physic lab report Conductivity
Conductivity
Purpose: To explore the idea of conductivity and construct an experiment to determine the conductivity of water using graphs of collected data.
Introduction: Conductivity, σ, is the inverse of resistivity, ρ. It is the ability of a material to conduct electricity rather than the material’s resistance to the flow of electricity. Metals have the highest conductivity, with silver being the most conductive, followed by copper. The measurement of conductivity is an important concept in electrochemistry and is used to determine ionic concentration in water, one measure of water quality, among other things. Current density, J, is related to the strength of the electric field, E, through conductivity: 
. “σ” then is current density 
divided by field strength 
.
The units of conductivity are:
. The SI unit for this is Siemens per meter, or mhos per meter. Mhos, you may notice, is simply Ohms spelled backwards, and is sometimes represented by an inverted omega: Ʊ.
Procedure: Using a nifty blue box, a power source and some copper electrodes, you can set up an electric field in various materials, and with an ammeter you can determine the current flow between the electrodes.
Unfortunately, measuring conductivity is not quite as simple as measuring other electrical properties because your meter has no way of determining either the strength of the electric field, nor of the cross sectional area of the conductive material/solution. The strength of the electric field can be calculated by dividing the electrical potential difference between the two electrodes by the distance between them: 
. As long as ΔV is measured in volts and D is measured in meters, your result will be in standard units for electric fields. The current can be read off the ammeter, and the cross sectional area can easily be calculated. (How will you construct a circuit that will enable you to read voltage across the material at the same time as current through it?)
It would then seem simple enough to put those values into the above equation and calculate the conductivity, but there are issues that complicate determining conductivity with a single measurement. The contact between the electrode and the material undergoing testing can produce field effects that are unrelated to the conductivity of the material. The best way to determine the conductivity, then, is to take a series of measurements with various E field strengths and plot the current density –vs- field strength on a graph such that the slope of the graph will equal conductivity.
Analysis: Using the materials and equipment supplied, determine the conductivity of distilled water and then water with salt added (or tap water) by using excel to graph the relationship between current density 
and electric field strength . Do the same for some top secret physics compound.
Suggested additional investigation: According to the above equations, current density in a given material in a constant electrical field should not be affected by the cross sectional area of the material. Experimentally demonstrate that this is true (or not).
Lab Evaluation – Please provide feedback on the following areas, comparing this lab to your previous labs: How much fun you had completing this lab; How well the lab prep period explained this lab; The amount of work required compared to the time allotted; Your understanding of this lab; The difficulty of this lab; How well this lab tied in with the lecture. Please assign each of the listed categories with a value from 1-5, with 5 being the best, 1 the worst. Comments supporting or elaborating on your assessment can also be very helpful in improving the future labs.
Date Last Modified 17 February 2015 Page Number 3
