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Because. Here's a word best avoided in physics. Whenever it appears one can be almost certain
that it's a filler word in a sentence which says nothing worth saying, or a word used when one
can't think of a good or specific reason. While the use of the word because as a link in a chain
of logical steps is benign, one should still replace it with words more specifically indicative of the
type of link which is meant. See: why.
Illustrative fable: The seeker after truth sought wisdom from a Guru who lived as a hermit on top
of a Himalayan mountain. After a long and arduous climb to the mountain-top the seeker was
granted an audience. Sitting at the feet of the great Guru, the seeker humbly said: 'Please,
answer for me the eternal question: Why?' The Guru raised his eyes to the sky, meditated for a
bit, then looked the seeker straight in the eye and answered, with an air of sagacious profundity,
'Because!'
Capacitance. The capacitance of a capacitor is measured by this procedure: Put equal and
opposite charges on its plates and then measure the potential between the plates. Then C =
|Q/V|, where Q is the charge on one of the plates.
Capacitors for use in circuits consist of two conductors (plates). We speak of a capacitor as
'charged' when it has charge Q on one plate, and -Q on the other. Of course the net charge of the
entire object is zero; that is, the charged capacitor hasn't had net charge added to it, but has
undergone an internal separation of charge. Unfortunately this process is usually called
charging the capacitor, which is misleading because it suggests adding charge to the
capacitor. In fact, this process usually consists of moving charge from one plate to the other. The
capacity of a single object, say an isolated sphere, is determined by considering the other
plate to be an infinite sphere surrounding it. The object is given charge, by moving charge from
the infinite sphere, which acts as an infinite charge reservoir ('ground'). The potential of the
object is the potential between the object and the infinite sphere.
Capacitance depends only on the geometry of the capacitor's physical structure and the dielectric
constant of the material medium in which the capacitor's electric field exists. The size of the
capacitor's capacitance is the same whatever the charge and potential (assuming the dielectric
constant doesn't change). This is true even if the charge on both plates is reduced to zero, and
therefore the capacitor's potential is zero. If a capacitor with charge on its plates has a
capacitance of, say, 2 microfarad, then its capacitance is also 2 microfarad when the plates have
no charge. This should remind us that C = |Q/V| is not by itself the definition of capacitance,
but merely a formula which allows us to relate the capacitance to the charge and potential when
the capacitor plates have equal and opposite charge on them.
A common misunderstanding about electrical capacitance is to assume that capacitance
represents the maximum amount of charge a capacitor can store. That is misleading because
capacitors don't store charge (their total charge being zero) but their plates have equal and
opposite charge. It is wrong because the maximum charge one may put on a capacitor plate is
determined by the potential at which dielectric breakdown occurs. Compare: capacity.
We probably should avoid the phrase 'charged capacitor' or 'charging a capacitor'. Some have
suggested the alternative expression 'energizing a capacitor' because the process is one of giving
the capacitor electrical potential energy by rearranging charges in it.
Capacity. This word is used in names of quantities which express the relative amount of some
quantity with respect to a another quantity upon which it depends. For example, heat capacity is
dU/dT, where U is the internal energy and T is the temperature. Electrical capacity, or
capacitance is another example: C = |dQ/dV|, where Q is the magnitude of charge on each
capacitor plate and V is the potential diference between the plates.
Centrifugal force. When a non-inertial rotating coordinate system is used to analyze motion,
Newton's law F = ma is not correct unless one adds to the real forces a fictitious force called the
centrifugal force. The centrifugal force required in the non-inertial system is equal and opposite to
the centripetal force calculated in the inertial system. Since the centrifugal and centripetal forces
are concepts used in two different formulations of the problem, they can not in any sense be
considered a pair of reaction forces. Also, they act on the same body, not different bodies. See:
centripetal force, action, and inertial systems.
Centripetal force. The centripetal force is the radial component of the net force acting on a body
when the problem is analyzed in an inertial system. The force is inward toward the instantaneous
center of curvature of the path of the body. The size of the force is mv2/r, where r is the
instantaneous radius of curvature. See: centrifugal force. [ Pobierz całość w formacie PDF ]