Why Do Grass Seeds Trace Out the Electric Field?

[quietrain]

does anyone know why grass seeds are able to trace out the E-field?

i was reading this http://www.oberlin.edu/physics/catalog/demonstrations/em/efield.html

i assume the oil is to reduce friction, and high voltages around 40kV are needed so the wimhurst machine is used?

but it doesn't really say why grass seeds react?

thanks

 

[shoestring]

I imagine that many things that are easily polarized and has the right size and shape could work. Grass seeds don't have to be manufactured, and perhaps that was the easiest and cheapest analogy to iron filings that Jefimenko could think of when he did his experiments in the early 60s. Perhaps he found some in his garden shed. I'm just speculating, and I don't know if he was the first one to use it, but he published an article about it (http://adsabs.harvard.edu/abs/1962AmJPh..30...19J").

Conducting particles like metal filings might cause short circuiting, too heavy particles might not move as easily, the needle-like shape of the grass seeds probably promotes the effect, not sure what the oil is for, and I'm just speculating anyway. :-)


Edit: Can't help wondering if that means crop circles have something to do with dielectric properties of the grains! ;)

 

[quietrain]

erm, what do you mean by grass seeds getting polarized?

because say iron filings, they trace out magnetic field lines cause they are conductors right?

but what about grass seeds? what is it about them that makes them trace out E-field lines?

 

[shoestring]

erm, what do you mean by grass seeds getting polarized?

because say iron filings, they trace out magnetic field lines cause they are conductors right?

but what about grass seeds? what is it about them that makes them trace out E-field lines?

No, the iron filings are magnetized by the magnetic field. By analogy, anything that can be electrically polarized by an electric field would create a similar effect (if the field is strong enough). Many materials are polarizable, think for example of experiments with static electricity where pieces of paper are used. Paper is electrically polarized by an electric field, and most likely grass seeds too.

If only the field is strong enough, the seeds will orient themselves along the field. Even if they'd been spherical, the field lines might show up as strands of dipoles attracted to each other. However, it'd probably require a stronger field before any pattern of field lines is visible as strings of attached shperical seeds, so I think the shape of the grass seeds is a main reasoin why they're used in these demonstrations.

 

[quietrain]

oh you mean that if the E-field is strong enough, i can cause the atom in a seed to have its +ve nucleus get attracted to the -ve of the e-field
whilst having the -ve electrons of the atom be attracted to the +ve of the e-field

so i get polarization and hence the seeds align themselves to the e-field?

 

[shoestring]

Or because molecules with a permanent dipole moment, like water molecules for example, are oriented by the field. That too would make the grass seed into an induced dipole.

 

[quietrain]

:O
you mean if i use water it would trace out the electric field lines too?

so grass seeds may have permanent dipole moment too?

 

[shoestring]

:O
you mean if i use water it would trace out the electric field lines too?

so grass seeds may have permanent dipole moment too?

The field lines would be invisible, and water usually contains ions that make it into a conductor, so grass seeds are definitely better.

I don't think the grass seeds get a permanent dipole moment, but they probably contain molecules with permanent dipole moment, which will tend to align with the electric field, and thereby contribute to the induced dipole moment of the grass seed.

There are substances with permanent electric polarization, but they're probably not that easy to use in experiments like this, because their polarization so easily becomes neutralized by charges in the surrounding.

If one could use really strong permanent electric dipoles instead of grass seeds, weaker fields could be used. But, as long as the dipoles have to be induced, the fields must be quite strong.

 

[quietrain]

thank you!

 

[jkeohane]

Actually the question is better than you might think it is. I think we need a chemist to answer it correctly.

Consider a cigar shaped dielectric. On a microscopic level, an electric field will induce a dipole moment in the direction of field, not the same direction as the elongation. Thus, one would expect there to be no torque, because the magnetic moment is parallel to the electric field.

Now consider a cigar shaped conductor. On the inside of this conductor, there is no electric field, because the charges would rearrange themselves to guarantee it. The same logic can hold for the surface charges. If there is a force on any individual charge, it would just move inside the conductor, and again no torque.

So, the questions I don't know the answer to are:

1. Is the dipole moment somehow induced, and if so why does it need to follow the elongation direction of grass seed?

2. Or, does grass seed somehow have a permanent electric dipole moment?

Any ideas?