Electrostatic field of the earth

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The electrostatic field of the earth , also known as electrostatic earth field , electric field of the earth or electric earth field , results from the fact that the earth's surface has a negative electrical excess charge of a few hundred kilocoulombs , while the ionosphere is equally positively charged. Put simply, the electrically conductive earth's surface and the ionosphere represent the plates of a large capacitor with the electrically non-conductive earth's atmosphere as a dielectric in between.

Currents within the earth's field

Since the earth's atmosphere is not a perfect insulator (if only because of the ions formed by ionizing radiation), the charge separation leads to a so-called "fair weather current " with a current density of the order of pA / m ² , which would make the earth's field disappear within a very short time. Different mechanisms are discussed how the observed charge separation is delivered. Certainly a significant contribution is made by lightning between thundercloud and earth, which transport negative charge to the earth's surface. Overall, the earth's electric field remains stationary; on average, the field strength in the air (a few meters above the ground) is 130 V / m . ${\ displaystyle {\ vec {E}}}$

Distribution of electrical charge

Around 1000 ions per cubic centimeter can be found near the surface of the earth , with one cubic centimeter containing 3 · 10 ¹⁹molecules at normal pressure . The atmosphere there is therefore only weakly ionized, it contains both positive and negative ions . However, the number of positive ions predominates. The ion density increases with altitude and has its maximum in the ionosphere . A considerable part of the gas molecules there is ionized by UV radiation from the sun.

Due to the excess of positive charges in the lower layers of the atmosphere, the electric field of the earth is partially shielded, so that the field strength decreases rapidly with the height above the earth, much faster than in a context. At the level of the ionosphere and magnetosphere , the field strength has already dropped to a few volts per kilometer. ${\ displaystyle {\ frac {1} {r ^ {2}}}}$

Determining the earth charge

From the field strength 130 V / m and the context

{\ displaystyle Q = - \ varepsilon _ {0} \ varepsilon _ {r} \, E \, A}

the excess charge of the earth can be estimated. Here Q is the electrical charge of the earth, ε _r is the dielectric constant and thus approximately the permittivity of air , E is the amount of the field strength measured and averaged in the air and A is the earth's surface with 5.1 · 10 ¹⁴ m ² . The result is that the time-averaged charge of the earth is Q = −0.6 megacoulombs .

Acceleration of Ions

Ions are accelerated by the earth's field - in addition to being accelerated by gravity. The acceleration is obtained from the equation

{\ displaystyle {\ vec {a}} = {\ frac {{\ vec {E}} \, Q} {m}}}

where the acceleration of the ion due to that of the earth's field and m is the mass of the ion. ${\ displaystyle {\ vec {a}}}$

For example, the acceleration of a single positively charged oxygen atom , which occurs due to the solar winds in the ionosphere, is about 80 million times as great as the local acceleration due to gravity due to the earth's electrostatic field . However, the ions accelerated in this way are slowed down by the collision with other atoms in the atmosphere, which is why the ions accelerated in this way usually have speeds of 100 m / s (360 km / h).

Measurement

With a cloudless sky in flat terrain, electrical field strengths between 100 V / m and 300 V / m can be determined from top to bottom with potential probes or with plate capacitors or so-called rotation voltmeters that can be rotated around the horizontal axis . During thunderstorms, however , the charge separation in the clouds also creates significantly higher field strengths of 25 to 35 kV / m, which can be observed on the ground.

literature

Adalbert Prechtl : Lectures on the basics of electrical engineering, Volume 1 . Springer Verlag, Vienna 1999, ISBN 978-3-211-82553-2 .

Individual evidence

^ ^A ^b ^c Wolfgang Demtröder: Experimentalphysik 2: Electricity and optics . Springer, 2008, ISBN 978-3-540-68210-3 ( limited preview in Google book search).
^ Richard Lenk, Walter Gellert (Ed.): Brockhaus ABC Physik, Volume 1, FA Brockhaus, 1972, p. 347