## 20150311

### Presentation: circuit basics

Wet plywood. Wires clipped to nails. Wires connected to a source of 15,000 volts. What could be more beautiful than this basic circuit? Or dangerous? (Video link: 15,000 volts.")

In this presentation we will take a first look at the basics of basic circuits. As with the set-up above, some of these very basic circuits are also very dangerous, so unless you absolutely know what you're doing (that is to say, you know enough physics to understand the perils involved), do not try these at home!

The most basic circuit we can build will have an ideal "electromotive" (emf) source of voltage connected to a resistor, such that charges can flow continuously around and around. Peculiarly the definition of current is the amount of positive charge (in coulombs (C)) that circulates per time (in seconds (s)), and these units of C/s are defined as amps (A). But as discussed in a previous presentation, in a conductor it is actually the negatively charged electrons that are free to move. So by convention we refer to "current" flowing clockwise through this circuit, while the electrons actually circulate in the opposite counterclockwise direction through this circuit. Just keep watching this GIF animation for a while until you get used to those current direction concepts. More on emf sources and resistors below, when you're ready.

An ideal battery uses chemical reactions that exchange charges in order to release electric potential energy, giving potential (that is, electric potential energy per charge, or voltage, measured in volts (V)) to the charges that circulate in a circuit.

Different chemical reactions will release different amounts of electric potential energy per charge, and thus different amounts of voltage. Note that these different batteries all have the same "AA" size, but the different chemical reactants inside (nickel metal hydride (NiMh), alkaline, or lithium) release different amounts of voltage (∆V = 1.2 V, 1.5 V, or 3.6 V, respectively). (Ideally the amount of voltage provided will be constant; but later we'll consider the effects of depleting the reactants inside "real" batteries, and the effect this has on their actual voltage output over time.)

Some larger voltage batteries are made up of a combination of individual batteries, in order to "stack" the voltage output ∆V, which is cumulative provided that their terminals are connected (+) to (-), etc. The result of stacking six individual 1.5 V alkaline batteries results in a single 9.0 V battery, as seen in several different stacking configurations.

The other part of a basic circuit is a resistor, which uses up voltage (electric potential energy per charge) as current flows through it. Different types of materials and shapes and sizes will result in different resistance values, measured in ohms (Greek letter Ω). (This is the inverse of conductance, so a good conductor (such as a metal wire) will have a low resistance value, while a poor conductor (such as an insulator) will have a high resistance value.)

If several resistors (here, Christmas light bulbs) are strung together in a line, forcing current to flow through each one in turn, then the equivalent resistance is their individual resistances added together. (This is not the only possible way to wire together resistors, but we'll stick to this basic configuration for now.)

Ohm's law can be applied to a basic circuit to determine how much current will flow in it, given the total amount of voltage from ideal batteries, and the equivalent resistance of all the resistors. Note how the different units are related in Ohm's law: a volt over an ohm is equivalent to an ampere, etc. (After a certain point you will just have to trust that all these units will work out in the end.)

So let's look at some very basic, very dangerous circuits.

We can use a wire (which has a very low resistance) to complete a basic circuit, connecting it to the (+) and (-) terminals of a 9.0 V (ideal) battery. Note the very small spark of current that leaps across the gap just as the wire completes the circuit. Now an absurd configuration of 244 9.0 V batteries are stacked with (+) terminals to (-) terminals. (How much emf voltage is that?) When a wire is connected to complete this stacked battery circuit, how does the amount of emf voltage compare to the single 9.0 V battery circuit? How does the amount of current compare to the single 9.0 V battery circuit? (Video source: "Fun with a few 9V batteries. (244 of them).")

Our next very dangerous basic circuit involves deionized water, itself a relatively poor conductor, as there are no free charges in it to transport current, so connecting a basic circuit of water with a household 120 V outlet as an emf source (where the light bulb is used to indicate the amount of current that is flowing) doesn't yield much current. When salt is poured into the water, introducing charged sodium (Na+) and chloride (Cl-) ions, how did the amount of resistance of this circuit change? How did the amount of current through this circuit change? (Video source: "Experiment electricity with saltwater.")

Our last very dangerous basic circuit is a power transformer used as an emf source, with a metal screw used to complete the circuit. This will result in a "short circuit," which is due to a very large or very small resistance? Does a very large or very small current result? (Video source: "The Metal Melter.")

In subsequent presentations we will go over more specific rules of circuit analysis for more complex configurations of emf sources and resistors, and power dissipation.