B Confused regarding Electricity Basics; Helping Kid with Project

[Bluesy]

TL;DR Summary

Not sure if I'm measuring conductivity or something else; general confusion

Hello,

My 10 year old is doing a science project in which he places a zinc nail and copper nail in a liquid, hooks each nail up to a voltmeter and then measures the recorded voltage for each substance.

Liquids include lemon juice, distilled water and ammonia.

The book from which he got the idea seemed to indicate that the higher voltage means that a given liquid is more/less conductive.

Questions: is he really measuring conductivity in this experiment?

My understanding is that a voltmeter does not really introduce electricity to the circuit so it is measuring...something. Would it be more accurate to say that the voltmeter is measuring the magnitude of the electrical field induced by the reaction of a given liquid with the zinc and copper?

Thank you.

 

[Baluncore]

Not measuring conductivity.
It is measuring the electrochemical voltage of a chemical cell.
More than one cell, would be called a "battery" (of cells).
https://en.wikipedia.org/wiki/Lemon_battery
https://en.wikipedia.org/wiki/Electrochemical_cell

 

[Bluesy]

Thank you.

We might forget the zinc/copper and voltmeter and use something like this (https://www.amazon.com/dp/B08R39SKXS?tag=pfamazon01-20

United Scientific™ CNDT01, Electric Conductivity Tester​

) to test conductivity. Along with pH strips to show acid/base.

Conductivity seems simpler somehow.

 

[Baluncore]

The most important thing about measuring electrical conductivity of a liquid, is to do it with a low voltage, AC signal. That will prevent polarisation of the electrodes.

The next most important thing is to have a standard geometry conductance cell, so the electrodes contact the same volume of liquid each time.

The problem with the CNDT01 is the 10 LED bar-graph, that is too crude to be both accurate, and to cover a wide range.

 

[Bluesy]

The most important thing about measuring electrical conductivity of a liquid, is to do it with a low voltage, AC signal. That will prevent polarisation of the electrodes.

The next most important thing is to have a standard geometry conductance cell, so the electrodes contact the same volume of liquid each time.

The problem with the CNDT01 is the 10 LED bar-graph, that is too crude to be both accurate, and to cover a wide range.

Thank you again. If the CNDT01 would show higher conductivity readings for the acids and bases and lower readings for the distilled water then I would be happy enough, as my son could do a very simple table with very basic conductivity results and pH levels.

I don't know enough about the specs (or how to interpret the specs) to know if the CNDT01 would accomplish this or not.

I'm tempted to buy it and try it out, however if lemon juice/ammonia/bleach are too "strong" to get a reading then I will pass.

Or maybe we follow the battery path...

Thank you for your help.

 

[nasu]

If the purpose of the experiment were to measure the conductivity, there will be no reason to use electrodes made from different metals. For this you will use something that does not react with the solution like platinum or graphite. Or use a specialized probe for conductivity measurement of water. Copper and zinc electrodes strongly suggest voltaic element (battery element). The voltage measured had very little to do with the conductivity of the solution and a lot with the chemistry. If the book suggests otherwise maybe use a better book. What book is that anyway?

 

[Bluesy]

If the purpose of the experiment were to measure the conductivity, there will be no reason to use electrodes made from different metals. For this you will use something that does not react with the solution like platinum or graphite. Or use a specialized probe for conductivity measurement of water. Copper and zinc electrodes strongly suggest voltaic element (battery element). The voltage measured had very little to do with the conductivity of the solution and a lot with the chemistry. If the book suggests otherwise maybe use a better book. What book is that anyway?

Excerpts are attached. Thank you!

 

[Tom.G]

The experiment pages you posted are showing the operation of a battery (actually a single cell of a battery, since formally anyhow, a battery is a collection of cells).

In my opinion the reason a battery is an early introduction, is that kids find it more interesting that a fruit can generate electricity; this will often lead them to ask more questions. In other words, you got them hooked!

Another part of the same experiment could be:
After the voltage reading is taken and documented, with the meter leads still connected to the fruit battery, disconnect the leads at the meter and touch them to your tongue. The voltage level is way too low to be dangerous but there will be a tingling sensation and the taste of lemon juice.

As another step, you could have two of those fruit batteries connected in series so their voltages add. Just wire the Copper electrode of one to the Zinc electrode of the other. Then connect the meter to the two free electrodes. You should get twice the voltage of the first reading.

By the way, since you will be connecting assorted wires, electrodes and meter together, try an on-line search for "clip leads." These are short lengths of flexible wire with spring-loaded clips on each end. Makes it much easier to keep connections together when you run out of hands!

The Volt is named after Alessandro Volta, who made the first source of continuous voltage in 1800.
See: https://www.britannica.com/biography/Alessandro-Volta

(above link found with:
http://www.google.com/search?hl=en&=who+was+alessandro+volta)

Have Fun!
Tom
p.s. Please let us know how it all turns out, we like to learn too!

 

[nasu]

The text is a little confusing and possibly the author is confused too. But at this level, all that mateters is to have a fun experiment.
The voltage measured across the terminals depends on the chemistry of the juice. The maximum current provided by the cell depends on the internal resistance which in turn depends stongly on the area of the electrodes. I doubt that with two nails it can light an incandescence light bulb. Maybe some LED, for a short time. It's very easy to measure some "open circuit" voltage (nothing connected but the voltmeter) between two electrodes dipped in any watery solution . To get a significant current on a load is not so easy.
Did you really get significantly different voltages with the same electrodes but different types of fruit/vegetables? Or is pretty much the same volatge but the current you can draw is different (which will translate into brighter lghtbulbs, if tehy really light on).

The book mentions a battery tester as an alternative to the multimeter or maybe as the same thing.
A battery tester (like the one used to test car batteries) is more complex than just a multimeter. If you really had a battery tester, then you could measure the internal resistance of the potato/fruit "battery". It simulates a load and measures the current through that load as well as the voltage drop relative to the open circuit voltage.

 

[Bluesy]

We used the conductivity meter posted above yesterday.

By doing so we eliminated the fruit battery dimension of the experiment and focused solely on the conductivity of electrolytic solutions (mostly acids and basis).

The conductivity meter only uses whole numbers, 1-10, and it seemed to read either a "1" (distilled water), "9/10" (for pH = 1 or pH = 13-14), and maybe an "8" for other pH values.

My understanding is that it is a very basic hobbyist/educational meter, and I didn't have great expectations but was grateful Amazon shipped it so fast and that it was relatively affordable.

Since the conductivity values (abstract 1-10 and not siemens per meter) were lumped with basically 3 values (1, 9ish, 8) for pH levels across maybe 6 or 7 pH values, I wonder if the chart will not reveal any gradual shifts in conductivity along the pH scale as we hoped.

Baluncore, you wisely warned me above about disappointing results with the instrument. However, it's probably good enough for 10 year old's science fair at this point, however I wonder if we could do anything else to get a feel for conductivity?

For example, I cross posted this topic with a more general science (but much less active) forum, and someone suggested using the multi-meter to measure ohms, as less resistance equals more conductivity.

If that were true, then we could possibly get more detailed values across the pH scale. I assume I would just use two copper wires, alligator clips and the multi-meter.

We might try it ...however, as Baluncore mentioned, ideally we would have an a low AC current.

In case someone replies before we test with the multimeter, does anyone see glaring issues going the resistance (ohms) route as a roundabout way to measure conductivity in the electrolytic solution?

Finally, in explaining to my son what is going on, is the following accurate?:

"An electric current/electromagnetic field needs a "bridge" to follow (? or "uses"?) in order for it to move from point A to point B.

The "bridge" it uses (follows?) is made up of charged particles. In many solids (like most metals), the bridge is often made up of electrons. In plasmas (he is interested in plasma), it is made up of ions and electrons. In liquids, it is made up of ions.

The strong acids and bases have lots of ions, so they make good bridges. The weak acids and bases have fewer ions, so they are not as good of bridges. Finally, the distilled water has basically no ions so it makes a poor bridge."

I used to think that electrons "carried" the energy but after watching things like it seems more accurate to say they somehow guide it along?

Thank you!

 

[Tom.G]

"An electric current/electromagnetic field needs a "bridge" to follow

I recommend that "electromagnetic field" be removed from the description.

Like gravity, a field, be it electric or magnetic, does not need anything as a 'bridge'. Each can and do exist in the vacuum of space.

The rest looks fine!

Cheers,
Tom

p.s. A magnet will attract a piece of Iron from a distance due to its magnetic field.
A balloon, after being rubbed on wool cloth, will attract scraps of torn-up paper due to its electric field.

 

[Bluesy]

I recommend that "electromagnetic field" be removed from the description.

Like gravity, a field, be it electric or magnetic, does not need anything as a 'bridge'. Each can and do exist in the vacuum of space.

The rest looks fine!

Cheers,
Tom

p.s. A magnet will attract a piece of Iron from a distance due to its magnetic field.
A balloon, after being rubbed on wool cloth, will attract scraps of torn-up paper due to its electric field.

Thank you!