Drift

The movement of charged particles in a substance caused by the application of an electric field. If the number per unit volume of specific particles with charge e created at a certain location ( x,y,z ) in a substance (gas, liquid, solid) to which an electric field E is applied is n ( x,y,z ), then the density of charge is en( x,y,z ). Here, specific particles refer to positive ions with a positive charge of + e , negative ions with a negative charge of -e , or electrons produced by the ionization of atoms and molecules. When an electric field is applied, these charges move in the direction of the electric field according to whether they are positive or negative, and if the average speed is w , then

j ^± = ± n ( x , y , z ) ^± ･ e w ^±

where ± corresponds to the positive and negative charges. The movement of charged particles in a material is not related to the application of an electric field, and the resulting positive and negative currents are

∓ D ^± e grad n ( x , y , z ) ^±

Here, D is the diffusion coefficient. Therefore, for j ^± , this diffusion term should be added to the previous term,

j = j ⁺ + j ^-
Furthermore, when there are several types of particles with positive and negative charges, the currents for each type must be added together. Drift current is generally much larger than diffusion current. In a substance to which an electric field is applied under normal conditions, charged particles have various directions and move violently with a thermal velocity u , the average of which is several hundred times larger than w , and after colliding with an atom, they collide with another atom when they have traveled the mean free path, and are scattered in various completely unrelated directions. If the average of u is taken to be the diffusion coefficient, then according to the kinetic theory of gases,

D = (1/3)
The time between two successive collisions is on the order of /, and the distance S traveled during this time in the direction of the electric field E with an acceleration of eE / m ( m is the mass of the particle) can be found. Since such progress occurs / times per unit time, w is given by S /. For a gas, is inversely proportional to the pressure P , so if L is the mean free path at unit pressure, then

= L / P
Write

This μ is called the drift mobility. As an example, under the condition of E / P = 1, the electron in Ar has w = 0.61× ¹⁰⁴ ms ^-1 , for Ar ⁺ it is w = 10.3 ms ^-1 , and for Ar- ^it is w = 12.9 ms ^-1 . Comparing these drift velocities w , we can see that the value for electrons with small mass is several hundred times larger than that of heavier ions, and is roughly the same for positive and negative ions.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

電場の印加によって起こる物質中での荷電粒子の移動．電場Eをかけられた物質(気，液，固体)のなかの，ある場所(x,y,z)につくられた電荷eをもつ特定粒子の，単位体積当たりの数をn(x,y,z)とすると，電荷の密度はen(x,y,z)である．ここで，特定の粒子とは正電荷＋eの正イオン，負電荷－eの負イオン，または原子・分子のイオン化で生じた電子をさしている．これらの電荷は電場の印加によって電場方向に正負に応じた移動を起こし，その平均の速度をwとすると，

j ^± ＝ ±n(x,y,z)^±･ew^±

の電流を生じる．ただし，±は電荷の正負に対応する．物質中での荷電粒子の移動で電場の印加に関係のないものに拡散の現象があり，これによる正負の電流は，

∓D^±e grad n(x,y,z)^±

となる．ここで，Dは拡散係数である．したがって，j^± としては前記の項にこの拡散項を加えるべきであり，

j ＝ j ^＋ ＋ j ^－
である．また，正負それぞれの電荷の粒子がいく種類もあるときは，それぞれの種類ごとに合算しなければならない．ドリフト電流は拡散電流より一般にはるかに大きい．通常の条件で電場を印加された物質中では，荷電粒子はいろいろの方向をもち，その大きさの平均はwより数百倍大きい熱運動速度uをもってはげしく動きまわり，ある原子との衝突のあと平均自由行程程度動いたとき次の衝突を行い，まったく関連のないいろいろな方向に散乱されていく．uの平均をとすると，拡散係数は気体分子運動論により

D ＝ (1/3)
である．引き続く2回の衝突間の時間は/ の程度であり，この間に電場Eの方向に加速度eE/m(mは粒子の質量)で移動する距離Sが求められる．単位時間でのこのような進みは /回あるから，wはS/で与えられる．は気体の場合，圧力Pに反比例するから，Lを単位圧力における平均自由行程とすると

＝ L/P
と書け，

と書くことができる．このμをドリフト移動度という．一例をあげると，Ar中の電子はE/P ＝ 1の条件下で，w ＝ 0.61×10⁴ m s^－1，Ar^＋ はw ＝ 10.3 m s^－1，Ar^－ はw ＝ 12.9 m s^－1 である．これらのドリフト速度wを比べると，質量の小さい電子の値は重いイオンに比べて数百倍大きく，正負イオンではだいたい同じであることがわかる．

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