Electricals And Electronics Engineering (EEE): 02/01/09

Electric field

In physics, the space surrounding an electric charge or in the presence of a time-varying magnetic fieldhas a property called an electric field (that can also be equated to electric flux density). This electric field exerts a force on other electrically charged objects. The concept of an electric field was introduced by Michael Faraday.

The electric field is a vector field with SI units of newtons per coulomb (N C⁻¹) or, equivalently, volts permetre (V m⁻¹). The SI base units of the electric field are kg•m•s^-3•A^-1.

Electromagnetism

Electric current produces a magnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire.

Ohm's law

Ohm's law predicts the current in an (ideal) resistor (or other ohmic device) to be the applied voltage divided by resistance:

$I = \frac {V}{R}$

where

I is the current, measured in amperes

V is the potential difference measured in volts

R is the resistance measured in ohms

Drift velocity

The drift velocity is the average velocity that a particle, such as an electron, attains due to an electric field. In general, an electron will rattle around in a conductor at the Fermi velocity randomly. An applied electric field will give this random motion a small net velocity in one direction

In a semiconductor, the two main carrier scattering mechanisms are ionized impurity scattering and lattice scattering.

Because current is proportional to drift velocity, which is, in turn, proportional to the magnitude of an external electric field, Ohm's law can be explained in terms of drift velocity.

Drift velocity is expressed in the following equations:

$J_{\it drift} = \sigma \cdot v_{\it avg}$ , where $J d r i f t$ is the current density , $σ$ is charge density in units C/m³, and v_avg is the average velocity of the carriers(drift velocity);
$v_{\it avg} = \mu \cdot E$ , where μ is the electron mobility in m²/(V·s) and E is the electric field in V/m.

The drift speed of electric charges

The mobile charged particles within a conductor move constantly in random directions, like the particles of a gas. In order for there to be a net flow of charge, the particles must also move together with an average drift rate. Electrons are the charge carriers in metals and they follow an erratic path, bouncing from atom to atom, but generally drifting in the direction of the electric field. The speed at which they drift can be calculated from the equation:

$I=nAvQ \!\$