At 08:02 PM 7/3/2012, Finlay MacNab wrote:
To clarify:
An electrolyte does not conduct. Chemical reactions occur at the
electrodes that accept and give up electrons. Current flows through
the metal conductors between the anode and cathode.
An electrolyte does conduct. That is, there is movement of charge.
That is all that conduction means. Finlay, you are not being careful.
I suggest you try it.
When I say that the voltage drop occurs withing around 1nm of the
electrode (the debye length), that is only the case for low voltage
experiments on the order of the red-ox potentials for a given
electrochemical reaction.
Sure. The experiment is a low voltage experiment, by the way. The
palladium deposition in this work is often done below the potential
at which heavy water will evolve deuterium at the cathode.
At 6kV this would not necessarily be true.
You aren't kidding. The thing would explode if somehow you maintained
6 KV across the cell.
Because the ions in the electrolyte of much much lower mobility
than electrons in a metal conductor they may not be able to
effectively screen the high applied fields, especially if the
solution is being mixed (a quick search of the literature did not
yield a relevant example at high field) . If the fields were
oscillating, the E field would definitely be felt within the
electrolyte (this is what I would have done).
Well, there is work with oscillating fields, but they are oscillating
the electrolytic current.
You seem to have a concept of an electric field that is different
from how such fields are understood by electronic engineers and
physicists. You ask a question below that is actually quite easy to answer.
When you say:
The situation has nothing to do with "free charges that can migrate
to the surface" of anything. The mode of conduction is irrelevant.
There is no surface here, not that is defined. There is a conductive
medium, it has a certain resistance. Current flows through it when
there is a potential across it (actually such electrolytes can also
generate potentials, I won't go there.). Ohms law is obeyed.
I fail to understand what you mean. The only reason that the field
inside the electrolyte can be zero is if charge carriers migrate to
the surface of the cell to screen the bulk of the electrolyte from
the externally applied field.
No, any region of low potential "screens" the field. You are making
it much more complicated than it is. Imagine a line between the two
high-voltage plates. Imagine an equipotential region inside the
electrolyte, parallel to the plate, the line crosses that region.
Let's assume, to keep this simple, that the equipotential region is
larger than the high voltage plate. How can the high voltage on the
other side of this equipoential region affect *any* region beyond the
equipotential region?
This is DC, remember. There is a very high voltage drop across the
acrylic, about 3 KV. That's a done deal! The voltage doesn't then rise up!
I don't believe your example of probing the electrolyte with two
probes and a bridge is relevant to this experiment, since the
external electrodes are not in contact with the electrolyte, no
chemical reaction can take place, and so no current can flow, the
field can only be screened by the build up of charged ions at the cell walls.
This is how to measure voltage! Because no charge movement is
involved, the whole issue about charge carriers is irrelevant. That's
why a bridge is used, in fact. In practice, what is needed is a very
sensitive current meter, which is zeroed out by applying the
reference voltage and adjusting it until the current is zero.
A "bridge" here just means that current is measured, and the
experimental voltage is measured by opposing it with a known voltage,
such that no current flows in the circuit.
Maybe you can explain it in a way i can understand.
Sure, I hope.
What would happen if you had two metal plates separated by an air
gap, then you applied a 6kv bias between them, and then put your two
probes into the air gap?
Air conducts electricity. If the air conducts uniformly, the
resistance of the air will be even and the voltage will uniformly
decline, linearly, between the plates. The bridge will measure the
voltage accordingly.
In the subject example, there are three regions between the plates.
Two regions are filled with acrylic, which has very high resistance,
higher than air, if I'm correct. And then there is the electrolyte,
which has relatively low resistance.
The voltage gradient is directly proportional to the current times
the resistance per unit length.
That's simply another version of Ohm's law.
In an elecric circuit, we do not need to know what voltages are
present elsewhere in the circuit to know the relationship between
current, voltage, and resistance, in one leg of the circuit. Electric
field strength is just another name for voltage gradient.
