Upward motion of electrical arc

[zumulko]

Can anyone please explain or suggest why the arc from video (link below) rises up?

http://blog.makezine.com/archive/2010/12/now_thats_a_switch.html


zumulko.

 

[torquil]

First a path of ionized air is created, which can pass a lot of electrical current. The heat generated by this current creates the light that you see. The heat also warms up the surrounding air, which starts to rise due to its lower density. The ionized path follows it upwards.

If hot air were denser than "normal", the spark would fall downward instead.

 

[K^2]

In micro-gravity, the arc still expands, so it's not just due to the rise of hot air. The second reason is the magnetic field created by the effective current loop pushing the loop to expand. The two effects work together to make the arc look like this.

 

[zumulko]

Thank you for your quick replies.

K^2,

how does exactly the magnetic field causes the upward motion?

Is it about the interaction of circular magnetic field created by and around arc with the Earth magnetic field?

 

[K^2]

No, it's just that the loop of wire carrying a current has an outward force trying to expand the loop. Has nothing to do with Earth's field.

The loop of wire produces a magnetic field B, which is perpendicular to the wire. A current perpendicular to magnetic field experiences a Lorentz force. If you carefully follow the right-hand rules, you'll see that on the loop of wire, the Lorentz force is such that tit tries to expand the loop.

 

[zumulko]

K^2,

let me analyze it by assuming the straight line current flow due to the arc.

The current produces loops of magnetic field which lie in the plane perpendicular to the direction of current. The magnetic field vectors are tangent to the loops, never at right angle to the direction of current. For the Lorentz force to be exerted on the wire the magnetic field vector must have a component which is at right angle to the current's direction. Since I see no such component I can't really see the effect of pushing the arc upwards by itself generated magnetic field.

You wrote also that "the Lorentz force is such that tit tries to expand the loop". What you mean by expansion? That the arc's cross section area becomes larger?

 

[K^2]

You cannot analyze this on a straight line flow, since the whole effect is due to a loop.

 

[zumulko]

K^2,

thank you. Although I still can't see the magnetic lift I don't want to torture you anymore with my questions.
Last thing, could you give me some reference so that I can read about details of the effect?

 

[kcdodd]

This is actually what is called a kink instability in plasmas. A current in a perfectly straight line will not experience any forces from itself. However, if you deform the straight line current it will begin to *exaggerate* the deformation, making it bigger and bigger. That is why the spark is so jagged and not a smooth curve. However, there has to be some initial deformation, which may be as torquil suggested caused by the hot air rising around the spark to get that large scale upward kink. Otherwise I would think it would just be in some random direction.

 

[zumulko]

Thank you all for help.