http://www.ck12.org Chapter 15. Electrostatics
- Electric charge is a conserved quantity that comes in two kinds: positive and negative.
- Conductors are materials in which electrons can move freely, while insulators are materials in which electrons
cannot move freely. - Coulomb’s law states that the force between two charges is directly proportional to the product of the charges
and inversely proportional to the square of the distance between the chargeF=kq^1 r 2 q^2 , wherekis a constant of
proportionality known as Coulomb’s constant. - The electric field~E, at a point in space, is the quotient~E=~Fq, whereqis a small positive test charge and~Fis
the force experienced by that small positive test charge at the point in space whereEis measured. - The magnitude of the electric fieldEdue to a point chargeqat a distancerfromqisE=krq 2 , wherekis a
constant of proportionality known as Coulomb’s constant. - The electric field between two parallel-plate conductors is considered uniform far away from the plate edges
if the size of the plates is large compared to their separation distance. - The potential energy of a chargeqat a point between two parallel-plate conductors isPE=qEx, a reference
point must be given such asPE=0 atx=0. - A point chargeqhas electric potential energyPExand electric potentialVxat pointx. Thus,PEx=qVx
- The word voltage is used when we mean potential difference.
- It is common to writeV=Ed, whereVis understood to mean the voltage (or potential difference) between
the plates of a parallel-plate conductor anddis the distance between the plates. - The work done by the electric field in moving a charge between two parallel plate conductors isWf ield=
−q∆V. The work done by an external force isWexternal f orce=q∆V. - Voltage can be thought of as the work per unit chargeV=Wq; that is, how much work is required per unit
charge to move a charged particle in an electric field.