Vertical Flame Diversion

Electricity and Magnetism Level 3

In normal conditions, when a flame is lit, it points upwards.

What would happen if we place the flame in a uniform electric field? We can create a uniform electric field using two parallel plates, and applying voltage across them.

A. The flame is attracted to the positive plate.

B. The flame is attracted to the negative plate.

C. Nothing happens.

Note Assume an ordinary gas, wood, or candle flame

A C B

1 solution

Michael Mendrin
Jan 27, 2017

This really depends on the composition of ions in the flame plasma, and how the flame looks like depends on what's being burned. For the most part, ordinary gas flames have the bulk of ionic matter as positively charged ions, i.e., the negative ions are mostly electrons ${e}^{-}$ , while the positive ions are mostly the heavier reactants such as $\ce{C{O}_{2}}$ stripped of electrons. For such flames, the negative ions drift towards the positive plate, but the overwhelming mass portion of the flame drifts towards the negative plate, and this is the part that you "see" when radiant.

Not exactly a trivial question with a clear and unique answer. A typical ordinary flame is only barely a plasma, i.e., only a small percentage of the reactant molecules are actually ionized, but it's enough to pull the flame to one side.

This phenomenon can be used in fighting fires .

Brian Charlesworth - 4 years, 4 months ago

Wow, that sure sounds like a Nikola Telsa project, fighting forest fires with giant electrical bolts? It takes a lot of electrical horsepower to move around big flames. But I know that controlling the shape and path of combustion in machinery such as power plants and other industrial processes has promise for higher efficiency or finesse.

Michael Mendrin - 4 years, 4 months ago

Yeah, that paper makes for a good read; there's a lot of potential here. Combining this with sound waves would allow for even greater levels of effectiveness.

Brian Charlesworth - 4 years, 4 months ago

What if a blue oxygen rich flame is used ? Will that change the answer ?

Aniruddha Bagchi - 4 years, 4 months ago

In fact it does make a difference whether the flame is lean or rich, i.e., oxygen poor or rich. Since ionized oxygen is negatively charged, an oxygen rich flame will tend to pull the other way, towards the positive plate.

Gaseous oxygen ${O}_{2}$ in high temperatures will simply "crack up" into ionized oxygen $2{O}^{-}$ . But, again, only a tiny fraction of free gaseous oxygen will do so in an ordinary flame since the temperature is actually very low as compared to "true plasma", which typically have temperatures from millions to billions of degrees. See where "flames" fall into this picture.

Michael Mendrin - 4 years, 4 months ago

Thanks for replying.

Aniruddha Bagchi - 4 years, 4 months ago

What about a red calcium ion flame ? Is there any generalised answer relating to any common property of flame types that makes them behave in such a way ?

Aniruddha Bagchi - 4 years, 4 months ago

Since ionized calcium is positively charged, which is also typical of ionized heavy reactants in ordinary gas flames, the flame will pull towards the negative plate. But see below about oxygen.

Michael Mendrin - 4 years, 4 months ago

This definitely shows that it is not at all unique.

Kartik Sharma - 4 years, 3 months ago

Vertical Flame Diversion

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