# Resistance of wet sponge with voltage

• kma
In summary, the conversation discusses the effect of water on electrical resistance of materials, particularly wood and sponge. The speaker shares their method of calculating resistance and asks about the potential impact of dissolved salts and electrode surface on resistance. It is mentioned that at high voltages, the Wien effect may come into play and change the resistance. The conversation also touches on the use of AC impedance measurements instead of DC resistance measurements and the potential for irreversible reactions to affect resistance.

#### kma

hi, in recent science projects I have been studying the effect of water on electrical resistance of materials that absorb water such as wood and sponge (the reason for the majority of my weird questions regarding water and electricity). I am calculating resistance by reading the current going through it at 9v, then dividing the voltage by current and for the sponge I got 87621 ohms. My question is would resistance stay the same as I go up the voltages to say 300 volts (I have no intention testing it with that lol) if not how much would it reduce by? I heard that the chemicals in the water may affect resistance as voltage goes up but would there be no space for that to happen in the water that is soaking an object due to the very low volume?

kma said:
I heard that the chemicals in the water may affect resistance as voltage goes up but would there be no space for that to happen in the water that is soaking an object due to the very low volume?
The dissolved salts are part of the water. It takes a reverse osmosis filter to stop the dissolved salt remaining free in the water.

Another question would be; How well filled are the pores in the sponge? Some pores may contain air, not salty water.

1, No, you don't have 87621 ohms. You have at best around 90 kΩ, your measurements didn't allow for higher accuracy.

2. In general solution response to the applied voltage is not linear - that is, it doesn't follow Ohm's law. In some narrow voltage ranges it does, but a lot depends on what is happening on the electrode surface, the only way current can "enter" the solution is through some electrode reaction, these depend on the solution composition and applied voltage, plus they are limited by diffusion and is many cases by the reaction kinetics itself. You could be able to record such a curve using your setup if you have access to variable voltage power supply (with some luck you can even record the curve using just a voltage divider).

3. At really high voltages things will get even more complicated due to a Wien effect.

Borek said:
1, No, you don't have 87621 ohms. You have at best around 90 kΩ, your measurements didn't allow for higher accuracy.

2. In general solution response to the applied voltage is not linear - that is, it doesn't follow Ohm's law. In some narrow voltage ranges it does, but a lot depends on what is happening on the electrode surface, the only way current can "enter" the solution is through some electrode reaction, these depend on the solution composition and applied voltage, plus they are limited by diffusion and is many cases by the reaction kinetics itself. You could be able to record such a curve using your setup if you have access to variable voltage power supply (with some luck you can even record the curve using just a voltage divider).

3. At really high voltages things will get even more complicated due to a Wien effect.
What sort of voltage will the wein effect kick in? And also how much does what is happening on the electrode surface typically change the resistance?

@kma -- in what situations would you need to use an AC impedance measurement instead of a DC resistance measurement? Hint -- look into biomedical impedance measurements through tissues...

berkeman said:
@kma -- in what situations would you need to use an AC impedance measurement instead of a DC resistance measurement? Hint -- look into biomedical impedance measurements through tissues...
Arent those two the same values?

kma said:
What sort of voltage will the wein effect kick in?
kV or even MV. What actually matters is not just voltage, but electrical field, a lot depends on how distant the electrodes are.

en.wikipedia.org/wiki/Wien_effect

kma said:
And also how much does what is happening on the electrode surface typically change the resistance?
Quantify "how much". But in general: if the voltage is low enough to not produce any electrode reaction there will be no charge transfer, so no observable current, so technically calculated resistance will be infinite.

kma said:
Arent those two the same values?

No.

Apart from many other things: when applying DC you keep some reaction going on, that changes the solution composition and in effect changes its resistance. Typical measurements use AC to avoid that affect, based on the assumption that if the reaction goes back and forth quickly the composition of the solution stays the same. This is only approximately true, as many reactions are irreversible.

berkeman
kma said:
Arent those two the same values?
Not necessarily -- see the post by @Borek above.

Borek said:
kV or even MV. What actually matters is not just voltage, but electrical field, a lot depends on how distant the electrodes are.

en.wikipedia.org/wiki/Wien_effectQuantify "how much". But in general: if the voltage is low enough to not produce any electrode reaction there will be no charge transfer, so no observable current, so technically calculated resistance will be infinite.
No.

Apart from many other things: when applying DC you keep some reaction going on, that changes the solution composition and in effect changes its resistance. Typical measurements use AC to avoid that affect, based on the assumption that if the reaction goes back and forth quickly the composition of the solution stays the same. This is only approximately true, as many reactions are irreversible.
What i mean by how much, is how different would the resistance in kohms be at approx 300v compared to 9v, ie if at 9 i get 9kohms if resistance reduces how much would it typically reduce by as you go up to around 300v, and from then up until wein effect, eg. would it reduce by half?

And how do I work out the impedance? How different is the value compared to resistance?

kma said:
What i mean by how much, is how different would the resistance in kohms be at approx 300v compared to 9v

No idea about exact values. This is one of these cases where measuring is easier than theoretical predictions. They will be highly dependent on the solution composition.

berkeman
Borek said:
No idea about exact values. This is one of these cases where measuring is easier than theoretical predictions. They will be highly dependent on the solution composition.
Is there a safe way of doing that measurement though, and would i expect it to be reduced by much?

Like I've posted this video on the forum before, this person did an experiment with high voltages, and whilst there is a reduction of resistance there isn't much... Is this roughly what I'd expect to happen in most circumstances with tap water and wet things (with tap water)?

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