Current and Resistance

This lecture is based on HRW, Sections 27.1-3, 27.6

Current is simply a net flow of charged particles in some direction
Current is defined as the number of coulombs per second (amperes) through some surface or area. The direction of current is defined to be in the direction of flow of positive charge, even though in most cases negative charges (electrons) are actually moving.
Under the influence of electric fields within a conductor, electrons move slowly through the material; drift speeds of much less than one mile per hour are common in metals.
Materials in which the current is linearly proportional to the electric flux are called ohmic.
For such materials, the constant relating voltage to current may be expressed either as conductivity, or resistivity.
Given a wire of specified length and cross-section, one can define resistance as the ratio of current to voltage within that wire.

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