# Building a circuit to measure the resistance of conductors

#### I3renda

Joined Oct 11, 2011
3
I have an assignment that is about electricity, circuits and I am totally confused on how to build the circuit to measure the current and voltage i think to find out the resistivity of conductors. the conductors my group and i are using are aluminium, iron, copper, zinc and i forget the last one. the teacher has mentioned that the resources she has for the experiment are all in the same size and stuff so i won't have to worry about measuring out the different metals to keep the variables the same. what i'm wondering is what would the diagram look like connecting the wires, alligator clips, power pack, switch, and other stuff needed to measure the current and voltage to find out the resistivity. I have no understanding of anything electricity and circuit wise so correct me if i'm wrong. I've been to about all relevant websites about circuits and stuff and i don't understand a thing. I've seen videos on youtube about measuring a wire's current and voltage but the resources the guy used in the video, our school does not equip. Here are the list of equipment our school has:

1. alligator clips / Wires (specifically for circuit making) i dont know if alligator clips and the wires are the same thing
2. Power pack
3. Multimeter
4. Voltmeter
5. Ammeter
6. Metals as conductors (pretty much the basic ones, a school might have, just not silver or gold)
7. Light bulbs or are they called LEDS
8. resistors
10. Breadboard equipment (wires, resistors, LEDS basic equipment breadboards require to build a circuit)

i have no idea if there would be light bulbs (LEDS i dont know) in the circuit. Let alone know how to draw the diagram, that is why i'm in desperate need of understanding the whole concept. Our teacher has been away and we have a substitute that knows nothing so i have to make do, trying to understand how to draw the diagram and how to make the circuit, measure everything, find the resistance of the conductors, compare and discuss why some metals are better conductors. I've got the written part of the assessment part down, i just need help creating the circuit. My group members are also researching about this. Could you please relate this to Ohm's law and write down the equation and stuff.

If any of you could possibly lend a hand that would be much appreciated. thank you.

#### shortbus

Joined Sep 30, 2009
10,049

Joined Jul 7, 2009
1,583
Well, we won't do the assignment for you (although some newbie on the forum will probably take the bait). But here's an outline of things to think about:

Your goal is to measure resistivity. Thus, you need to write the defining equation for resistivity to guide what you'll do experimentally. Generally, when you're running an experiment, you get the dependent variable in terms of the independent variables

$$z = f(x_1, x_2, ..., x_n)$$

where the x's are the independent variables. Then you measure the independent variables so that you can calculate the dependent variable. Thus, for example, if you were measuring the density of rectangular solids, you'd measure the three linear dimensions and the mass, calculate the volume, then divide the mass by the volume to get the density. Here, your dependent variable is resistivity.

Once you know the defining equation, you should be on your way to figuring out what equipment/supplies you need to make an experiment.

If you're still confused, then you first need to understand the basic principles of resistivity. This will be explained in any elementary high school or college physics text.

Joined Dec 26, 2010
2,148
It seems unlikely that a school practical for students completely unfamiliar with the material would be planned without providing quite explicit instructions. This is, apart from anything else, a question of safety:

"George, don't do that, no George, not with the ammeter, no really George IT DOESN'T GO THERE...BANG!!! "

Of course, no responsible school should give students really meaty power supplies to play with, but a bent meter pointer (or its modern digital equivalent) is still possible if we get our currents and voltages mixed up.

Here is my guess of how it might be done. If you were presented with your specimens in the form of wires of known length and thickness, you would quite likely use a four-contact voltmeter / ammeter method, perhaps with extra resistance added in series to ease setting the current. This is necessarily vague as I've never seen your setup.

A power supply with a constant-current feature might be used to supply current to the specimen directly. Otherwise, if only a defined-voltage supply is available, series resistors may be required to help define the specimen current, especially for copper wire which may have an inconveniently low resistance.

The current may be measured with an ammeter, or a current range on a DMM, or even perhaps from a readout on the power unit itself, but the latter may not be so accurate.

Two crocodile* clips make contact to the specimen to pass the current. For best accuracy, two separate crocodile clips should pick up the voltage for the voltmeter to avoid contact resistance problems ("Kelvin" connection), but at your school level this may not be required.

This link shows the idea, but note that here no extra resistance is shown: it may not be needed, depending on the available supply voltage and the specimen resistance. http://www.allaboutcircuits.com/vol_6/chpt_3/14.html

Quite possibly you will be asked to record a series of pairs of potential difference (V, Volts) and current (I, Amperes). You could then plot a graph - this should be a fairly good straight line, but may bend if the wire heats appreciably. Anyhow, V/I = R , which should be similar for each pair of points. You may be expected to something like a regression line, but I don't know what your teacher wants.

*For some unknown reason we have "crocodile" clips, in this country, not "alligator".

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#### I3renda

Joined Oct 11, 2011
3
Our teacher makes us wait for her, to check if our circuit is done properly before turning on the powerpack, a precaution 2 make sure nothing blows up. thanks for helping, i sorta get what i'm doing now, my real teacher came back and explained everything in more detail. the stupid assignment task sheet doesn't even specify the things we need 2 do. all it says is: 'Investigate the resistivity of different conductors.' seriously, from that sentence, many people can interpret that in many ways. anyways, thanks

#### I3renda

Joined Oct 11, 2011
3
our teacher gave taught us how 2 find out the resistivity of the conductors using ohm's law or something like that. thanks for helping with the density part, i think our group needs 2 do that. cuz we have 2 experiments were doing, one that has 4 metals that are in shape of a rod, all teh same size. i guess 2 figure our the density for that using the information you provided, pie X radius squared then multiply by the height of the rod, then of course weigh the rod then divide like you said? for the second experiment, we have the same metals but this time its in a shape of a rectangle and its about 1 mm thin. and we're trying to find out if the density or how much of the metal would affect its resistivity. but i'm not sure about what the outcome would be. would it be that the bigger the metal or the more heavier or dense the metal is, the more resistance or less? i still don't know how that works.

Joined Dec 26, 2010
2,148
our teacher gave taught us how 2 find out the resistivity of the conductors using ohm's law or something like that. thanks for helping with the density part, i think our group needs 2 do that. cuz we have 2 experiments were doing, one that has 4 metals that are in shape of a rod, all teh same size. i guess 2 figure our the density for that using the information you provided, pie X radius squared then multiply by the height of the rod, then of course weigh the rod then divide like you said? for the second experiment, we have the same metals but this time its in a shape of a rectangle and its about 1 mm thin. and we're trying to find out if the density or how much of the metal would affect its resistivity. but i'm not sure about what the outcome would be. would it be that the bigger the metal or the more heavier or dense the metal is, the more resistance or less? i still don't know how that works.
Pls try not 2 use "textish": it's 2 hard 4 sum of us old f@rts 2 read. On this forum, standard English is, erm...de rigeur.

Ohm's law tells us that the current I in amperes passing in (certain) conductors is directly proportional to the potential difference V in volts appearing across the conductor. Dividing V by I gives the resistance R in ohms. So R = V/I, I = V/R, V = IR

http://en.wikipedia.org/wiki/Ohms_law

Resistivity is a bit different. You can find the resistance of a conductor by getting measurements of V and I, and calculating R=V/I. To find the resistivity rho (measured in ohm metres)* of the substance from which the conductor is made, we need another equation.
R = rho.L/A, where L is the object's length in metres , A is its cross-sectional area in metres squared. Thus rho = R.A/L

http://en.wikipedia.org/wiki/Resistivity

*The ohm metre is a unit of resistivity, but an ohm-meter can also refer to a device for measuring resistance. Don't confuse the two!

Resistivity versus density is another matter. You could get a clue by looking up the densities and resistivities of, say, alumin(i)um, copper, and lead in a textbook, or on the internet, but that might be a bit like reading the end of a detective story before the beginning. I do know something about how this story should come out, but I won't spoil it for you - except to say that real detective stories don't always have the neat endings you see in some films or TV programs.

#### MrChips

Joined Oct 2, 2009
31,068
Pls try not 2 use "textish": it's 2 hard 4 sum of us old f@rts 2 read. On this forum, standard English is, erm...de rigeur.
btw im with u 2. LOL