# Homework Help: Resistance in water in motion

1. May 25, 2004

### ponjavic

Im a high-school student from Sweden currently competing in a worldwide physicstournament. Our task is to work with a problem both with theory and experiments. A professor here in Sweden told me that our problem is very similar to magnetoresistance.

The question is as follows:

Flow
Using a dc source, investigate how the resistance between two metallic wires dipped into a flowing liquid depends upon the speed and direction of the flow

The assignment consists of both building a theory revolving the connection between velocity, resistance and direction and also to prove these theories with experiments.

We have done some experiments and can conclude the following.

For water that runs in a U-shaped pipe solely accelerated by gravity:
If the electricity is led across the pipe the resistance grows with the velocity.
If the electricity is led alongsides the pipe the resistance grows or sinks with the velocity depending on the direction of the nodes in other words where we have - and +.

This is obviously unpure water as pure water would give no difference when switching - + and + - in the second scenario.

Certainly it seems as though scenario number one is the most normal and this on applies to magnetoresistance. We have an electric current and we apply a peripendicular force to it to change the resistance.

What I can conclude is that electricity in water is created by positive ions accepting electrons and negative ions releasing them e.g hydroxide and oxonium.
What I can't understand is why movement would negate some of these events. Why would a force change the resistance? Does it make the ions spend more energy? Do some ions not make it as far as to the electrode or what is it that happens?

I have no posibility (right now) to check wether pure water would change resistance when put through scenario one but I believe that it will. I also believe that it wouldn't change in scenario 2 because the number of + and - ions would be the same.

I was wondering if you could give us ideas on how to make formulas from magnetoresistance work for our case and also on how to conduct the experiments.

We started with the U-pipe by changing angles but we had big problems with that. We decided to have the pipe horizontally on the floor instead and by attaching the electrodes to a toy train or similar let them run through the water.

Any suggestions, theories or ideas on where we could find help, may you not be able, would be highly appreciated.

//Aleks Ponjavic

2. May 25, 2004

### Simon666

How did you come to your last conclusion? As the OH- and H+ ions are of completely different size and nature, a different propagation speed is not some sort of miracle. You would get the same results with pure water if you ask me.

3. May 25, 2004

### ponjavic

I guess i subconsiously tried to create what my teachers stated but when I rethink it you're absolutely right.
A thing that bothers me now is that when i fill a box with water and move the electrodes around i always have the same resistance. It seems as though the other experiment gave results simply because there was a constant change of flow and depth.
Can it be that motion have no effect at all? Seems unlikely since this is a competition.

Last edited: May 25, 2004
4. May 25, 2004

### Simon666

If you move the electrodes around, the ions will not have to move "upstream" or "downstream" or anything except in a small area around the electrodes where the water is moved, so the situation is not equivalent to holding the electrodes stationary and moving the water.

5. May 25, 2004

### ponjavic

Mr simon is there anyway i could chat with you as the presentation is coming soon and the forum way will take too long? You seem to know exactly what i need.
BTW we are allowed to ask other people for advice as that's part of the competition.

6. May 25, 2004

### Simon666

Check your PMs, I'll send one.

7. May 25, 2004

### Simon666

Btw, if anyone else has an idea, please post it. I'm not having my courseat hand and it has been years since I practised physical chemistry.

8. May 25, 2004

### Integral

Staff Emeritus
If I recall correctly the resistance of a large fluid bath is more dependent on the GEOMETRY (ie Spherical, pointed, etc) of the probes then their separation.

In tap water there are many current carriers not just H and OH, the conductivity of water is due, in a large part, to the other ions present.

What do you mean by "pure" water? De Ionized water will have resistance in the range of 10 Meg Ohms. This of course is very hard to maintain.

I Would suggest adding a known conductor (NaCl ?) measuring static resistance and then a range of flow rates (assuming that you do have a reasonable accurate flow meter?)

9. May 26, 2004

### Simon666

Btw, I've proposed a simple model - most likely too simple - to try to explain some findings. It is on my website at http://allserv.rug.ac.be/~skdmeule/WaterResistance.doc [Broken]. Please take a look at it, point out mistakes, faulty assumptions...

Last edited by a moderator: May 1, 2017
10. May 26, 2004

### ponjavic

When calculating the velocity i use a piece of paper putting it in the water in the inclined pipe. I have a starting point and a finish point. By dividing D/T ill geet the average speed. I'm not sure how accurate this is at it should not be a constant velocity. Maybe sqrt(2)*D/T should be used instead?

11. May 26, 2004

### ponjavic

Another thing:
as the water runs through the pipe at higher velocities the area between the electrodes will decrease, the height of the water will decrease. Will the amount of ions passing through still be the same? Will the higher speed of the water compensate this?