Transcript Electric Charge

Electric Charge
Charge
Conductors and Insulators
Coulomb’s Law
Permittivity Constant
Spherical Conductors
Charge Quantization
Conservation of Charge
pps by C Gliniewicz
Early Greeks first discovered that amber, when rubbed, caused straw to jump
onto the amber. This was the first observation of electric charge and electrical
forces. One can observe sparks when shoes are dragged across a rug and then
another person or a piece of metal is touched.
Electric charge is an intrinsic property of matter. There are positive and negative
charges. If the charges are equal or balance each other, the material is electrically
neutral. Charges with the same electrical sign repel each other. Charges with
opposite signs attract each other.
The labels for the charges were arbitrarily assigned by Benjamin Franklin. He
easily could have exchanged the signs. Photocopying uses electrical charges to
transfer the toner to paper.
Conductors are materials through which electrical charges can move freely.
Nonconductors or insulators are materials through which charges cannot move
easily. Semi-conductors are materials between conductors and insulators.
Superconductors are materials that are perfect conductors wich allow the charge
to move without any hindrances.
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A conductor, since it allows the free movement of charge, will provide a means
for charges in another conductor to move to a place with a different charge. The
earth is a repository of charges. A conductor can allow charge to move to the
earth.
Electrons have a negative charge and protons have a positive charge. Only the
electrons move since they circle the nucleus which contains the protons. Metals,
like copper, have their outermost electrons loosely held allowing them to move to
other atoms which are close by.
Objects become negatively charged when excess electrons are accumulated. If
objects loose electrons the object becomes positively charged because there are
more protons than electrons in the object.
If one adapts their eyes to darkness for ten or more minutes and then another
person bites into a wintergreen LifeSaver, a blue flash can be observed. As the
sugar crystals are broken into pieces, different charges end up on each piece and
the charge jumps between the pieces causing the blue spark.
The screen of a computer or television accumulates a charge and dust and
bacteria are attracted to the screen. Touching the screen can transfer bacteria.
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Suppose that two particles have charges q₁ and q₂ and are separated by a distance
r. The electrostatic force of attraction or repulsion between them is
in which k is a constant.
Charles Augustin Coulomb completed experiments in 1785 which led him to his
law. The law is very similar to Newton’s Law of Gravitation. A positive value
means repulsion and a negative value means attraction.
The coulomb is the SI unit of charge. Electric current is the time rate of change of
electric charge.
A shell of uniform charge attracts or repels a charged particle that is outside the
shell as if all the shell’s charge were concentrated at the center of the shell.
If a charged particle located inside a shell of uniform charge has no net electrostatic
force acting on the particle due to the shell of charge.
When first discovered by Franklin, electric charge was thought to be a fluid. We
now know that electric charge is quantized.
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The elemental charge on an electron or proton has the same magnitude, but
opposite charge. That charge is 1.602×10⁻¹⁹ Coulombs and is designated by the
symbol, e.
In a system containing electric charges, the total number of charges remains the
same. No experiment has yet shown that electric is not conserved. Therefore, one
can add conservation of electric charge to conservation of energy, momentum and
angular momentum.
Nuclear interactions all show this conservation. Examples are shown below.
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