Electric Circuits


Electricity has been with us since our beginnings, even if those humble cavemen weren't able to realize it. This electricity comes in many shapes and sizes, from the electricity that flows from the clouds, to the neurons that control the skeletal muscles in the arms. At one point, when people became more aware of this electricity around them, it fascinated and frightened them, some even came to believe it was some form of magic. Even though electricity has been around us for a long time, we have not come to realize what it really was until a few hundred years ago. Benjamin Franklin is credited with the discovery of electricity; however, this is not actually the case. An Italian scientist, Alessandro Volta, was the first person to make a successful battery, a device which provides direct current to a closed circuit. Later, Michael Faraday realized that you can make an electrical current by passing a magnet through a copper wire. Thomas Edison accelerated the field of electricity with his innovation design for a light bulb, a sealed glass bulb with a filament that creates light when current is passed through it, however it was expensive. They where run by battery, so Edison got to work to find a more practical way to supply energy, seeing as how batteries did not last very long and had to be replaced. He built one of the first working power plants and was able to supply a small town with the electricity needs. However, that is all that it could supply, the Direct Current that it used was not able to supply to those further from the factory. Fortunately the invention of the
Image of a circuit on a pc board.  Image Courtesy of Narrow-bandwidth Television Association
Image of a circuit on a pc board. Image Courtesy of Narrow-bandwidth Television Association
alternating current,
extended the range at which all electric currents could be carried, allowing for more efficient power plants, and more people touched by an electrifying revolution. However grand this revolution came from electricity, There where those who feared this change, and even poets thought of the electrical light as unromantic compared to a gas lit lamp. However, those poets couldn't stop the technological revolution that unfolded, with further advancements and enhancements to the simple circuits created by some of the most notable scientists in history.[1][2]



A Simple Circuit


In essence, a light bulb and a battery is one of the easiest and simplest electrical circuits to make. All it requires is a battery, a bulb, and some wire or other conducting metal that can be shaped easily. The most important thing when creating the circuit is to have a complete circuit, as in everything is connected. Now, Where do we connect the wires? In the light bulb there are two wires that are connected by a filament, those wires travel down and are connected to the base of the bulb. One of the wires is attached to the very bottom, while the other is attached to the larger mass of metal at the base. Connect the two terminals of the battery to the individual connectors on the light bulb and you have light, disconnect one of the terminals and the light disappears. How this works is simple, in the battery the electrons will want to move through the circuit to get to the other end of the battery. While along the conducting wires the electricity will only move in a single direction, however, inside the battery current if moving in both directions at the same time. Along the way the current will pass through the filament of the bulb, where they will lose their charge and spit light photons out. This will create light and heat. Then the current flows from the filament to the other end of the battery, where they will be recharged in the battery and flow through the circuit until the connecting wires are disconnected, breaking the circuit. For those who need more of an explanation, the flow of an electric current through a circuit can be compared to water flowing through a pump. The battery pushes current through the wires, like a pump pushes water through pipes. Water, like electricity will flow all the way back to the pump as long as all the pipes are connected and running back into the place that the pump is drawing water from. Then the pump will force the water back through the pipes until it runs out of power or until at least one of the pipes are closed off, like how disconnecting the wires from the battery or the bulb will close off the circuit and current stops flowing through. [3] [4] [5]

Electric Circuits


An electric circuit is simply a closed pathway that creates a loop with a power source, meaning that there has to be a complete circuit, which has no breaks between connectors. Again, water is very similar to electricity as it has to be connected completely, without any gaps, to flow through the pipe. However, there are distinctions, unlike water in a pipe, when there is a break in an electrical circuit it will cause the electric current to stop flowing through the circuit. Now, electricity can flow through the circuit, but why would we need the current to flow through it? As current flows through a circuit it will pass through any load, an object in a circuit that produces work, that it is wired to get to the end of the circuit. Now this load could be anything from a DC motor to the coils in a space heater, but the thing they have in common is that they both utilize the electrical charge to produce work.

There are a few types of circuits that exist. One of them is a series circuit, this is set up to where the electrical current has multiple loads to pass through, but they are aligned to where the current only has a single path it can travel along. Series circuits would be used in applications that need to be as small as possible, like Christmas lights. Another would be a parallel circuit, where the current has multiple loads, but also has multiple paths that it has to take. These are typically larger than a series circuit, but it provides extended flexibility and reliability. It is possible to have a series-parallel circuit. This circuit would incorporate both a series circuit and a parallel circuit within the same circuit.

There is a multiplicity of laws that pertain to electrical currents. One of the most notable ones is Ohm's law. But to understand Ohm's law there are three fundamental components that stand in the way. The first is current, this is the movement of electrons in the circuit. Next there is voltage, the pressure of the electrons applied to the circuit. Lastly, there is resistance, which is a measure of the opposition an object gives to current passing through it. Now for Ohm's law,
equation_(3).png
where V = voltage in volts, I = current in amperes, and R = resistance in ohms
. Basically, this says that the current is proportional to the voltage divided by the resistance.

[6][7][8]

Series and Parallel Circuits


elec_ill76.JPG
Image courtesy of Contractor Talk
Electric circuits come in many shapes and sizes, two wires and a battery, or the more complex circuit boards that CPUs use to function. There are two fundamental circuit designs: the series circuit and parallel circuit.

The Series Circuit


A series circuit is a circuit that has multiple loads and a single path to go through. Such as a circuit that is connected with a battery and three light bulbs. First a current has to connect to one load, then the other, and finally it will flow through the last load and back to the battery. Every time the current passes through the load it will encounter resistance. Since it only has one path through multiple loads, using Ohm's Law , the current can be calculated with this
equation_(4).png .
Meaning that the resistance produced by every load will be incorporated in lessening the current. For example, if we have a 40V battery and three loads that created a resistance of 2 ohms each, then the current will be
equation_(5).png,
giving a current of about 6.7 Amps. As for the voltage, it is divided among the loads. If the resistance of every load is equal, than the voltage in every load throughout the circuit will be the same. However, if one or more of the loads has more resistance, then it will use more voltage. Using the last circuit, except replacing one of the 2 ohm resistors, any device that provides resistance to the current, with a 4 ohm resistor, as the voltage dissipates in the first resistor, using 10V, the second one uses another 10V, then the last 4 ohm resistor will use the remaining 20V.[9 ]

The Parallel Circuit


Like the series circuit, the parallel circuit passes through more than one load. However, the circuit gives the current more than one path to complete the circuit with. Since it has multiple paths, the current will encounter less resistance by moving through all of the paths at the same time. The current in a parallel circuit depends on the different resistances, but a parallel circuit will create less resistance on a current. For example, a series current with three 2ohms of resistances in each load will have 3x the resistance, 6ohms. However, a parallel circuit with three 2ohms of resistance will have 1/3 of the resistance. Using Ohm's law to calculate this, we would take the individual resistances of the loads and use their reciprocals, then add them and divide the voltage by it.
equation_(6).png.
As an example, using a 40V battery, and three loads that have a resistance of 2 ohms, it would be
equation_(7).png,
giving the current a final value of 60 amps. [10 ]


Schematic Diagrams



Explaining a circuit to another can become confusing. So what's a way to work around this? Draw it! But of course not everyone can draw very well, a battery may end up looking like a resistor and then the entire circuit is messed up. The way to avoid mishaps and convey the circuit as intended is to use schematic diagrams. These use simplified drawings to represent the various parts that can be found in a circuit. The circuit schematic will not show where parts are placed on a circuit, but where everything is connected at. One of the most commonly used symbols in schematic diagrams is the wire, battery, and the resistor. In the schematic the wire is represented simply by a single line extending from on part to another. It would be pointless to draw a wire that leads to nowhere, so when there is a wire there is always something connected to both ends. The symbol for the resistor doesn't stray far from a line, instead of a straight line it makes a zig-zag, except for the European symbol, which is represented by a rectangle. As for a battery, it is represented by two parallel lines, one shorter than the other to represent the negative terminal of the battery.

Almost everything that can be put on a circuit has a symbol with which to identify it with. Some of the other symbols on a circuit, that are commonly used are; switches, which look like a switch, diodes, which are triangles with a line on one side, and light bulbs, which are represented by a circle with a curved line in the middle.[11]

Compound Circuits


As circuits get more complex and more resistors are added to the mix, it becomes difficult to know exactly what is going on in the circuit. Especially compound circuits, which are circuits that incorporate at least one parallel and series wiring in the circuit. But, simplifying the circuit by using schematic diagrams will visually represent a compound circuit in it's entirety. Then it can be broken down into simpler visualizations until a single resistance can be deduced from a jumble of them. Take the circuit diagram step by step, look for any individual pair of resistors in a series or parallel circuit, and simplify it. By simplifying two resistors the equivalent resistance is found, meaning that the two individual resistors are the same as one resistor with the simplified resistance. To simplify the resistance in two resistors in a series, the individual resistances are added together. Such as if there is a series with a 5 ohm resistor, and a 6 ohm resistor then the equivalent would be 11 ohms. Regardless of the number of resistors in the series or the amount of resistance that the resistors contribute, all that is needed to find a simplified equivalent value is to add the resistances together. The parallel portions in a compound circuit are where it get's tricky. Whenever the resistances of the different resistors are equal, than the equivalent resistance can be calculated by dividing that resistance by the number of resistors. So, a parallel with two 40 ohm resistors will have an equivalent resistance of 20 ohms. However, when the resistances of the resistors are not equal, another method has to be taken to find the equivalent resistance.
equation_(8).png
With this the equivalent resistance in a parallel circuit can be calculated. As an example, a parallel circuit with one resistance of 5 ohms and the other with a resistance of 20 ohm will put in the equation, the equivalent resistance will be 4 ohms. After simplifying all of the parts of the compound circuit, the end result will be a single resistance that is equivalent to the entire circuit. [12][13]

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References


1. Introduction to Electricity
2. History of Electricity
3. HowStuffWorks "How Light Bulbs Work"
4. How to Make a Battery Light a Bulb, eHow
5. WHICH WAY DOES THE "ELECTRICITY" REALLY FLOW?
6. What Is an Electrical Circuit? wiseGEEK
7. The Basic Electrical Circuit
8. Ohm's Law
9. The Series Circuit
10. Series and Parallel circuits
11. Circuit Diagram
12. Circuit Connections
13. Compound Circuits