Electrical Circuits

The most fundamental requirement for electrical technology to be useful is that you have to be able to get electrons to move when and when you want them to. In order for electricity to flow it has to have a Source and a Path. The source provides the electrons and the motivation (voltage) to make them move. Examples of sources are batteries, generators, fuel cells and solar panels. A path can be any material with high electrical conductivity (conductor) that runs from the source and to the ground or back to the source. Common examples of conductors are copper, aluminum and gold. Take away, or disconnect, either the source or the path and electricity will not flow.

Short Circuit

A short circuit is created when a conductive path is created between the source and ground or back to the source. A short circuit allows electricity to flow with very little resistance. The energy from the source is converted into electrical energy (electricity) when the electrons flow through the conductor. If your using a battery a the source, it will be dead very quickly. Energy can not be created or destroyed, only converted from one form to another (Law of Conservation of Energy). Without anything else in the circuit to use that electrical energy it has to be release somehow. The electrical energy is converted into heat energy. The conductor and the battery will heat up and the heat energy will be radiated or conducted away. Because short circuits create a lot of heat they are a severe fire and burn hazard. To avoid these negative consequences many safety components have been developed to prevent short circuits. Wires are covered with insulation (low conductivity) materials like plastic. Fuses, circuit breakers, and 'ground fault indicators' are all used to stop the flow of electricity if a short circuit is detected.

Complete Circuit

A complete circuit has a source and a path, but also has a component called a Load that uses the electrical energy. Loads can convert electrical energy into mechanical, thermal, or electromagnetic energy. For example, you could create a circuit that connects each side of a battery to a light bulb with wires. The electricity flows from the battery, through the light bulb (producing light) and then back to the battery. This is like the circuit you might find in a flashlight. The electrical energy is used by the light bulb to produce light. Because the wire (a conductor) still has some resistance, some of the energy will still be converted into heat and the light bulb most likely produces some heat (infrared radiation) in addition to the visible light. The efficiency of a circuit tells you how much of the energy is being uses and how much is wasted. Florescent light bulbs are more efficient than incandescent light bulbs, but modern LED light bulbs are more efficient than both of them. Incandescent light bulbs produce the most heat and LED light bulbs produce the least amount of heat.

Good Complete Circuit

While a complete circuit can be safe and can get work done, it will not work well for long unless you add in some Control components. Take that flashlight circuit as an example. It will produce light, but without a switch to turn it on and off it will make light when you don't need it to and the batteries will soon be dead and no longer make light when you do need it. To make this a good complete circuit you need to add a switch so that you can turn it on whenever you need it and off to save the battery for later. Some examples of control components are switches, resistors, potentiometers, capacitors, transformer (not the science fiction robots), transducers, and transistors. The purpose of all of these is to get the right amount of electricity to flow the right places at the right times.

Series and Parallel Circuits

Series Circuits

In a series circuit a path is created that allows electricity to flow from the source, through each component and back to the source such that there is only one path for the electrons to take. The electricity flows through a series of components lined up one after another. This is the simples kind of circuit to understand because you have given the electricity no choice in when it goes. Calculating Voltage, Resistance and Current in series circuit is rather easy. When two or more batteries are connected in series, their voltages are added together (two 9volt batteries in series produce 18volts). Resistors connected together in series are also simply added together to get the total resistance for the circuit (100 ohms + 50 ohms + 80 ohms = 230 ohms). Once you know the total voltage and total resistance you can use Ohm's Law to calculate the current flowing through the circuit.

Parallel Circuits

When a circuit is created that provides more than one possible path for the electricity to flow through it is call a parallel circuit. A simple parallel circuit might has to light bulbs connected to the same battery such that there are two paths the electricity can flow, one through each bulb. Some of the electricity flows through the first bulb and some of the electricity flows through the second bulb. By sharing the electricity in this way, and if both paths are identical, both bulbs receive the same amount of voltage from the battery. When batteries are placed in parallel their voltages are NOT added together but rather the life of the batteries are extended (two 9volt batteries in parallel will provide 9volts to the circuit for a longer time than just one 9volt battery). When you provide more then one path for the current to flow through multiple loads you actually lower the total resistance in the circuit. Calculating the total resistance in a parallel circuit is more complicated (1/R_t = 1/R₁ + 1/R₂ ...) but remember the total resistance will always be lower than the smallest resistance of any branch. For example if one branch has a light with 1000 ohms of resistance and another branch with only 100 ohms of resistance, then the total resistance will be less than 100 ohms. It is even more complicated to calculate the current in parallel circuits. Once you know the total voltage and total resistance you can calculate the total current flowing through the circuit, but that will not tell you how much current is flowing throw any one load. The current in each branch of the parallel circuit will be inversely proportional to the amount of resistance in that branch compared to the total resistance (the branch with 100 ohms of resistance will have 10 times more current than the branch with 1000 ohms of resistance).

Parallel & Series Circuits

As you work with more and more complex electrical circuits it is very comment to find circuits that have components in both parallel and in series. To calculate the voltage, resistance and current in such a complex circuit you must first find the total resistance in each of the parallel circuits before you can calculate the total resistance for the entire circuit. Starting with the total voltage you then have to calculate the voltage drop in each of the parallel circuit.