Drift velocity

In <a href="/facts/Physics/a7aH2y08">physics</a>, drift velocity is the <a href="/facts/Average_velocity/Q3o1LVDe">average velocity</a> attained by charged particles, such as <a href="/facts/Electron/L0KBo5cW">electrons</a>, in a material due to an <a href="/facts/Electric_field/tt4MooBs">electric field</a>. In general, an electron in a <a href="/facts/Electrical_conductor/BSuyupLP">conductor</a> will propagate randomly at the <a href="/facts/Fermi_velocity/S4Q8ShGR">Fermi velocity</a>, resulting in an average velocity of zero. Applying an electric field adds to this random motion a small net flow in one direction; this is the drift.

Drift velocity is proportional to <a href="/facts/Current_(electricity)/Dc1xdnUX">current</a>. In a <a href="/facts/Resistance_(electricity)/hxSt0UAj">resistive</a> material, it is also proportional to the magnitude of an external electric field. Thus <a href="/facts/Ohm%27s_law/PjfEyTgI">Ohm's law</a> can be explained in terms of drift velocity. The law's most elementary expression is:

u
        =
        μ
        E
        ,
      
    
    {\displaystyle u=\mu E,}

where u is drift velocity, μ is the material's <a href="/facts/Electron_mobility/aN3bco00">electron mobility</a>, and E is the <a href="/facts/Electric_field/tt4MooBs">electric field</a>. In the <a href="/facts/MKS_system_of_units/fMpV1vDw">MKS system</a>, drift velocity has units of m/s, electron mobility, m2/(<a href="/facts/Volt/KF18MXP2">V</a>·s), and electric field, V/m.
When a potential difference is applied across a conductor, free electrons gain velocity in the direction, opposite to the electric field between successive collisions (and lose velocity when traveling in the direction of the field), thus acquiring a velocity component in that direction in addition to its random thermal velocity. As a result, there is a definite small drift velocity of electrons, which is superimposed on the random motion of free electrons. Due to this drift velocity, there is a net flow of electrons opposite to the direction of the field. The drift speed of electrons is generally in the order of 10−3 meters per second whereas the thermal speed is on the order of 106 meters per second.

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Drift velocity