Urgent question regarding resistance of water

Question

I have mentioned before that I am currently in the middle of a university project where I am discussing electrical safety of water.

I want to work out what the current would be at mains electricity voltage (240V.) I have done many calculations and have eventually gotten values by calculating resistance at low voltages and then by Ohms law finding out what the voltage would be at 240 volts.

One thing I know is that the molecules of the water change when electricity is applied. Surely it affects the values.

• Would the value deviate much from the calculation I have done?
• What actually happens to the molecules at 240V compared to 3V?
• Does higher voltag 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? I am sorry if this is not worded correctly, but I am 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

I did a 9V experiment and got was 2.3 milliamps which means a resistance of approx 3913 ohms. This means at 240V if resistance stayed the same, I’d get a current of 61 milliamps. I’ve been told even with the changes it’ll still be around that ballpark from the calculation.

Is this a valid enough answer to make it not worth doing a 240V experiment as I don’t really want to do something that dangerous for both myself and my electricity.

Answer 1

Resistivity of "water" is not fixed since you're not working in lab conditions with some reference water whose ion contents are well measured and known.

The "water" you deal with can be anything from a bathtub in a beach house with salty mist blowing in from the ocean through the open window, through highly mineralized water in the mountains, sodium-infused water from an ion-exchange water softener, etc.

There's a broad range of conductivity, and to get worst-case estimates, you have to choose some particularly conductive water that occurs in reasonably likely circumstances.

I am currently in the middle of a uni project

What does your project supervisor suggest? That's where you should be asking that question too for sure!

I’ve been told even with the changes it’ll still be around that ballpark from the calculation, is this a valid enough answer to make it not worth doing a 240v experiment

That depends on what you care about. If you care about some idealized notion of water resistance then the resistance can be assumed constant, up till dielectric breakdown occurs.

If you care about effective resistance with realistic electrodes interacting with the water, then surface reactions at the electrodes come into play, and the electrodes may well be injecting ions in the water and making it more conductive. So, in a very short time frame, the resistivity won't depend on the voltage. As you go from milliseconds to seconds, things will get quite dynamic and the fixed conductivity assumption doesn't match reality well anymore.

The differences between different types of water is irrelevant here

I am discussing electrical safety of water

Any such "discussion" is quite vacuous without looking at the range of conductivity and the various electrode-caused effects that change the picture. For example, imagine galvanic corrosion leading to a breach within a water heater, exposing the heating coils (elements) to water. The coil metal starts dissolving in water rather vigorously at that point and there will be lots of local heating speeding up the reaction. When you pour such water out of the heater, it may well look gray or black, and be quite a bit more conductive than the water that went into the heater. As you should imagine, that has some bearing on your discussion.

Answer 2

We usually refer to the conductivity of water rather than resistance. As noted it varies widely depending on what is in it.

There is a rather good backgrounder at this page.

Note that metallic electrodes in solvents (of which water is clearly a member) act as an EDLC (Electrolytic Double Layer Capacitor) so with the bulk conductivity, they exhibit a differentiator effect when inserted in the liquid.

You can read about EDLCs at TDK

For a single electrode pair, the current should scale directly with the voltage up to a certain point; the behaviour in a multi-electrode (one power, others receivers) is more complex.