Textbook Readings Week 1 1 1 1 7 Action at a distance The worldwide view that the interaction of two material objects requires no mechanism other than the objects themselves and the empty space between them Total eld The total eld due to both the constant electric eld and the eld of the point charge Scalar Field A function that gives us a single value of some variable for every point in space Example temperature variation atmospheric pressure Vector Field A quantity that has both a magnitude and a direction in space Example uid ow velocity momentum acceleration force gravity Gravitational Force Electric elds Electrostatic force The force between electrically charged objects at rest The electrostatic force between charged objects falls off as the inverse square of their distance of separation and can either be attractive or repulsive Coulomb Force The Electric Field at P Magnetic Fields Magnetic Fields are another example of vector elds The magnetic eld of a bar magnet has lines traveling from the north pole to the south pole Grass Seeds and Iron Filings Representations If you scatter grass seeds in a strong electric eld they will orient themselves with the long axis of the seed parallel to the local eld direction Two types of electric charge positive and negative The smallest unit of free charge known in nature is the charge of an electron or proton which has the magnitude of 1 602 x 10 19 C The electric force exerted by q1 on q2 is given by Coulomb s Law The Principle of Superposition When more than two charges are present the net force on any one charge is simply the vector sum of the forces exerted on it by the other charges Electric Field Lines A graphical representation of the electric eld in space The direction of eld lines is radially outward for a positive charge and radially inward for a negative charge The properties of electric eld lines The direction of the electric eld vector E at a point is tangent to the eld lines The eld lines must begin on positive charges or at in nity and then terminate on negative charges or in nity The number of lines that originate from a positive charge or terminate on a negative charge must be proportional to the magnitude of charge No two eld lines can cross each other Force on a Charged Particle in an Electric Field Consider a charge q moving between two parallel plates of opposite charges the charge will experience a either upward or downward Coulomb force Electric Dipole 2 1 2 7 An electric dipole consists of two equal but opposite charges q and q separated by a distance 2a By de nition the dipole moment vector p points from q to q and has a magnitude p 2qa where q 0
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