The townsend (symbol Td) is a physical unit of the reduced electric field ( ratio ''E''/''N''), where

E

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

and

N

is concentration of neutral particles. It is named after John Sealy Townsend, who conducted early research into gas ionisation.

Definition

It is defined by the relation

1\ \text = 10^\ \text \cdot \text^2 = 10^\ \text \cdot \text^2.

For example, an electric field of

E = 2.5 \cdot 10^4\ \text/\text

in a medium with the density of an ideal gas at 1 atm and 0 °C, the

Loschmidt constant The Loschmidt constant or Loschmidt's number (symbol: ''n''0) is the number of particles (atoms or molecules) of an ideal gas per volume (the number density), and usually quoted at standard temperature and pressure. The 2018 CODATA recommended val ...

n_0 = 2.6867811 \cdot 10^\ \text^

gives

E/n_0 \approx 10^\ \text \cdot \text^2,

which corresponds to 1 Td.

Uses

This unit is important in gas-discharge physics, where it serves as scaling parameter because the mean energy of electrons (and therefore many other properties of discharge) is typically a function of

E/N

over broad range of

E

and

N

. The concentration

N

, which is in

ideal gas An ideal gas is a theoretical gas composed of many randomly moving point particles that are not subject to interparticle interactions. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is ...

simply related to pressure and temperature, controls the

mean free path In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...

and

collision frequency Collision frequency describes the rate of collisions between two atomic or molecular species in a given volume, per unit time. In an ideal gas, assuming that the species behave like hard spheres, the collision frequency between entities of speci ...

. The electric field

E

governs the energy gained between two successive collisions. Reduced electric field being a scaling factor effectively means that increasing the electric field intensity ''E'' by some factor ''q'' has the same consequences as lowering gas density ''N'' by factor ''q''.

References

* Electrical breakdown {{physics-stub

Definition

Uses

See also

References