Resistance

Resistance is a feature of a material that determines the flow of electric charge.  When charges have been separated onto positive and negative “terminals”, and suddenly the two terminals are touched together, the charges will move very quickly to equalize the charge.  In this case, there will be a large spark which corresponds to a huge instantaneous current (transfer of charge over a very short time).  On the other hand, if the terminals are in a vacuum, no current will flow between them.  Any intermediate situation will give some intermediate current.  The “resistance” of a given piece of material placed between two terminals with difference in voltage V is defined as:  V = i * R¹, where i is the current which flows between the two terminals when the resistance R is connected.  R can always be defined in this way.  For other materials, R is nearly independent of temperature, the voltage across it, and the current through it.  Materials with variable resistance are fascinating both for their function and their construction.  If you go on to design electrical circuits you will work with such variable resistor components as diodes and transistors, or you might worry about the breakdown voltage of gases at which current starts to flow.  But in our lab today, we will start at the beginning with materials called “resistors”, for which the “resistance” R is independent of current and voltage.  That is, a graph of voltage vs current would be a straight line, with slope R.