Image description: In a plasma lamp, the image of a field appears. The electric arcs we observe somewhat follow the lines of force of this field, creating the illusion of a filament structure. The electric arcs are a manifestation of this field, but they are not the field itself. What we see are the effects of the electric field on the ionized gas.
The plasma lamp, readily available on the internet, is a device that creates spectacular visual effects. But beyond the light spectacle, we can glimpse, through this object, the fascinating concept of "field", making the plasma lamp a true little physics laboratory.
Plasma is the fourth state of matter where atoms are ionized, meaning that electrons are separated from atomic nuclei, thus creating ions and free electrons to move. This generally occurs at high temperatures or in the presence of an intense electric field.
In a plasma lamp, a noble gas (such as neon, xenon, argon, or krypton) is typically used. Although noble gases strongly resist forming chemical bonds (because the outermost electron shell is saturated), they are chosen for their ability to easily ionize under the influence of an electric field.
Inside the plasma lamp, there is therefore a rare gas, at low pressure, subjected to a high voltage generated by a central electrode. The voltage applied to the central electrode typically ranges from 2 kV to 20 kV, depending on the design and size of the lamp. This high voltage ionizes the gas atoms and influences the behavior of the plasma by modifying its electrical, thermal, and dynamic properties, particularly intensity and color. The shape of the glass lamp is usually spherical to allow for uniform distribution of the plasma.
When voltage is applied to the central electrode, an electric field of several kilovolts is created between this electrode and the walls of the glass sphere. The intense electric field ionizes the gas atoms, creating free ions and electrons. The electrons are then attracted to the central electrode, creating a visible flow of electrons through the luminous filaments. The ionized areas create conductive paths for electric current, allowing the plasma filaments to form and move through the gas. These filaments often appear as flashes or arcs of plasma that move outward from the central electrode.
The colors of plasma filaments depend on the type of noble gas used.
Neon produces a red-orange light, argon produces a violet light, krypton produces a blue light, while xenon produces a blue-violet light.
Note: The color of the emitted light depends on the energy difference between the electronic energy levels of the atom. When an electron "jumps" from a higher energy level to a lower one, it emits a light photon whose energy exactly corresponds to the energy difference between the two levels. This energy translates into a specific color.
The image offered by a plasma lamp, with its luminous filaments that seem to dance in all directions, evokes the idea of a field, as if the filaments were following lines of force.
The electric field is a general concept that describes the influence of charges on the surrounding space. It can be represented by field lines that indicate the direction of the force that would be exerted by a positive charge placed at a given point. These field lines are oriented radially from the central electrode.
The central electrode creates an intense electric field that extends throughout the volume of the lamp. The plasma, being an excellent conductor of electricity, allows charged particles to move freely.
However, electric fields overlap. This means that the total electric field at a point is the vector sum of the electric fields created by all the present charges. The superposition of electric fields, the instability of the plasma, and the interactions between charged particles explain the non-linearity of the filaments and their irregular shapes.
The image we perceive from the lamp is dynamic because the charges are constantly in motion, following fluctuations in the electric field. Thus, the luminous filaments follow the electric field lines, providing a direct visualization of the field configuration.
Each point in space inside the lamp is subjected to an electric force that has a precise direction. This direction is tangent to the field line passing through that point, which can be interpreted as a vector. However, the electric field is a vector field, meaning that a physical quantity (a vector) is associated with each point in space.
This visualization is a simplification of reality because plasma is a complex medium where many physical phenomena interact. Nevertheless, the plasma lamp offers a simple and elegant way to represent the concept of vector field.
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