Voltage vs Current curve isnt going through origin for Playdoh resistor, why?

[RJBRJ]

Hi, I'm doing a IB Physics internal assessment on how the length of a tube of playdoh in a straw relates to the resistance of that tube. I tested it by connecting a 150 ohm resistor to one end of the tube of playdoh and connected the two to a power supply.

My data at the different lengths all came out nicely and the graph to find the resistance at each length was almost perfectly linear; however, none of the graphs passed through the origin--they all intersect the x-axis at 1.25 to 1.5 volts. Why is this? Shouldn't it always pass through the origin, if it is Ohmic?

Thanks A TON for any help!

--
RJ

 

[berkeman]

Hi, I'm doing a IB Physics internal assessment on how the length of a tube of playdoh in a straw relates to the resistance of that tube. I tested it by connecting a 150 ohm resistor to one end of the tube of playdoh and connected the two to a power supply.

My data at the different lengths all came out nicely and the graph to find the resistance at each length was almost perfectly linear; however, none of the graphs passed through the origin--they all intersect the x-axis at 1.25 to 1.5 volts. Why is this? Shouldn't it always pass through the origin, if it is Ohmic?

Thanks A TON for any help!

--
RJ

Welcome to the PF.

Could you please post your data, and a sketch of your setup, including the instruments that you used? We should be able to answer your questions fairly quickly with a little more info.

Love the Playdoh angle!

 

[Phrak]

How do you make contact with the tube of playdoh at each end?

 

[RJBRJ]

Here is a diagram of my setup:

(http://i53.tinypic.com/290snc7.jpg)

I attached my data in an xlx file with the graph for each trial.

I connected the 150 ohm resistor to the playdoh by pushing the wire coming out of the resistor into the playdoh just under a centimeter deep and the connection to the ammeter from the playdoh was made by pushing the metal probe into the playdoh about a centimeter. I can better explain it with this picture:

(http://i56.tinypic.com/20had0i.png)

To perform each trial, I removed the metal probe from the end of the playdoh, cut a few centimeters of the tube off, and reinserted the metal probe for the next test.

 

[Phrak]

Disconnect the lead from the DVM, short the DVM leads, and measure the current.

 

[vk6kro]

The battery you have shown should be a variable DC power supply? Is that right?

The current shown cannot be in Amps because the 150 ohm resistor alone would drop the current to 40 mA at 6 volts.

Assuming the current is actually in mA, you can work out the resistance of each length of sample by dividing the voltage by the current and subtracting the value of the 150 ohm resistor.

Doing this for the first (16.45 cm) sample gives:

2.00 Volts .0.32 mA ..6100.00 ohms
2.50...0.57...4235.96
2.99...0.83...3452.41
3.50...1.10...3031.82
4.01...1.37...2777.01
4.50...1.64...2593.90
5.00...1.92...2454.17
5.50...2.20...2350.00
6.00...2.47...2279.15

The sample's resistance drops with increase in voltage so it is not ohmic.

Is this ordinary play-doh or the type that has graphite embedded in it?

 

[OmCheeto]

The battery you have shown should be a variable DC power supply? Is that right?

The current shown cannot be in Amps because the 150 ohm resistor alone would drop the current to 40 mA at 6 volts.

Assuming the current is actually in mA, you can work out the resistance of each length of sample by dividing the voltage by the current and subtracting the value of the 150 ohm resistor.

Doing this for the first (16.45 cm) sample gives:

2.00 Volts .0.32 mA ..6100.00 ohms
2.50...0.57...4235.96
2.99...0.83...3452.41
3.50...1.10...3031.82
4.01...1.37...2777.01
4.50...1.64...2593.90
5.00...1.92...2454.17
5.50...2.20...2350.00
6.00...2.47...2279.15

The sample's resistance drops with increase in voltage so it is not ohmic.

Is this ordinary play-doh or the type that has graphite embedded in it?

It made my head hurt last night too.(must be milli-amps)

But I've never tested play-doh before.(maybe salt flour water mixture is conductively non-linear with varying voltage?)

Though I did find some entertaining DIY conductive play-doh videos.

https://www.youtube.com/watch?v=y0LCTLKV2II

But the little demons who made the video did not tell us what the frequency to resistance factor was. Argh!

 

[Adjuster]

The voltage / current plots do seem quite straight, so these results could be characterised as fixed resistances (found from the slope of the line) in series with offset voltages (found from the intercept). I can think of a few things that could cause an apparent offset:

1) The ammeter may have a zero error, and so does not read zero at zero current. This is more likely with an analog(ue) meter.

2) Similarly, The voltmeter may have as a zero error.

3) The Playdoh resistor really has a voltage offset, possibly due to electrolytic effects. Are there different metals contacting it at each ends (the resistor wire end and the probe)? Could electrolysis of water have something to do with it ?

 

[RJBRJ]

The battery you have shown should be a variable DC power supply? Is that right?

Yes that's correct, sorry I forgot to mention it previously.

And, yes, the data is in mA, not A, sorry I didn't mention that in the file either.

Is this ordinary play-doh or the type that has graphite embedded in it?

Nope, its the regular stuff, fresh and straight from the store.

Both the ammeter and voltmeter are digital, and we did a few basic labs with them previously that had perfect data, so I don't think that there is a zero error, but I can't be totally sure.

I do not remember, nor have access to my original lab set-up, so I'm not sure if the metal contacts were different types of metals. Also, the play-doh was just slightly moist, so I doubt there was much, if any electrolysis occurring.

OmCheeto: That is a really interesting video!

vk6kro: Nice observation, I graphed the Voltage vs Resistance, where resistance was found by V/I-150, and strangely it showed a Power trend-line. Which still leaves me wondering, why in the world would this happen?
Here's the graph:

(http://i55.tinypic.com/fmigkm.png)

 

[vk6kro]

[PLAIN]http://dl.dropbox.com/u/4222062/V%20vs%20I%20for%20salt%20soln.PNG

I tried a dilute NaCl solution with identical electrodes and got the result above.
It shows the current through the solution vs voltage across it.

The V vs I relationship becomes very non linear at low voltages.

It would be worse with dissimilar metals as electrodes as there could then be a generated voltage across the play-doh, which is a dilute salt solution.

 

[Phrak]

vk6kro, a resistor lead is tinned with some possible copper exposure on the end if clipped. A typical DVM probe lead looks like nickel plate. With the salty playdoh, it looks like a high resistance battery to me. RJBRJ could test this hypothesis with the method I mentioned above.

 

[vk6kro]

I doubt if it matters now. He just wanted to know why his data was not graphing as he expected.

Clearly a salt solution could cause non linear conductivity with identical electrodes, or even a stray EMF with dissimilar electrodes.

So, it would be more unusual if the results had been linear.

There is another type of resistive putty using embedded graphite and this does give resistances that are related to the diameter and length of the shapes that are made of this stuff.
Very messy on your hands, but a good product.

 

[Phrak]

I doubt if it matters now. He just wanted to know why his data was not graphing as he expected.

Clearly a salt solution could cause non linear conductivity with identical electrodes, or even a stray EMF with dissimilar electrodes.

So, it would be more unusual if the results had been linear.

There is another type of resistive putty using embedded graphite and this does give resistances that are related to the diameter and length of the shapes that are made of this stuff.
Very messy on your hands, but a good product.

Certainly the conductivity was nonlinear but the, now missing, Exel data of I vs. V were (engineering) linear. That is, E = IR - V. The only things in my understanding that could produce this with Playdoh under normal conditions would be the development of a metal-oxide rectifier through corrosion of one conductor and/or an inadvertent battery produced from dissimilar metals across a solution of ions.

 

[desy.kris]

V = E - Ir
Ir = -V + E
I = -V/r + E/r

if you look at it carefully, it's actually:
I = mV + c
which is
y = mx + c

the c is non-zero constant.. it's E/r. therefore the graph does not pass through the origin