Electric field strength of a point charge:
Electric field is a vector quantity; it has both magnitude and direction. The resultant electric field due to several point charges can be determined using the same method as was used in Coulomb's Law problems. Calculate the strength of the electric field due to each point charge at point P. Determine the direction of the electric field by determing the direction that a test charge placed at point P would take. Use vectors to determine the magnitude and direction of the resultant electric field at point P.
Two points are said to differ in electric potential if work is done to move a charge from one point to another point in an electric field.
The electric potential at a distance r from a single point charge can be derived from the expression for electric field due to a point charge. The expression for absolute potential:
Electric potential is a scalar term. When finding the electric potential due to a collection of point charges, you need only add the potentials together with no concern for direction. You must, however, include the signs of the charges when calculating absolute potential.
In a conductor, charges move until all parts of a conductor are at the same potential. If a large and a small sphere have the same total charge, the large sphere will have a lower potential. If a large and a small sphere have the same potential, the large sphere will have the greater charge.
Electrostatic charges are only found on the outside of conductors.
When a DC voltage source is connected across an uncharged capacitor, the rate at which the capacitor charges up decreases as time passes. At first, the capacitor is easy to charge because there is little charge on the plates. But, as the charge accumulates, more and more work is needed to move additional charges on the plates because the plates already have charge of the same sign on them. As a result, the capacitor charges exponentially, quickly at the beginning and more slowly as the capacitor becomes fully charged. At any time, the charge on the plates is given by:
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