Measuring resistance of metal cubes with 9v battery

Discussion in 'The Projects Forum' started by Jennysa, Nov 17, 2009.

  1. Jennysa

    Thread Starter New Member

    Nov 17, 2009
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    My son has a science project to measure the resistance of 5 metal cubes at different temperatures using a 9v battery. Metal cubes are all the same density and are: zinc, aluminum, iron, brass and copper. He is confused as to how to do this. He has a Sperry digital Multimeter. Questions: what is the appropriate Ohm setting? Where does he place the black and red leads in relation to the metal cube and battery to measure resistance?
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Greetings:

    The ohmmeter probes should be placed on opposite faces of the cube. The range should be set to the LOWEST scale. However, this is going to read a VERY low value. I wonder if the ohmmeter in question is a special low-resistance type, or a conventional DMM. A standard DMM won't read fractions of an ohm too accurately. But I'll give you the benefit of the doubt and assume the meter you're using CAN read very low resistance values.

    Let us know how it works!

    Eric
     
  3. Jennysa

    Thread Starter New Member

    Nov 17, 2009
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    Hi Eric,
    Thank you for your timely response. The Ohmmeter is brand new and I have to confess I am totally ignorant about this (I'm a business/marketing type!). Just so I understand, should my son place the metal cube on top of the battery and then place the probes on opposite faces of the cube. The probes should never be placed on the battery terminals (that's a question!)
     
  4. Jennysa

    Thread Starter New Member

    Nov 17, 2009
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    Hi Alberto,
    Thank you for your reply and your drawing! That helped me to understand. Someone had suggested we perch these small cubes, which are less than an inch square, on top of the battery. Would that position obtain a proper reading or the same kind of reading as using the reister wires?
    Thanks again,
    Jenny
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Jenny,
    No, that would not work. The cube would create a dead short across the battery terminals. The battery would quickly get hot, and it may rupture.

    A 100 Ohm resistor is a pretty heavy load for a 9v battery. It would have to have a rating of 1 Watt to just barely be within specifications; it will get hot pretty rapidly. Accurate readings will be difficult, because the 9v battery will become drained quite quickly.
     
  6. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    There is a lot with this question that doesn't seem to make sense to me.

    First, I find it hard to believe that you have solid metal cubes of zinc, aluminum, etc. that all have the same density. Something is wrong there. Are they solid cubes? Wires inside of cubes (e.g., black boxes)?

    Second, let's assume the resistance across a cube is a few milliohms. You add a 100 ohm resistor in series to provide a reasonable load. Now, you are trying to measure the temperature coefficient for the milliohm component. In other words, you will be measuring changes on the order of 10E-6 (i.e., one ppm). What is the accuracy of your voltmeter? What is the noise level for a 100 ohm resistor? How will you control its temperature sufficiently, so its changes do not completely swamp out the much smaller changes from the metal cube?

    It sounds like an impossible experiment starting with an impossible premise (same density). My first step would be to clarify what the experiment really is supposed to be.

    John

    Edit: Alberto posted while I was writing. Change milliohm to microohm. You are now trying to measure one part in 10E-9 (ppb). No way.
     
  7. Duane P Wetick

    Active Member

    Apr 23, 2009
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    It is stated that the metal cubes are all the same density? How is that possible? Did he mean the same physical size? If they are 1 cm. on a side, then measuring across will give a volume resistivity in ohms/ cm. at a particular temperature. It seems that a precision measuring apparatus (and a trained technician) is necessary to get any meaningful data, even a comparison at different temperatures.

    Regards, DPW
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    During coffee, I had time to cogitate on this question. Here is one approach:

    1) Be sure the question is asking for the temperature coefficient for electrical resistance of the metals. That is, be sure it is not asking for something like the coefficient of expansion. e.g.., change in density with temperature.

    2) If it is electrical resistance, then going to a high current may get rid of some of the exponents and problems, but I still see it as an almost impossible task with large cubes. I would start by measuring the density of each cube. Then, cut off a small portion (like an edge) and smooth the edges reasonably well. Now pound or roll that small piece into a thread or ribbon that has a resistance you can measure with your meter. Establish good electrical contact as close to the ends as practical. Weigh and determine its dimensions and distance between contacts.

    At that point, you may be able to do the experiment directly using the ohm function on your meter. I would suggest a large heat-sink for adjusting temperature, like a bowl of water (distilled or deionized) or thermos. You might use antifreeze or light weight oil instead of water for the heat conductor. Test resistance at several temperatures ( 4 to 5) and let stabilize at each temperature before taking readings. Your data along with the known dimensions of the ribbon will allow you to calculate its temperature coefficient for resistance.

    If you cannot get decent readings directly, then consider using a bridge to measure the changes. Look up Wheatstone bridge. That approach will be more complicated, but has the effect of removing the fixed resistance values, so you will only be measuring the changes in resistance.

    Good luck. John
     
  9. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Yes, the possibility that the OP meant equal masses was considered. The problem is still very difficult to solve. I think a Wheatstone bridge is probably the only practical way get at it without changing the problem (i.e., removing a portion of each block).

    As for homework being method, rather than result oriented, I agree with that principle. But direct measurement by inserting a 100 ohm resistor in series with micro- to nano-ohm changes is the wrong method. There are too many variables that would need to be controlled too tightly for that to work. Moreover, the consumer Sperry digital voltmeter just doesn't have that level of precision, nor is any voltmeter likely to have 10E-9 precision.

    John

    Edit: My apology to Alberto. I only took a quick glance at Alberto's schematic. On further review, you will still measure zero volts (or artifact) across the cubes, particularly with only 90 mA of current.
     
    Last edited: Nov 18, 2009
  10. Jennysa

    Thread Starter New Member

    Nov 17, 2009
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    Hello to everyone who has replied to my questions,
    First, let me thank you all for giving this 'simple middle school experiment' so much thought - I honestly think I'm more confused than when I asked my first question. Let me see if I can give you more information so that you can give me a simple (electricity for dummies) response.
    We purchased a kit from a science store of 5 metal cubes of the same density. They are not the exact same size, but fairly close - the aluminium being the largest (lowest density). I provided that info on their density as an FYI only, as it could be considered a control, if you will. My son wants to see if the resistance changes in these cubes when they are subjected to different temperatures, using a 9v battery. His plan is to measure resistance at room temperature, and do 3 measurements per cube. Then, he wants to put all 5 cubes in the freezer for half an hour and do the same 3 measurements of resistance per cube, then put them in the over at 200 degrees for 15 minutes and again obtain 3 readings of resistance per cube. What is the simplest way to measure resistance for him? He is 13 years old - found this experiment to be interesting, but in reality he hasn't studied electricity yet. If this were your kid brother, or son, how would you suggest he do this to get the job done and not overwhelm him with more information than he can handle or that is needed at this age? We don't even know which setting we should be placing the ohmmeter on! WHew, I hope that helps. Thanks again all - and your help is REALLY appreciated. I should say the plan was for my son's grandfather to help with this as he is a whiz at electricity, but he is now gravely ill, which left this to me (mom who has an MBA in int'l business marketing - no electricity courses there!). I'm calling for a lifeline here! Jenny
     
  11. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Hi Jenny,

    I think your son has good intentions, but the problem is difficult or impossible, as described above.

    Measuring the resistance of each cube with that meter will give you ambiguous results at best, and probably just plain wrong results, because so much will depend on the quality of the contact between the probe and the metal cube. I am very much opposed to teaching children at any age "wrong science." See post #13

    What good science could you do? Do you have a thermos bottle? Do you have a good thermometer?

    1) You could measure the change in density of each cube with temperature. Again, that will involve measuring a very small change.

    2) You could measure the "specific heat" of each metal. For example, if you heat each in boiling water (100°C) and put it in a thermos full of water (say 200 to 400 mL -- be sure to keep the amount of water constant for each metal and experiment), then let it equilibrate. The temperature rise of the water will vary with the metal. Divide that change by the weight (i.e., mass) of each metal to cancel out differences in size. (Your actual result will be something like degrees temperature change per gram of metal.) Then do the same after cooling the metals in the freezer, etc. You can use a glass bowl in an insulated styrofoam container instead of the thermos.

    If you want to throw in something to do electronics, you could use a thermistor or thermocouple and your digital meter to make the temperature measurements. I would just use a simple digital thermometer.

    BTW, I am a grandpa and have gone through a bunch of science fairs with my 4 children and am now looking forward to helping the grandchildren. Sorry to hear about father/father-in-law.

    Given the diverse backgrounds of people on this forum, I am sure other ideas will surface throughout the day.

    John
     
  12. PIC_User

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    Sep 22, 2008
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  13. Jennysa

    Thread Starter New Member

    Nov 17, 2009
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    I appreciate your suggestions and candor. I will ask my son to talk to his teacher about extending the time and possibly doing a different experiment.
     
  14. jpanhalt

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    Jan 18, 2008
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    FYI,

    I looked up specific heats. Don't know if you are used metric or imperial/common units. The units can be daunting-- something like calories/gram per degree C. Or the equivalent in joules. I wouldn't worry too much about the exact units. Basically, what they say is that for each degree C that one gram of the metal changes, it is able to heat or cool 1 gram of water so any degrees.

    Example: 1 g of aluminum cooling 1 °C would heat 1 g of water 0.21°C

    The relative heats are: aluminum, 0.21; iron, 0.12; zinc = brass= copper, 0.092. So, that means if you heat the aluminum to 100°C it will cause the temperature of the water to rise almost twice as much as the same mass/weight of iron would produce, etc.

    Of course, aluminum is a lot lighter than iron (less dense), so your results with the same sized chunks will be quite different. But once he does the calculations, he will appreciate why the definitions are important to know.

    John
     
  15. steveb

    Senior Member

    Jul 3, 2008
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    Just a quick suggestion.

    Why not simplify the experiment to aluminum and copper only (or maybe add steel too)? Aluminum, copper and steel is readily available in wire form. A thin and long wire will have resistance that is much easier to measure.

    The wire can be insulated and spooled and then heated and cooled, to provide two measurement points.

    Using caution, the resistance can be measured in boiling water, using a plastic bag to keep the electrical part dry.

    The resistance can also be measured in a water ice mixture. This will give two fairly well controlled temperature points to measure; 0 C and 100 C.
     
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