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Conductivity of different types of metal wires (help with my kid's science experiment)

Posted on 2013-05-15
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Last Modified: 2013-05-16
Hey guys,

I'm helping my kid with a Science project and to be honest I need a hand here...

The experiment he chose was to test the conductivity of different types of metal wires (we have copper, steel, aluminum, and nickel)

We put an RC battery at one end (7.2 Volt NiCd, 1800 mAh) , grounded the negative of the battery and Volt meter, and then basically stuck different wires into the positive clip of the battery and tested the wire at the other end with the volt meter to measure the Volts DC output.

I had thought since different metals have different conductivity that there would be some difference among the wires, with copper showing the most conductivity.

But - in all cases except for the aluminum wire, the measurements of the output out the wires were exactly the output out the battery directly.

So the copper, steel and nickel wires all had the same conductivity.

That wasn't what I was expecting so I'm a bit stumped.  This doesn't invalidate the experiment I don't think, as long as we can come up with a good explanation for why this is.

Problem is I have no clue. Is it because at such low battery power they are all equally conductive? (except the aluminum).  I need to somehow explain why they all showed no resistance (except aluminum)

Any help/guidance/explanation would be much appreciated.

Thanks
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Question by:serveradm
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LVL 17
ID: 39170027
The difference will be very slight. You might detect a difference if you connect the battery to two lengths of wire and connect your test piece between the other two ends of the wires forming a short circuit. Working quickly as things will be getting warm, connect a sensitive voltmeter across the two ends of the test piece. I used to use a moving coil meter with no padding resistors, litterally just the coil of the meter was the circuit, and I would detect about one degree of movement with a nine volt battery and about 20 feet of wire and measuring over one foot at a time. This was in a multiple wired loom looking for resistive shorts so I only needed to see if there was current flow or not. Not, meant that I was past the short and current flow meant I was before it. You might just see a difference in the reading, but it will be tiny
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by:Thibault St john Cholmondeley-ffeatherstonehaugh the 2nd
Thibault St john Cholmondeley-ffeatherstonehaugh the 2nd earned 56 total points
ID: 39170038
Sorry, typing in a small window on my phone and ran out of space.
What you are trying to measure is the resistance of something designed to carry electricity, there may be differences but they will be very slight, especially over short lengths. If you had a hundred yards or so of each sample there would be more chance.
Otherwise, I wonder if a wheatstone bridge would be enough to detect a difference.
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by:d-glitch
d-glitch earned 111 total points
ID: 39170081
This is a difficult but not impossible measurement to make at home.  If you are trying to compare material properties, make sure the wire diameters and lengths are the same.  Long thin wires will have higher resistance and will be easier to measure.
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by:duttcom
duttcom earned 56 total points
ID: 39170156
Alumiunum forms an oxide very quickly and easily; an oxide which is an electrical insulator. This may explain why you observed some resistance with the Aluminum wire.
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by:ozo
ozo earned 112 total points
ID: 39170249
The resistance of the wires would be small compared to the internal resistance in the battery.
A Wheatstone Bridge should work better to show the differences in resistance  of the wires.
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by:aburr
aburr earned 55 total points
ID: 39170322
First note what d-glitch said
" If you are trying to compare material properties, make sure the wire diameters and
"So the copper, steel and nickel wires all had the same conductivity."    NO



" If you are trying to compare material properties, make sure the wire diameters and lengths are the same "

These directions MUST be followed. (If you are looking for a 1% difference your lengths must be measured to 0.1%)
The diameters must be measured with a micrometer or better.
Wire diameter is measured by gauge number (copper from, say, 14 to 22) It might be enough to get the same gauge for each wire.
There are several ways you can proceed, most require measuring voltage very carefully. You need a digital voltmeter with at least 3 digits. (Harbor Freight sells really cheap ones. Their accuracy may not be the best but you only need precision.)
A wheatstone bridge has been suggested but would take a page of directions and one type would require that you have three samples of each type of wire. I will write out the instructions if you are still interested.
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by:serialband
serialband earned 55 total points
ID: 39170515
You'd need decently long and thin wires to be able to noticeably measure the difference in the resistance on a cheap multimeter.  The resistances are so low that you won't be able to see any difference in very short lengths with a very low voltage.
Here are some sites that list the resistances of various guage copper household wire and the various material conductivity.
http://www.cirris.com/testing/resistance/wire.html
https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity#Resistivity_of_various_materials
http://physics.info/electric-resistance/


The wheatstone bridge is described on wikipedia with a nice diagram.
https://en.wikipedia.org/wiki/Wheatstone_bridge
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by:ozo
ID: 39170794
If you are looking for a 1% difference
The relative difference isn't that small.
It should be more than 200% between most of the metals mentioned.

But I agree that things like oxide surface coatings can make a bigger difference than the
bulk conductivity, so you do need to be careful.
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by:
d-glitch earned 111 total points
ID: 39171411
I was also stuck with just an iPod last night, when I really could have used a keyboard and a scanner.

I have attached a drawing showing how to measure the resistance of a piece of wire on the bench.  You need a 5 cent resistor and a meter capable of measuring millivolts.
ExEx-Wire-Resistance.png
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by:serveradm
ID: 39171433
Thank you very much for ALL your replies and d-glitch for that drawing!  That makes a lot of sense.

My only issue now will be to try to find 1 Meter+ wires in different lengths (of differing types of metal wire)

I went to home depo and they basically just had copper wire.  The other wires I striped from things around the house/garage.  

I wonder if maybe Radio Shack might have different cables, or where else I can check.
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by:d-glitch
ID: 39171480
Picture hanging wire is probably steel but you might find aluminum as well.

If you can find stranded wire and unravel it to get very thin single conductors it will make the measurement a little easier.

You don't really need any specific length, but longer will be easier to measure.
You won't know what lengths you need until you set up and try to make some measurements.

If the wire diameters are the same, but the lengths aren't, you can convert to ohms per inch, or meter, or mile.

If the diameters are different, you can calculate bulk resistivity for each material.
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by:andyalder
andyalder earned 55 total points
ID: 39172535
d-glitch has already pointed out the error in that you need to add a resistor since at the moment the only one in the circuit apart from the wire is the internal battery resistance but I would add this; aluminium is one of the most conductive metals available, it is the conductor in overhead power cables (they have a steel core but that's just for strength).

Aluminium oxide doesn't conduct that well though and it's that which you are making contact with since aluminium is always covered with oxide. Useful chart at http://www.tibtech.com/conductivity.php.
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by:ozo
ozo earned 112 total points
ID: 39173025
We put an RC battery at one end (7.2 Volt NiCd, 1800 mAh) , grounded the negative of the battery and Volt meter, and then basically stuck different wires into the positive clip of the battery and tested the wire at the other end with the volt meter to measure the Volts DC output.
This setup sounds even worse than what I thought you were doing.  The biggest resistance in the circuit would be the internal resistance of the volt meter.  And since a good volt meter will want to have an internal resistance as high as possible, the small additional resistance in the battery and wire will make little difference.
Do you have an amp meter or an ohm meter?
If not, you would want to measure voltage across a resistor, whether it is across a known resistor or the unknown you are interested in.
Again, a Wheatstone Bridge should be the easiest way to detect differences in resistance.
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