losing or gaining electrons.
CONDUCTORS VS INSULATORS
The behavior of an object which has been charged is dependent upon whether the object is made of a
conductive or a nonconductive material.
A material with easily detached electrons (which can then move
through the material somewhat freely) is referred to as a conductor.
Conductors are materials which permit electrons to flow freely from
atom to atom and molecule to molecule. Metals are good conductors
because some electrons in the atoms are free to move throughout the
metal. The Earth is also a good conductor.
In contrast to conductors, insulators are materials which resist the
free flow of electrons from atom to atom and molecule to molecule
because their electrons are strongly attached. If charge is transferred
to an insulator at a given location, the excess charge will remain at the
initial location of charging. So insulators are good at generating and
holding a surface charge because the particles of the insulator do not
permit the free flow of electrons.
Conductors: silver, copper, gold, aluminum,
iron, steel, brass, bronze, mercury, graphite,
dirty water, concrete
Insulators: glass, rubber, oil, asphalt, fiberglass, porcelain,
ceramic, quartz, (dry) cotton, (dry) paper, (dry) wood,
plastic, Styrofoam, wood, air, diamond, pure water
What does this chart mean?
Methods of Charging Objects
THE LAW OF CONSERVATION OF CHARGE
THE LAW OF CONSERVATION OF CHARGE
Charge is neither created nor destroyed. When objects are
charged, electrons are transferred from one object to another
or they move within the object. No matter what type of
charging method is used, the amount of charge present in the
system before charging will be the same as the amount of
charge present in the system after the charging. This is
referred to as the law of conservation of charge.
Well, how do objects get charged to begin with? There are
three methods of charging: friction, induction and conduction.
FRICTION
Friction results in a transfer of electrons between two electrically neutral objects which are
rubbed together. The two objects become oppositely charged as a result of the transfer of
electrons.
Different materials are made of different atoms – with different electron affinities. Simply
put, the property of electron affinity refers to the relative amount of love which a material
has for electrons. If atoms of a material have a high electron affinity, then that material will
have a relatively high love for electrons.
A triboelectric series is a listing of materials in order of their relative attraction for the
electrons of another material. Materials listed near the top of the table have a stronger
affinity for electrons than those located below them. When any two materials in the table
are rubbed together, the one which is higher can be expected to pull electrons from the
material which is lower. As such, the materials highest on the table will have the greatest
tendency to acquire the negative charge. Those below it would become positively charged.
Triboelectric
Series
strong affinity
Celluloid
Sulfur
Rubber
Copper
Brass
Amber
Wood
Cotton
Human Skin
Silk
Cat Fur
Wool
Glass
Rabbit Fur
CONDUCTION
Charging by conduction involves the contact of a charged object to a neutral object. When a charged object
touches a neutral object, electrons flow from one object to the other attempting to equalize the charge on
both objects. The objects need to be conductors. Conduction doesn’t work as well with insulators because
they do not allow charge to spread out.
INDUCTION
Charge induction occurs when a charged object causes
electrons on the surface of a second neutral object to
either move toward or away from the charged object
without the two objects actually touching. This is called
separation of charge and serves to create areas in the
second object that are unevenly charged. The unevenly
charged object is said to be polarized. Because the charged
object doesn’t touch the neutral object, there is no transfer
of charge (electrons).
Effects on conducting materials…
When the originally neutral object is a conductor, its electrons are loosely
bound so they can move from one part of the object to another part. If a
negatively charged object is brought near a neutral conducting object (1), it
will cause electrons to move away from the negatively charged object and the
positive charges to move toward the negatively charged object (2). Overall the
object is electrically neutral; it's just that the positive and negative charge has
been separated from each other. We say that the charge in the object has
been polarized within the object.
If the originally neutral object is a grounded conductor (3), the negative charge
will flow to the earth and the entire object will become positive (4). If is NOT
grounded, it will return to its neutral state once the charged object moves
away.
Effects on insulating materials…
Electrons aren’t free to move in an insulator – but that does not mean that an insulator doesn’t experience
polarization. Polarization in an insulator is a result of an alignment of the charge within each individual
molecule. So when induction happens to an insulator, the particles in the insulator realign themselves creating
a polarized object. Charge builds up and stays there (creating static electricity).