How to demonstrate the linearity of resistance of water vs. test voltage?

[kma]

Homework Statement

Show that resistance of water is mostly linear and ohmic at different voltages

Relevant Equations

V = IR

Hi I have a question. I am currently in the middle of a uni project where I am discussing electrical safety of water and I want to work out what the current would be at mains electricity voltage. (240V) I have done many calculations and have eventually got values by calculating resistance at low voltages and then by ohms law finding out what the voltage would be at 240 volts (May have hinted at this with some posts I have done in the past.

However one thing I know is that the molecules of the water change when electricity is applied and surely it affects the values. My question is would the value deviate much from the calculation I have done and what actually happens to the molecules at 240v compared to 36v? Does higher voltages simply accelerate the processes that happen at lower voltages? If so how can I recreate what would happen to the water at 240v so I can get a value without actually having to apply 240v? Would arcing through water happen at that voltage as well? Sorry if not worded this correctly but panicking as I have this due in very soon so need to settle this soon and I need a value I can use at a high voltage so I can accurately say what current I’d get at high voltage

With lots of consideration I have two ways to proceed and queries on both. First stage is to use fluid dynamics or other simulation software that can give me an accurate result on this. However OpenFOAM is extremely complicated to use and there is no way I can learn it in time to get a valid answer. My question is is there any CFD software or virtual labs that is accurate where I can do this experiment virtually (also that has a free trial)? I’ve investigated Labster, is this an accurate piece of software?

Secondly I am coming to the conclusion that I am probably going to have to do this experiment for real. But this fills me with a lot of anxiety, as you’d probably expect handling 240V would do. Ideally I would like to do it in an environment for this sort of thing but the lab at my uni is shut for the summer (spent too long trying to avoid doing this experiment for real) so that takes that option away. What’s the best environment to do this in, is there anywhere that is designed for this sort of thing, should I do it with an experienced electrician (I don’t know many…)

Or would it be better to do neither and just assume the resistance of water at 240V is the same at 9V? In past posts I have done on various forums, I have been told I should do some sort of experiment to prove that because of factors like electrochemistry changing...

 

[hutchphd]

What the current would be through what at mains voltage?????.
At the very least you should have a very current- limited source to do such experiments. Mostly what you will find is that pure water is a very bad conductor but any ionic contamination will change that drastically. Seems like a bad idea to me and a very well researched subject.
Also you have not elucidated the protocol other than "play with water and 240 Volts". Not a good mix.

 

[kma]

What the current would be through what at mains voltage?????.
At the very least you should have a very current- limited source to do such experiments. Mostly what you will find is that pure water is a very bad conductor but any ionic contamination will change that drastically. Seems like a bad idea to me and a very well researched subject.
Also you have not elucidated the protocol other than "play with water and 240 Volts". Not a good mix.

What the current would be through tap water at mains voltage, some say that scaling up and working it out based on ohms law is not the right thing to do because of electrochemistry at different voltages...

 

[hutchphd]

Look it up.

 

[Gordianus]

Working with mains voltage is dangerous and requires proper trauning.
Let's resort to: https://forum.allaboutcircuits.com/...ater-resistance-changing-with-voltage.190600/

 

[kma]

Working with mains voltage is dangerous and requires proper trauning.
Let's resort to: https://forum.allaboutcircuits.com/...ater-resistance-changing-with-voltage.190600/

Yes I have gathered it likely does follow ohms law, but lots of people say I need to demonstrate it for the purpose of this project...

 

[nasu]

I have seen sparks through water at this voltage (AC), for a distance between electrodes of about 3-4 cm. So it may happen. It all depends on what water happens to be between electrodes. Unless you have pure, deionized water every time you do the experiment, you may have different "waters" in different experiments. Imupurities on the electrodes or the vessels as well as quality (source) of the water may change the results. Did you get consistent results when you measured at 9V?

 

[kma]

I have seen sparks through water at this voltage (AC), for a distance between electrodes of about 3-4 cm. So it may happen. It all depends on what water happens to be between electrodes. Unless you have pure, deionized water every time you do the experiment, you may have different "waters" in different experiments. Imupurities on the electrodes or the vessels as well as quality (source) of the water may change the results. Did you get consistent results when you measured at 9V?

What do you mean when you say sparks do you mean arcing? and what are you saying may happen? I got consistent resistance similarities when comparing 9v to 36v

 

[nasu]

An electric arc is not the same as an electric spark. I mean sparks, not an electric arc. I did not see anything lik an arc.
https://en.wikipedia.org/wiki/Electric_arc

The sparks may have been through water vapours rather than liquid water. The water will start boiling pretty fast at this voltage, if the water is tap water. Actually electric kettles can be built like this, with the current going through water rather than through a high resistance wire dipped in the water. In the ones I saw (home made models, did not see comercial ones), a cup of water will start to boil in seconds. Pretty cool but possibly nor very safe.

 

[kma]

An electric arc is not the same as an electric spark. I mean sparks, not an electric arc. I did not see anything lik an arc.
https://en.wikipedia.org/wiki/Electric_arc

The sparks may have been through water vapours rather than liquid water. The water will start boiling pretty fast at this voltage, if the water is tap water. Actually electric kettles can be built like this, with the current going through water rather than through a high resistance wire dipped in the water. In the ones I saw (home made models, did not see comercial ones), a cup of water will start to boil in seconds. Pretty cool but possibly nor very safe.

So sparking is from the heat?

 

[Steve4Physics]

Hi @kma. A few points which might help...

1. You should probably be considering the resistivity of water, not resistance. The resistance in a given situation will typically depend on the water’s resistivity, the size of the contact-areas and the distance between contact-areas.

2. In Post #1 you said this is part of a “...a uni project where I am discussing electrical safety of water...”. It would probably help if you said what course you are doing and a bit about the context; for example, if it’s about the risks of using certain types of equipment in some specific environments.

3. Presumably you are referring to 50/60Hz AC (not DC). You did say “mains” – but it might be worth stating this explicitly.

4, Presumably youj are only interested in tap water as you mentioned it specifically.

5. The title of your thread is: “How to demonstrate the linearity of resistance of water vs. test voltage?”. But you probably mean: “How to demonstrate the independence of resistance of water vs. test voltage?”. You are basically asking if the resistance (or more generally the resistivity) remains constant. (This is equivalent to asking if water obeys Ohm’s law.)

6. If water doesn’t obey Ohm’s law it may not matter. For example if resistivity deviates only a few percent over the range of interest, that may not matter enough to affect the releevant safety considerations.

[Minor edit.]

 

[kma]

Hi @kma. A few points which might help...

1. You should probably be considering the resistivity of water, not resistance. The resistance in a given situation will typically depend on the water’s resistivity, the size of the contact-areas and the distance between contact-areas.

2. In Post #1 you said this is part of a “...a uni project where I am discussing electrical safety of water...”. It would probably help if you said what course you are doing and a bit about the context; for example, if it’s about the risks of using certain types of equipment in some specific environments.

3. Presumably you are referring to 50/60Hz AC (not DC). You did say “mains” – but it might be worth stating this explicitly.

4, Presumably youj are only interested in tap water as you mentioned it specifically.

5. The title of your thread is: “How to demonstrate the linearity of resistance of water vs. test voltage?”. But you probably mean: “How to demonstrate the independence of resistance of water vs. test voltage?”. You are basically asking if the resistance (or more generally the resistivity) remains constant. (This is equivalent to asking if water obeys Ohm’s law.)

6. If water doesn’t obey Ohm’s law it may not matter. For example if resistivity deviates only a few percent over the range of interest, that may not matter enough to affect the releevant safety considerations.

[Minor edit.]

Sorry for the late reply. 6 is basically what I want, to see if it roughly obeys ohms law and I am interested in tap water. Its an engineering course and this project talks about electrical safety in the workplace, particularly wet conditions...

 

[kma]

Hi, sorry if this post seems quite stressy but I am extremely stuck on this right now. I don't have long to do this course and I have evaluated means of doing this experiment in a safe way (where I literally operate it remotely) but my family do not want me to test 240 volts with this. I have also evaluated CFD software but it is too complicated. However I STILL need an answer as to whether I can do 9v experiments and then scale it up with ohms law and calculating the current by using the resistance calculated with the 9v at 240v, whether it is a roughly accurate result... This involves tap water. Has anyone done an experiment like this before? I've linked this experiment many times but it seems to hint at ohms law being followed but the electrodes are far away from each other... I'm quite stressed as if I can't give a correct answer on this my whole project goes kaput and I can't change now... I cannot find much online...

 

[Steve4Physics]

I couldn't find much online either, but if you haven't already seen this it may be helpful: https://physics.stackexchange.com/questions/736424/what-causes-water-to-be-non-ohmic

Of course, that wouldn't be a suitable reference to include in a report. But it suggests that water at least approximately obeys Ohm's law over your range of interest.

If you can't do the experiment or find an alternative, you will simply have to state that you are assuming that resistivity of tap water measured using 9V DC is approximately the same when using 240V AC.

Minor edits.

 

[DaveE]

my family do not want me to test 240 volts with this.

Your family is smart. Any experiment like this should be done at safe voltages, say less than 30V or so.

 

[DaveE]

This experiment is really about water solutions. Pure water isn't a good conductor and it will be difficult to measure resistivity with a simple experiment. However, pure water is also a pretty unusual thing. There are nearly always contaminants dissolved which will dramatically alter the resistivity. So, in real life, you're not measuring water, you are measuring a solution of some unknown composition.

Here's my suggestion: Set up a safe measurement system to test resistivity and clean it really well, like flushing it with hot water for a while. Find the lower limits of detection by reading about the sensitivity and accuracy of your instruments. Make some measurements to verify this, like measuring dry air at low voltages. Then get some distilled water and try to measure it, it will probably also be below the limits of detection. Then make your own known solution with something like salt (NaCl). Make a series of measurements with different concentrations and record the results, maybe graph it.

If you did all of this and explained why you were really interested in contamination issues, I bet you'd get a good grade.

Lab work is really more about the experiment design, procedures, and documentation, than it is about the results. For example, look up the mass of an electron, you'll see something like 9.1093837015(28)×10⁻³¹ kg. It is the number of significant digits, and the "(28)" final uncertain digits that are the real issue with these measurements. They don't actually give you an absolute answer, they describe what they know and what they don't (the limitations of their experiment). This is the result of good lab work, even if your experiment has 5% errors.

In practice, this is a result that most everyone will just look up in someone else's research; someone that cared more than us, and had more money, time , and equipment to do it right. They are really testing your ability to do lab work.

 

[kma]

Your family is smart. Any experiment like this should be done at safe voltages, say less than 30V or so.

But how do i know about higher voltages, i just assume its approximately the same if i scale it up?

 

[hutchphd]

One way is to look for voltage dependence at a series of lower voltages (None of which potentially lethal) That would bolster your argument. Also maybe look at (nonlethal) AC voltages (does that matter?). Just don't do anything close to 240 V. If it is worth potential death then somebody qualified and trained can do the experient .
Thank god I only had 120V (and low explosives but that's another story) available as a kid. Even then I was lucky.

 

[kma]

One way is to look for voltage dependence at a series of lower voltages (None of which potentially lethal) That would bolster your argument. Also maybe look at (nonlethal) AC voltages (does that matter?). Just don't do anything close to 240 V. If it is worth potential death then somebody qualified and trained can do the experient .
Thank god I only had 120V (and low explosives but that's another story) available as a kid. Even then I was lucky.

At lower voltage, the resistance reduces but the reduction levels off and some believe its due to the fact im calculating resistance from current and voltage, and lower values give weirder results. Indications suggest it remains at the level its levelled off to, should i just assume that?

I have had some people say if I was to experiment with high voltages I should either use a trained electrician or make it safe in a way that I never go within 10 metres of anything thats live, (ie using a remote control plug) but I want to find this answer out without this...