What happens to resistance of tap water as voltage is increased?

[kma]

1
In recent days I have done a few experiments measuring the current of water as it goes up from 9 volts up to 36 volts, and following Ohms law to convert it to resistance. And I discovered a very interesting trend. In between 9 and 18 volts, there is a massive drop in resistance (by around a 40% reduction) but then as I go up to 27 volts, its a 5% reduction, and is even less of a reduction when reaching 36 volts. I've done this experiment a few times and this has continued to happen. This is it visualised on a graph;

I am curious to know why this happens, why there seems to be a reduction as I go from 9 to 18 volts, yet the reduction seems to reduce at 27 volts and reduce further at 36. Is there a reason that? And as I go further up the voltages (don't want to test with higher), does this continue with the reduction in resistance continuously reducing, and if not at what voltage does it change?
Specifically I kinda want to know what resistance can I expect at around 240 volts

 

[Baluncore]

Welcome to PF.
The rise in resistance could be because the area of the electrode in contact with the water is reduced by the gas produced by electrolysis of the water.
Is the water distilled, or salty ?
Are the electrodes carbon or metallic ?
What shape are the electrodes ?
Did you use DC or AC ?
You will need to use AC to prevent polarisation of the electrodes.

Specifically I kinda want to know what resistance can I expect at around 240 volts

Zero, then infinite. The water will boil violently to produce high pressure steam. That will probably destroy your experiment.

Electrical steam generators are regulated by controlling the water level needed to reach the 3 phase rod electrodes fixed above in the pressure vessel. They are rated in megawatt.

 

[kma]

Welcome to PF.
The rise in resistance could be because the area of the electrode in contact with the water is reduced by the gas produced by electrolysis of the water.
Is the water distilled, or salty ?
Are the electrodes carbon or metallic ?
What shape are the electrodes ?
Did you use DC or AC ?
You will need to use AC to prevent polarisation of the electrodes.Zero, then infinite. The water will boil violently to produce high pressure steam. That will probably destroy your experiment.

Electrical steam generators are regulated by controlling the water level needed to reach the 3 phase rod electrodes fixed above in the pressure vessel. They are rated in megawatt.

Thanks for the response, have a few questions and answers on what you said.

'The rise in resistance could be because the area of the electrode in contact with the water is reduced by the gas produced by electrolysis of the water.' The resistance did not rise, more it reduced, are you saying there was more gas reducing contact at lower voltages?
'Is the water distilled, or salty ?' It's tap water, so its not distilled
'Are the electrodes carbon or metallic ?' I am using metal probes connected to a multimeter
'Did you use DC or AC ?' DC, how can i do this experiment with AC, anything I need for that?
'Zero, then infinite. The water will boil violently to produce high pressure steam. That will probably destroy your experiment.' How would this momentarily result in zero resistance? Is there a way to stop that boiling happening so I can get a reading, and if so what resistance would I get

 

[Baluncore]

The resistance did not rise, more it reduced, are you saying there was more gas reducing contact at lower voltages?

No. The rate of fall was reduced at higher voltages, when higher currents would flow and more gas would be produced.

'Are the electrodes carbon or metallic ?' I am using metal probes connected to a multimeter
'Did you use DC or AC ?' DC, how can i do this experiment with AC, anything I need for that?

You are stripping metal from one electrode and plating metal onto the other. You must use carbon for the electrodes to avoid dissolving the metals.
Because you use DC, the system will always push an ion one way. If you used AC, current, the system would recover and show different electrical properties. I don't know why you are doing the experiment, so I cannot say if you should use AC or DC.

How would this momentarily result in zero resistance? Is there a way to stop that boiling happening so I can get a reading, ...

If you subject the water to a high voltage, what will limit the current ?
Surely you know that high voltages and water do not meet safely.

 

[kma]

No. The rate of fall was reduced at higher voltages, when higher currents would flow and more gas would be produced.You are stripping metal from one electrode and plating metal onto the other. You must use carbon for the electrodes to avoid dissolving the metals.
Because you use DC, the system will always push an ion one way. If you used AC, current, the system would recover and show different electrical properties. I don't know why you are doing the experiment, so I cannot say if you should use AC or DC.If you subject the water to a high voltage, what will limit the current ?
Surely you know that high voltages and water do not meet safely.

Wouldn't the water's resistance limit the current still? And how do i obtain an AC electrode that uses carbon? Also I don't see gas bubbles in the experiment

 

[Baluncore]

Do you understand electrolysis ?
https://en.wikipedia.org/wiki/Electrolysis_of_water

Do you understand that the resistance will depend on what species of chemicals are in the water ?

Do you also understand that the resistance will depend on the size and shape of the electrodes that form the cell ?

 

[kma]

Do you understand electrolysis ?
https://en.wikipedia.org/wiki/Electrolysis_of_water

Do you understand that the resistance will depend on what species of chemicals are in the water ?

Do you also understand that the resistance will depend on the size and shape of the electrodes that form the cell ?

Yes I am. And when you are talking about there being no resistance at higher voltages, are you referring to the dielectric breakdown of the water where I believe there's a certain voltage where the resistance no longer works? Also (I'm not as knowledgeable about this which is part of the reason I am asking this question) if its gas bubbles resulting in the rate of fall being reduced, would the gas produced result in the reading being unstable (it was pretty stable in the experiment i did). Also a similar resistance trajectory happened when I tested something that was wet where there's no space for gas bubbles to form?...

 

[Baluncore]

I think first you should find out why you did not notice the gas bubbles forming on the electrodes. Is there something wrong ?

Then maybe you can describe the voltage source you are using, and how you measure the cell current and voltage.

Next, describe the geometry and size of the cell, the electrode material and dimensions.

It may then be possible to answer some of your questions.

 

[kma]

I think first you should find out why you did not notice the gas bubbles forming on the electrodes. Is there something wrong ?

Then maybe you can describe the voltage source you are using, and how you measure the cell current and voltage.

Next, describe the geometry and size of the cell, the electrode material and dimensions.

It may then be possible to answer some of your questions.

I will put a picture here of my setup later when I get the chance.



Interestingly this guy did a similar experiment to what I'm wanting to do (but i don't want to actually experiment with as high voltages and with a bigger distance between the electrodes) and when calculating the resistance he got at different voltages, its basically the same even when he got close to 300v... I'm guessing the difference is he used AC with proper electrodes...



But even when he used DC, that resistance drop didn't happen...

 

[Baluncore]

I will put a picture here of my setup later when I get the chance.

I await your response.

 

[kma]

I await your response.

May be a while before I am able to do that but i can say its not a great setup to say the least tbh, i guess that's the reason the youtuber linked is getting very stable resistance even when getting close to 300v isn't it?

 

[kma]

I await your response.

Do you know why the YouTube video I linked, the resistance stayed relatively stable even up to 300 volts with tap water?

 

[Baluncore]

Do you know why the YouTube video I linked, the resistance stayed relatively stable even up to 300 volts with tap water?

I think it might have something to do with the fact that the cell is made from a very long, opaque plastic tube. That also explains why you did not see the bubbles.

Unfortunately, it is only now that we can guess what you might be doing. The orientation of the tube ends, and the current that has flowed during the experiment, will be critical to the electrode contact with the water and the cross-section of the water column. That is because gas may build up at the high points of the tube.

 

[kma]

I think it might have something to do with the fact that the cell is made from a very long, opaque plastic tube. That also explains why you did not see the bubbles.

Unfortunately, it is only now that we can guess what you might be doing. The orientation of the tube ends, and the current that has flowed during the experiment, will be critical to the electrode contact with the water and the cross-section of the water column. That is because gas may build up at the high points of the tube.

In my experiment I am simply putting two crocodile clips into a fairly small box of water...

 

[Baluncore]

Trying to find out what you are doing, and why, is close to impossible.

You have made it just too difficult. This process is such a waste of my time that I have stopped watching this thread.

 

[DaveE]

Sorry to spoil the party. But... Your tap water isn't the same as my tap water. This question isn't really about the water, it's about what's in the water. Pure water is a pretty good insulator, but you never get pure water from a "tap".

No one can answer your question since you're asking something like "why does some unknown substance act this way?" - We don't know either.

 

[Tom.G]

And how do i obtain an AC electrode that uses carbon?

From cheap flashlight batteries.

By 'cheap' I mean the low cost Carbon-Zinc batteries from your local discount store with a paper or cardboard outer surface, they are sometimes labelled as "Heavy Duty", Not 'Alkaline' batteries.

I suggest using either a "C" or "D" size cell because the Carbon rod in smaller cells is rather small and breaks easily.

The center electrode in these Carbon-Zinc batteries is Carbon. You will have to cut the body open to get to the center electrode. The body is a thin sheet of Zinc, which is a soft metal and relatively easy to cut.

Lay down a thick layer of newspaper as a work surface or do the cutting outdoors on a surface you won't damage.

The content is a chemical paste that is a mix of Ammonium Chloride and Manganese Dioxide. Being an acid, it is mildly irritating and somewhat corrosive, it will dry out your skin so plastic or rubber gloves are a good idea. Baking Soda will neutralize it. Wash any tools after contact to avoid corrosion.

Have Fun!

 

[kma]

Hi, its been a while since I posted here about what I have been trying to find out regarding resistance of water at high voltages. I may seem oddly obsessed with this but it is forming a big part of a science project I am doing revolving around electrical safety with water and wet things, using detailed resistance values to back things up.





I've linked these videos before, but I have since studied them a fair bit more and calculated resistance values at different voltages as per the video with AC. Here is what I found;

25V = 170000 ohms
49.8V = 171724 ohms
100V = 166666 ohms
150V = 166666 ohms
200V = 166666 ohms
225V = 164306 ohms
250V = 163265 ohms
287V = 163068 ohms

So for this I have come to the conclusion that whilst it doesn't quite follow ohms law the resistance does not decrease by much at all (in the end only by like 8,000 ohms.) My question is is this what would normally happen that it only strays from ohms law by a little bit and what is the reason it strays from it but not by much? Also why does resistance remain at the same value for a bit? And would increased salts or any other substance in the water affect how much it deviates from ohms law?

Hope I have made the question clear and understandable :D

 

[berkeman]

25V = 170000 ohms
49.8V = 171724 ohms
100V = 166666 ohms
150V = 166666 ohms
200V = 166666 ohms
225V = 164306 ohms
250V = 163265 ohms
287V = 163068 ohms

What were your error bars on these measurements?

 

[kma]

What were your error bars on these measurements?

What do you mean by error bars? Also note this wasnt my own experiment i was dividing current by voltage from the values in the video

 

[berkeman]

What do you mean by error bars?

When you do real life experiments, you need to understand the errors / tolerances in your measurements. And those should be included in any data that you publish.

If you are lifting this data from somewhere else, you should look to the original data to figure out what their published errors / tolerances were. If the did not make that available, I would steer away from their data...

 

[kma]

When you do real life experiments, you need to understand the errors / tolerances in your measurements. And those should be included in any data that you publish.

If you are lifting this data from somewhere else, you should look to the original data to figure out what their published errors / tolerances were. If the did not make that available, I would steer away from their data...

A little hard to get my own data seen as a 240v experiment is not something I'd do but I'd guess the tolerances here are lower than the values measured

 

[Lord Jestocost]

What happens to resistance of tap water as voltage is increased?​

One needs reliable measurements to answer this question.
In the case of conductivity measurements on liquids, one generally uses four electrodes instead of two, to be able to measure voltage dropped across a length of the liquid only and to be able to ignore the "resistive" effects of electrodes. See, for example, "Kelvin connection" for liquid conductivity measurements in
https://www.testandmeasurementtips.com/when-to-measure-conductivity-instead-of-resistivity-faq/