Download Electrostatics, Electricity, and Magnetism

Electrostatics & Electricity
Electrostatics is the interactions between electric charges. It is often called static
electricity. You are familiar with the terms positive and negative and should recall
that atoms are composed of positively charged protons, negatively charged
electrons, as well as neutral neutrons. During your study of atoms you learned that
protons were 2000 times more massive than electrons, however, the magnitude of
electric charge is the same for each. So, one electron and one proton result in no net
electric charge: neutral. The basic rule of electrostatics is the similar charges repel
each other and opposite charges attract each other. The forces that result from
electric charges are on the order of 1020 times larger than the force of gravity. You
have probably noticed the effect gravity more than the effect of electric force for
the basic reason that gravity only attracts, while electric forces can attract and
repel, thus canceling each other out. The amount of electric force experienced by
electric charges increases as the amount of charge increases and decreases as the
distance between the charges increases. The strength of the electric force in the
space around a charge is known as an electric field. The unit of electric charge is
called a coulomb (C) and is equal to the amount of charge resulting from
6.25 x 1018 electrons. Coulomb's law provides a mathematical relationship to
determine electrical force.
The value of Coulomb's law constant (k) is 9.0 x 109 N·m2/C2
Everyday objects often become charged in three basic ways: friction, conduction,
and induction. Charging by friction is simply the scraping off of electrons from
one object onto another by rubbing them together. The object losing the electrons
becomes positively charged and the object scraping the electrons becomes
negatively charged. Conduction simply involves contact between a charged and a
charged or neutral object. When the two objects touch electrons are transferred
away from a negatively charged object, or toward a positively charged object. If
one of the objects is neutral it either gains or loses electrons based on the charge of
the other object. Charging by induction involves a charged object coming near an
object. If a negatively charged rod were brought near a neutral conductor the
electrons will be repelled to the far side of the conductor leaving the closer side
positively charged, otherwise known as polarizing. If, while the conductor is
polarized, you were to provide a pathway to the ground for the electrons on the far
side (grounding) the electrons would be further repelled through that pathway. By
removing the negatively charged rod at this point, followed by releasing the
grounding action you produce a positively charged conductor due to the loss of
electrons. You can ground a conductor by simply toughing it with your finger, your
body is then the pathway to the ground. As it turns out, the earth itself is a
virtually infinite reservoir of electrons. Let’s reverse the induction example
provided above. Bring a positively charged rod were brought near a neutral
conductor the electrons will be attracted to the near side of the conductor leaving
the far side positively charged. Ground the conductor and more electrons will flow
onto the conductor due to the attraction of the positively charged rod. Remember
the electric force is extremely large. The result would be a negatively charged
conductor due to the gain of electrons. Another example of charge induction is
found in the production of lightning.
Clouds in a thunderstorm are often polarized; the top and bottom regions of the
cloud have opposite charges. If the bottom of a certain cloud were negative it
would induce a positive charge on the earth’s surface below. This induced positive
charge attracts more negative charge to the bottom of the cloud. If the electrostatic
buildup of electrons becomes too great there is a discharge of the electrons from
the cloud to the ground, which we know as lightning. Ground to cloud, cloud-to-
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cloud, and inter-cloud lightning are all results of the discharge of huge amounts of
electrons. You may experience mini-lightning if you buildup charge on yourself
and then touch another object, often a conductor like a doorknob or a car door, and
get shocked.
In either of these cases a change has taken place, which as we know requires
energy. There is an electric potential energy (EPE) much as there is a
gravitational potential energy (GPE). You must do work on a positively charged
particle in order to move it closer to another positively charged particle. This work
is equal to the EPE gained by the particle. A change in the amount of charge also
changes the EPE, much like changing the mass of an object changes its GPE. EPE
is not usually a useful quantity, but is incorporated in the idea of electric potential,
which is the EPE divided by the amount of charge present. Electric potential or the
potential difference are more commonly called voltage. The unit of voltage is the
volt (V) or 1 joule per coulomb. A lightning strike is on the order of 108 volts.
Potential difference specifically refers to the difference in electric potential
between two points. In order for electric charges to move, as in lightning, there
must be a potential difference. This is the electric version of water flowing
downhill. Water has to be uphill before it can flow downhill, a potential difference
due to location.
We have used lightning as a sudden and short-lived movement of electric charges,
electrons usually. One of the major scientific and technological advances of the
nineteenth century was the harnessing of the flow of electrons along conductors, or
an electric circuit. In a circuit, electric charge flows due to the movement of free
electrons in the conductors, usually metal wires. The flow of electric charge is
caused by a potential difference in the circuit and is called electric current. The
reason a circuit is different from an electric discharge is that there is an “electric
charge pump” in the circuit, a voltage source, such as a battery (which is actually a
series of electric cells). Think some more about the idea of water flowing.
Specifically, let’s look at a garden or park fountain. There is a reservoir of water
that is pumped up and out through the fountain. The water falls back to the
reservoir due to gravity and is again pumped up through the fountain. A battery
works much like the pump in the fountain in that it “pumps” electric charge back
up to a higher electric potential. The electric charge then “falls” through the circuit
to the bottom of the charge reservoir (all the materials that compose the circuit
provide the electrons) and is “pumped” back to a higher electric potential by the
battery. The rate of electric current is measured in amperes (A). One ampere is
defined as one coulomb of charge passing a point in one second. The current in a
circuit does not normally cause the circuit to be electrically charged. The electric
charge that flows is part of the circuit, which is almost always neutral in charge to
start with. Another interesting result of the electric charge being provided by all
conducting parts of the circuit is that once the circuit is completed all parts of the
circuit experience current at the same time. Unlike the flow a water, an electric
current produces an almost instantaneous electric field and current at all parts of
the circuit. Although the electric field nearly instantaneous, the individual electrons
have a speed of about 800,000 m/s but move randomly and only move along the
wire at a rate of about one meter every hour. This occurs in a circuit with a direct
current (DC), here the electric field lines are maintained in one direction in the
circuit. In a typical household circuit the current is an alternating current (AC),
here the direction of the electric field is changed 180 degrees at a regular rate,
much like a wave. In an AC circuit the electrons in the wires do not travel along the
wires at all, but simply sway back and forth with the electric field.
The simplest electric circuit is composed of a voltage source, conducting
material, and a resistor. A voltage source is often a battery, which is a series of
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electric cells. There are two types of electric cells, wet and dry. Both involve
chemical reactions between metal and a compound. A wet cell involves an aqueous
electrolyte, usually an acid, and metal plates. The chemical reaction between the
acid and the metal produces energy, which results in a potential difference between
the metal plates, terminals, of the cell. A dry cell works basically the same but with
a solid “paste” as opposed to a liquid. Everyday batteries are actually dry cells. In
order to have a battery, by definition, you must have multiple cells. A “AA” battery
is actually a “AA” cell. A resistor is just that, something that resists the electric
current. Much like a pipe to water, the thicker and shorter a wire is, the less
resistance it provides to the electric current. Resistance also varies with the
specific material and the temperature of the material; copper is less resistant than
steel and a higher temperature increases resistance. Resistance is measured in
ohms () and is related to current and voltage according to Ohm’s Law. Ohm's
Law states that the current in a circuit is directly proportional to the voltage
established across the circuit, and is inversely proportional to the resistance of the
circuit. As written in the equation to the right: I = current, V = voltage, and R =
resistance. Electrical devices such as light bulbs are resistors.
There are two basic types of circuits: series and parallel. In a series circuit there is
only one path for the current to flow. The amount of current is the same in all parts
of the circuit and the voltage is divided proportionally to the resistance of each part
of the circuit. A 3 Volt battery will provide a voltage around 3 Volts, and the
voltage drop across each resistor in the circuit will add up to that 3 Volts. A break
anywhere in the circuit will cause the current to stop flowing in the entire circuit.
A parallel circuit provides multiple pathways for the current to flow between the
same two points. As a result the voltage across each path is the same and is equal to
the voltage source. Each path behaves as a separate series circuit and the current in
each path is inversely proportional to the resistance of that path. As the number of
paths increase the total resistance decreases and the total circuit current increases.
A parallel circuit with three bulbs on three paths will drain a battery about three
times faster than the same three bulbs in a series circuit. If one of the paths is
broken (a bulb blows out) the current will remain in the other paths of the circuit.
Another important aspect of electrical components is their power, the rate at which
work is done. Electrical power is simply the product of electric current and voltage
within a circuit.
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