Discussion in 'The Projects Forum' started by opeets, Mar 16, 2015.

1. ### opeets Thread Starter Member

Mar 16, 2015
103
0
Don't know if this is the right forum or not but here I go anyway...

I need to come up with some ideas to help my 5th grade son on his science project. His proposal was to investigate, experiment, and document the properties of different electrical conductors. I have a background in computer engineering but my EE classes were well over 20 years ago and since then I've been a software developer so my knowledge base for electrical circuits is quite rusty.

I was considering constructing a basic circuit consisting of a battery, flashlight bulb, a digital multimeter, and conductors of varying material, thickness and length.

The simplest observation to be made with each conductor will be to observe how bright the bulb glows when connected to the battery.

I was considering taking this a little further by measuring the current flowing through the circuit (by connect the MM in series using gator clips) to show the relationship between current flow and intensity of the light bulb.

A couple of questions:

1) Does anyone see this not being a good approach?
2) Would a 6V battery be a good choice? Or should I use D batteries?
3) Any suggestion for a particular bulb?
4) Suggested conductors? So far I have one solid copper wire from a Cat5e cable and some steel wool.

2. ### Dodgydave Distinguished Member

Jun 22, 2012
4,998
745
A battery is ok, a psu will last longer, the thinner the wire the higher the resistance so less current will flow, and a the bulb will be dimmer, try the graphite from inside a pencil,thicker cables will make lower resistance, yes put the ammeter in series.

Last edited: Mar 16, 2015
3. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,754
760
You can use D cells or 6 V battery.

Conductor choice is difficult for such small load. U can use coil wire. thin wires will help. You need the ammeter in miliamps range.
You can use same wires and increase the no. by paralleling them to show how thicker wires reduce loss.
I will need to have a fairly long wire to show adequate drop

4. ### wayneh Expert

Sep 9, 2010
12,148
3,058
The problem with that plan is that the light bulb's range of resistance is too narrow.

Let's say it's a 3.6W bulb. That will be 10Ω filament that will draw 0.6A when supplied 6V. If your test material is also 10Ω, your bulb will be at half brightness. By 100Ω or so, your bulb will be very dim. At 1000Ω and up, which might be typical for your samples, the bulb won't light at all. Your numbers may be different, but the point is the same.

Using your meter to directly measure resistance or current will work fine because your meter has a very much larger range than a light bulb.

You should be able to get a larger range with light bulbs by using a range of different bulbs. For instance you might be able to light an LED at low current (1mA is enough to see a normal LED), several LEDs at higher current, and a regular flashlight bulb at current up to an amp.

5. ### opeets Thread Starter Member

Mar 16, 2015
103
0
Thanks for the input so far. What kind of analogy would you use to explain to a 11 year old that a thinner wire produces more resistance than a thicker wire of the same material?

Also agree that length of wire will be a difficult characteristic to experiment with. I am going to suggest that he modify his proposal so that the length element is taken out for practicality.

What about bulb type and a way method of affixing it? I still have my old breadboards and even an Analog/Digital Trainer Pad which has breadboards built into it.

My goal is to not go overboard by making it look like I did all the work for him. It is after all a 5th grade science project. I'm merely just planting ideas in his head and trying to find ways to show him (and eventually his teacher) easy ways to verify his assumptions.

6. ### Bernard AAC Fanatic!

Aug 7, 2008
4,176
397
A hard part is to find several different compositians all with same diameter. Choices might be Ni-Cr, stainless steel, iron, brass, aluminum, copper, & silver.

7. ### opeets Thread Starter Member

Mar 16, 2015
103
0
Doesn't need to be exact, just something to prove that he learned something. We're also looking to keep cost at a minimum.

8. ### Dodgydave Distinguished Member

Jun 22, 2012
4,998
745
The best analogy is to compare current with water in a pipe, the smaller the diameter of pipe the harder it is for water to get down, the bigger it is the easier water can flow, same for electricity,

it gets complicated when you use high frequency AC ,it then conducts on the surface of the cable...Skin Effect.

Last edited: Mar 16, 2015
9. ### wayneh Expert

Sep 9, 2010
12,148
3,058
You can definitely get a range of materials to test for very little cost, if not free. Just stroll through the local big box hardware store. You should be able to find copper, aluminum and steel wires, and I think you can find bronze and other metals as well.

Then all you need is a micrometer to measure diameter of the samples. I think keeping length constant is a good idea, unless you specifically want to show how resistance is proportional to length.

That wouldn't be bad, by the way, to use thin copper magnet wire (which you can pull off the back of the picture tube in any old junk CRT) to show how resistance varies with the length. Frankly that alone is a decent concept for a 5th grader.

10. ### opeets Thread Starter Member

Mar 16, 2015
103
0
Probably more EE101 questions but I just need some re-assurance:

1) When measuring the current I assume it will be constant throughout the circuit even though there are different materials that complete the circuit (the leads for the MM, the bulb circuitry, the battery, and the conductor currently being tested.

2) When the conductor provides less resistance (i.e. same material, but larger diameter) more current flows and the light bulb should glow brighter, right?

3) When I was playing around with this proposed circuit assembly yesterday I noticed that when I added the MM in series with the circuit the bulb dimmed. How would I explain this observation starting with...."the conductors used in the MM leads decrease the actual current in the circuit because...."

4) What difference would I expect using a 6V battery vs. a D size battery. In my testing yesterday, the 1.5V D battery pretty decently lit up a small bulb that I had using a 2 inch length of Cat5e solid copper wire and barely lit the filament when I used a single strand of steel wool. Had I used a 6V battery instead would I have seen much of a difference?

5) Would twisting an increasing number of thin strands of any given conductor be a crude way to simulate testing the same material of a larger diameter?

6) There is probably no simple answer to this one but would there be any approximate (5th-grade level) mathematical equation/expression/approximation that could be used to represent the difference in current flow between 1 strand of material vs. 4 strands of the same material twisted together? Not exact obviously, just ballpark. With the diameters of wire we are working with I doubt it would be noticeable with a basic MM.

11. ### Dodgydave Distinguished Member

Jun 22, 2012
4,998
745
1) the current wont be constant as the battery will be slowly decreasing in voltage.
2) correct
3) the reason the light dimmed with your meter in series will be the leads are of low quality , you need 20Amp leads or better,
4) bigger voltage lasts longer if its high in mAh capacity, use a bigger voltage bulb for bigger voltages,
5) yes more strands of wire in parallel, reduces the resistance

12. ### opeets Thread Starter Member

Mar 16, 2015
103
0
1) understood, but for a few seconds it will be relatively more or less constant, right?
3a) what are my leads likely made of and what you do you suspect they are rated at? will the leads you recommend show no noticeable difference in the bulb brightness?
3b) so the cheap leads introduce a greater resistance even though they are thicker than the conductor I am testing?
4a) What is the typical mAh for a 6V vs D battery?
4b) How do you match up a battery with a bulb for this type of experiment?

13. ### bertz Member

Nov 11, 2013
238
31
Here is the report my Grandson did for his 5th grade science fair project. It was an award winner! It's a little different than what you are contemplating, but perhaps it will generate some ideas. Obviously I built the conductivity cell and apparatus and mentored him, but he did the rest of the work.

Use the bulb as ballast and measure current directly with the MM. Good luck!

File size:
723.7 KB
Views:
35
14. ### opeets Thread Starter Member

Mar 16, 2015
103
0
Jeepers! This was for a 5th grade project? Smart kid.

15. ### bertz Member

Nov 11, 2013
238
31
"1) understood, but for a few seconds it will be relatively more or less constant, right?"
Use a LiPo battery, the voltage stays constant under load (at least until you reach the end of capacity) However this gets expensive since now you have to buy a special charger.
" what are my leads likely made of and what you do you suspect they are rated at? will the leads you recommend show no noticeable difference in the bulb brightness?"
Use #14 silicone wire
"3b) so the cheap leads introduce a greater resistance even though they are thicker than the conductor I am testing?"
copper wire is copper wire, so AWG #14 will have the same resistance per foot regardless of how much you pay for it.
"4a) What is the typical mAh for a 6V vs D battery?"
depends on the resistance of the circuit. See Ohm's Law in the tutorial section.
"4b) How do you match up a battery with a bulb for this type of experiment?"
Bulbs are rated by voltage. I suggest using a 6 volt bulb and a 6 volt lantern battery

16. ### n1ist Active Member

Mar 8, 2009
171
16
"1) When measuring the current I assume it will be constant throughout the circuit even though there are different materials that complete the circuit"

The current flow depends on the resistance. Different materials have different resistance, as does different cross-sectional areas of the same material. You can measure this with your multimeter directly, or compute it by measuring the current and the voltage drop across the sample.

"2) When the conductor provides less resistance (i.e. same material, but larger diameter) more current flows and the light bulb should glow brighter, right? "

Yes

" 3) When I was playing around with this proposed circuit assembly yesterday I noticed that when I added the MM in series with the circuit the bulb dimmed. "

Doubtful that it is the test leads. More likely, it is the current shunt in the meter. Ammeters work by putting a resistor in series with the circuit and measuring the voltage across the resistor. So part of your voltage is dropped across the bulb, and the rest across the shunt in the meter

" 4) What difference would I expect using a 6V battery vs. a D size battery. In my testing yesterday, the 1.5V D battery pretty decently lit up a small bulb that I had using a 2 inch length of Cat5e solid copper wire and barely lit the filament when I used a single strand of steel wool. Had I used a 6V battery instead would I have seen much of a difference? "

The steel wool has a higher resistance, so you need a higher voltage to get the same current to flow. You do need to be careful, since putting 6V across a 1.5V bulb will probably burn it out.

" 5) Would twisting an increasing number of thin strands of any given conductor be a crude way to simulate testing the same material of a larger diameter?"

Yes. That's how they make stranded wire. It's more flexible that way. Connecting them in parallel would also work as the current flow will divide among the parallel wires based on their resistance.

" 6) There is probably no simple answer to this one but would there be any approximate (5th-grade level) mathematical equation/expression/approximation that could be used to represent the difference in current flow between 1 strand of material vs. 4 strands of the same material twisted together? Not exact obviously, just ballpark. With the diameters of wire we are working with I doubt it would be noticeable with a basic MM. "

You would need to use thin wire. Look at http://en.wikipedia.org/wiki/American_wire_gauge to see the correspondence between area and resistance per foot. 100 feet of 30AWG wire is about an ohm.

"4a) What is the typical mAh for a 6V vs D battery?" This doesn't depend on the resistance, but on the chemistry of the cell. Check the manufacturer's web site. According to Wikipedia, D cells tend to be 8000-12000 mAh, and 6V lantern batteries 10500-26000 mAh. Circuit resistance (and therefore current draw) would dictate how long the battery will last.

/mike

17. ### opeets Thread Starter Member

Mar 16, 2015
103
0
Any suggestions on what type of bulb(s) I should get? Looking to go to Home Depot today. Will also pick up various types of wires to test.

18. ### bertz Member

Nov 11, 2013
238
31
I went back to your original post and you stated that the objective of the science fair project was to investigate properties of conductors. Well, there are basically two properties you can determine experimentally:
1. Resistivity
2. Conductivity or specific conductance

But conductivity is the reciprocal of resistivity so if you determine the one you've got the other.

An excellent project would then be to experimentally determine the resistivity and compare it to the standard resistivity. Then try to figure out how much of an error and why the error.

Attached is a diagram for a simple circuit to measure resistivity.

For example, let's say that the ammeter reads 132 mA or .132 amps and the voltmeter reads .o11 volts, then from Ohm's Law the resistance of the test material is 0.083 ohms.
Then by measuring the length and cross section of the sample wire you can calculate the resistivity. Don't be afraid to hit 5th graders with the math involved, you'd be surprised at what they can absorb.

Also attached are some references I suggest you bone up on prior to starting the experiment. Most science fair projects require research references. Also a good source for sample wire of various materials. Good Luck!
http://www.rapidtables.com/electric/Resistance.htm
http://hyperphysics.phy-astr.gsu.edu/hbase/tables/rstiv.html
http://www.rapidtables.com/calc/wire/wire-gauge-chart.htm
http://www.homedepot.com/p/OOK-50-ft-Aluminum-Hobby-Wire-50176/100192917

File size:
23.5 KB
Views:
27
19. ### opeets Thread Starter Member

Mar 16, 2015
103
0
Like the idea but I only have one meter (unless it's implied that the measurements are taken independently from the same meter).

20. ### bertz Member

Nov 11, 2013
238
31
Hmm, I can see that would be a problem. If you use the same meter, the voltage drops will change because the shunt in the MM also has some resistance. However, all is not lost. Do you have a Harbor Freight nearby? If so, shoot over there and pick up a MM for \$4.00. We all can use an extra MM!

If the Harbor Freight option doesn't work for you, there are tons of MM's on e-bay for cheap. The key is to get voltage readings to 3 decimal places in order to get meaningful results.