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Problem no. 3 "Plasma"

Measurements had to be done first. As an example for the results of our measurements we want to show you two diagrams. The measurements were quite difficult, because there were lots of fluctuations. So the results are only average values.
This diagram shows the dependence of the current from the horizontal distance of the electrodes.

experimental data: sharp electrodes on the same level in the flame voltage: 150 V, DC

This diagram shows another example of our measurements.

experimental data: sharp electrodes above each other intersection about 4 mm voltage: 150 V, DC

After our measurements we drew the following conclusions:

• This is a chaotic system with hardly reproducible measurement results.
• The problem can't be completely solved, only the influence of various parameters can be examined, such as:
  • the electric field (shape and arrangement of the electrodes)
  • the temperature
  • the gas flow and the composition of the gas

Now we want to show you, what we found out about the influence of these parameters.
Let's start with the gas flow:
For our research on gas flow, we "recycled" a theory and measurement results from the previous year. For those who can't remember: In the 12^th IYPT the problem no. 13 "Gas flow" concerned the gas flow in the flame of a candle. So we now show you what we found out one year ago:

This sketch shows you, how we measured the gas flow:

left mass: 2g; right mass: 1g; connected with a circular Al - plate (in the flame: Fe - plate); Temperature measurement with Pt-100 or thermocouple

The streaming force F_Str is an upward force on the right mass which produces an additional downward force on the electronic scales. This causes the scales to show a higher weight, so F_Str can be calculated using this "mass difference".
With the streaming force we are able to calculate the velocity of the gas flow using this formula:

Cw = 1,1 (for circular plates) A = r2.p (area of the circular plate) FStr ... has been measured rho ... density of air: calculated (see following)

Calculation of air density: using the ideal gas equation

p pressure [Pa] V volume [m3] n number of moles R gas constant (= 8,31) [ J / (K . mol) ] T temperature [K] M molecular weight rho density [kg/m3]

calculation of the molecular weight M:
values from literature: air at 0°C, 1 bar: r = 1,293 [kg/m³]

This diagram finally shows the result of the gas flow measurements: Velocity of the flow versus the vertical distance from the flame:

Furthermore we considered some theory about plasma. Useful statements can only be given about a balance plasma. For a balance plasma, 3 distributions have to be fulfilled:

• The Boltzmann-,
• the Maxwell-,
• and the Saha-distribution.

N number of molecules k Boltzmann constant (approx. 1,38.10-23 [J/K]) mM mass of one molecule T Temperature [K]

We wanted to find out, how many molecules are ionised. In order to calculate the minimum velocity for ionisation through hits, we set the ionisation energy equal to the kinetic energy of a molecule.
After having calculated the minimum velocity, we used the Maxwell distribution to calculate the fraction x of molecules which are faster than the minimum speed.
It is possible to calculate this for each kind of molecules in the plasma. Here are two results for example:
results: e.g. for O₂: x = approx. 3.10^-64; for N₂: x = approx. 9.10^-91
As you can see the fraction of ionised molecules is so small that it is possible to neglect it.

The Saha distribution:

The Saha equation indicates the ionisation balance between the electrons and the ions versus neutral particles:

me mass of the electron h Planck constant k Boltzmann constant T temperature Ei ionisation energy

Also this equation proofs that there are actually no ions in the plasma.

Influence of the electric field:

The electric field causes electrons to drift through the plasma with a current density j:

Moreover we used some formulas which we found in literature.

Using the shown formula, we can proof that the conductivity is proportional to one divided by squareroot of the mass.

By taking an element with an average mass from the periodic system of the elements we can show that the share of ions of the current is smaller than 0,4 %.
So we can restrict ourselves to the consideration of drifting electrons only.

The Richardson-Dushman equation indicates, how many electrons are emitted by the electrodes.

WA work function k Boltzmann constant T Temperature h Planck constant m rest mass of the electron

The Richardson-Dushman equation shows only the influence of the temperature and does not consider the point effect!

The electric field also causes the electrodes to emit electrons, as it is indicated by the Fowler-Nordheim equation.

E electric field strength F force Q charge ec elementary charge j current density

It shows that the current density is proportional to the electric field strength squared times Euler's number to the minus one divided by the field strength.
To determine the electric field strength we made a computer simulation. The following plots shall give you an idea how the field depends on the shape and arrangement of the electrodes.

Click on a thumbnail to enlarge...

	Field between a point electrode and a spherical electrode.
	Field on the surface of a spherical electrode.
	Potential distribution on the surface of two needel electrodes.
	Field between two spherical electrodes (quarter of the symmetry)

In conclusion the following things have to be said: The connection between our experiments and our theory is not possible. The current is determined by so many parameters which correlate so much that it is not possible to measure the dependence of the current from one single parameter. Therefore there can be no real connection between theory and experiments. Moreover, the phenomenon is much too complex so that there can be no final solution formula which describes the behaviour of the current.

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