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          Macroscopic matter is made of atoms and molecules bound together by electrical forces.  The atoms are composed of electrically neutral neutrons, positively charged protons, and negatively charged electrons.  In batteries, and other sources of electrical power, positive and negative charges are separated usually by some mechanical process.  Since unlike charges attract each other, work must be done to separate them.  


           You probably have already studied conservation of (mechanical) work and energy in Newtonian mechanics.  Electrical energy is simply another form of energy, and what we have found is that all kinds of energy can change form, from one kind to another, but the total energy of an independent system is conserved.  Recall that energy cannot be created or destroyed, but changed from one form to another.  Under the right conditions, we can get back the work we put into separating the charges.  A source of electricity is rated by the “voltage”, or work per unit charge which could be recovered if a unit positive charge moved from the “positive” side of the battery or electricity source to the “negative” side.  (or, which is what usually happens, a unit negative charge moved from the “negative” side of the battery to its “positive” side).

The flow of charge through a conductor is called the current and it is the rate at which electrical charges pass an observer’s station per unit time is called the electrical current.  We will not discuss conductors in depth here, but we will define a conductor as a material which allows charge to pass through it freely.  Since Voltage is work/unit charge and current is charge/unit time, we see that Voltage * Current is Work/time, which is power, or the rate at which work is done.  The utility company charges by the total work (or power * time).  You should know how much power an electrical device can use, and how much power your electrical power source provides, before connecting any device to an electrical power source.  Not knowing could lead to circumstance which are both dangerous and costly.

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.