Can flame be “Plasma”?

Last week, something slipped outta my mouth, the so-called “Plasma”. So today, we’re addressing Plasmas (It should be noted that there’s a wonderful list of different kinds of plasmas. For now, we’re dealing with the common neutral plasmas). What everyone knows is that gaseous atoms, at extreme temperatures get ionized and lose electrons, thereby leaving a collection of highly excited ions and free electrons, leading to the fourth state of matter called Plasma. That’s a terrible name, as there also exists a Plasma in biology, the watery fluid in blood having which has no cells, but serves as a medium where blood cells are suspended. Science stuff are named in a way to get confused. By the way, Plasmas are mostly neutral. Getting ionized doesn’t mean that they have excessive charged particles…

Air as a plasma…

Well, plasmas are almost an everyday experience. While man-made stuff like my $11 Plasma globe also exist, you can find a lot of natural phenomena that contain plasma. A proper definition of plasma require the number density (i.e) the number of free electrons per unit volume, which determines the ionization state of the substance. So, if our definition neglects this constraint, we can freely consider a lot of other phenomena as plasma. Seemingly weird statement that air can be a plasma (unless you’re a physics student), I can sense your yelling, “How could this be?”. In order to arbitrate this argument and smoothly bring this down to a peaceful consensus, we should digest the fact that if any compound can exhibit solid, liquid and gaseous phases, then they’ve got their sole right to peek into plasma too. The answer is of course, air can be plasma. We can notice this property in all kinds of electric discharges, which are very often. But, there should be a threshold limit to achieve this highly excited state. It’s often a misconception that heat is always necessary for this excitation. No, it’s not. Sufficient voltage is far enough. Else, we would’ve been smoked out if we switched on our TV.

Usually, we use Saha equation that relates the ionization state of the “plasmic” substance to its temperature. But, it’s a deeper chasm, which is not necessary for us now…

Sparks & Arcs…

So, let’s pay a visit to the breakdown voltage. Every single dielectric material (such as air) has a threshold limit to its non-conductivity, beyond which the atoms of the material get ionized and start to conduct electricity (i.e) an electric discharge, an arc, or whatever you’d like to call “occurs”. Nikola Tesla is well known for all his tricks with this wonderful phenomena (you do remember all his illusional stuff in “Prestige“, don’t you?) So, this breakdown voltage can be determined using the Paschen’s curve, which gives the relation between the pressure and breakdown voltage. From mosquito rackets, Tesla coils to the marvelous lightning, electric discharges are wholly based on this principle. It’s very amusing that the mosquito racket having a series of voltage multiplier circuits, that enhances the potential difference from a bare 230 V to a maximum of 30,000 V  DC can concentrate it into the grids of separation around 5 mm.

(Duh, I was just speaking about the breakdown voltage for air, which is around 30 kV/m).

Nothing “so sexy” as a candle flame…

Electric FlameWe’ve already found the answer to the question. Flames (not all) can be considered as plasmas. Because, all flames do have ions to some extent. If we managed to stick to the definition of neglecting the number density, the candle flame can be a good candidate for plasma. Because, we can observe a candle flame deflect under the influence of an applied electric field (low voltages). If it’s high enough and more importantly, confined to a small region such that it helps the breakdown, candle is always a perfect example for plasma (as long as you don’t ponder around by comparing it with the sun). As you can see in the Veritasium video, Derek manages to make candle flame conduct electricity.

Dumb as a grape…

Grape PlasmaThere are a lot of debates going allover the internet that grape (when cut half and put in an oven) produces plasma. An year ago, we had a discussion on the topic (Well, he’s a scientist). According to him, the grape’s skin is at the right size to focus the microwave right at the center (just like a parabolic dish or antenna), thereby causing it to vaporize, producing flame. So, it isn’t plasma.

Anyways, for our concern, let’s ignore this issue today, as the grape-plasma  hasn’t been clarified and settled over yet…

For now, let’s get on with this, most flames can be plasma…

Inspired by this answer at Physics Stack Exchange.


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