Current
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Table of Contents
#UCLA #Y1Q3 #Physics1B
Current
Key Definitions
Electric Current - flow of through a medium
Volume charge density - current per unit area
Current
Defined as
\(I=\frac{dQ}{dt}\approx\frac{\Delta Q}{\Delta t} [A]\)
The unit of current is the Ampere:
\(1 A = 1\frac C s\)
Direction of Current
Current is defined by the flow of positive charge:
\(I = \frac{d(+q)}{dt}\)
However, in a conducting wire, the negatively charged electrons are moving.
Thus, current flowing through a wire actually means the negative flow of electrons through a wire:
Drift Velocity
Free moving electrons collide with other electrons and atoms. The average velocity of free charges is called the drift velocity, $v_d$, and it is the direction opposite to the electric field for the electrons.
Collisions transfer energy to the conductor, thus requiring a constant supply of energy to maintain a steady current:
\(I=\frac{dQ}{dt}=n|q|v_dA\)
Mechanics of Current
On the subatomic level, electrons bounce around within a conducting wire at the order of $10^6 m/s$, but the thermal motion of electrons is random and creates no net current.
To produce non-trivial current through a wire, we need to apply an which will exert a net force on the charges by
The average velocity of the charges is the drift velocity and is about $10^{-4}m/s$
Current Density
Volume charge density - current per unit area:
\(J=\frac{dI}{dA}\)
Alternatively, given drift velocity: