Metals Conductivity and Ohms Resisters

[angieb16]

In a Fifth grade science fair project using a standard DC circuit board to detect conductivity. Is there a way to use ohms resisters in order to test varying degrees of conductivity that would be visible in the brightness of the lightbulbs that come with a kit?

[rmarz]

angieb16 - Which experiment are you working on? Conductivity is a quality to be measured or calculated, not detected. It is the inverse of resistance, (as in 1/R). I just don't understand your reference to brightness regarding the kits lightbulbs. There is certainly an effect on brightness that is influenced by putting resistors in series with a bulb, or LED that reduces current in the bulb.

Rick Marz

[angieb16]

I have done the experiment testing conductivity of certain objects (an eraser, a lego, paper clip, penny and aluminum foil), and have found that of course the metals are all conducters and light the light bulbs on the circuit board. But I was hoping to take the experiment a step further by testing the amount of conductivity of each metal by using resisters. However, what we are finding is that there isn't enough resistance difference in the metals for there to be a visable difference in the amount of light produced by the bulbs. I was hoping someone could show me a way to show a visable difference in the amount of light produced in order to prove my theory of which metal has the most conductivity.

[rmarz]

angieb16 - As you stated in defining your experiment there are materials that are very poor conductors of electricity (glass, rubber and plastic) and very good conductors (metals) and many materials in between. You might want to narrow your experiment to analyze some variability between good and fair conductors. To measure the resistivity or conductivity of various materials I think you would be better served to use a multimeter as a measurement instrument rather than observe the relative brightness of the current flow through an LED. The reasons are pretty obvious, the light emitted by the LED or even incandescent lamp is non-linear over a range of current. In addition, the human eye is very subjective in judging light levels. The combination really prevents good scientific analysis of the subject at hand, conductivity. There is another problem of the way conductivity measurement is standardized. The accepted standard is to measure the resistance or conductivity of a cubic block of the material. A cubic meter, cubic centimeter or other. The measurement is made across the opposite faces of this cube. This is very difficult, if not impossible to do easily. A better approach would be to test these materials in the form of samples of the same gauge wire of the same length. Perhaps you can obtain samples of silver, copper, iron, aluminum and nichrome (an alloy) wire in a very small diameter, perhaps #30. These could be measured with a multimeter in a resistance range and could be the basis of your thesis. If same gauge wire isn't available, perhaps you could a table of AWG tables to make the math corrections to evaluate different wire gauges or diameters. You can find AWG tables on the internet. Here is a link to the general subject.

http://en.wikipedia.org/wiki/Electrical ... nductivity

Rick Marz