In physics, Coulomb's Law is an inverse-square law indicating the magnitude and direction of electrical force that one stationary, electrically charged substance of small volume exerts on another. When one is interested only in the magnitude of the force (and not in its direction), it may be easiest to consider a simplified, scalar version of the Law
<math>
F = \frac{\left|q_1 q_2\right|}{4 \pi \epsilon_0 r^2}
</math>
where q1 is the charge on one substance, q2 is the charge on the other, r is the distance between them, and ε0 is a universal constant, the permittivity of vacuum. (See physical constants for more information. Note that 1/(μ0ε0) = c2
where the vector r connects the two substances, and the other symbols are as before.
(The r vector in the numerator indicates that the force should be along the vector connecting the two substances. |r| has been raised to the third power instead of the second in the denominator in order to normalize the length of the r in the numerator to 1.)
In either formulation, Coulomb's Law is fully accurate only when the substances are static (i.e. stationary), and remain approximately correct only for slow movement. When movement takes places, magnetic fields are produced that alter the force on the two substances. Especially when rapid movement takes place, the electric field will also undergo a transformation described by Einstein's theory of relativity.
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As the train neared Beldingsville, Pollyanna watched her aunt
restless, more and more gloomy; and Pollyanna was fearful of the time
that she would not have believed it possible that any one could have
were lusterless, her cheeks pallid and shrunken, and her forehead
corners, and her hair was combed tightly back in the unbecoming
before. All the softness and sweetness that seemed to have come to her
the old hardness and sourness that had been hers when she was Miss
aunt might have read her thoughts.
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"Well, I wish you'd get out my black veil. We're nearly there."
"But it's so hot and thick, auntie!"
"Pollyanna, I asked for that black veil. If you'd please learn to do
for me. I want that veil. Do you suppose I'm going to give all
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