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Series, PaXattat, & Combination Circuits

Benjamin Root

The best way to get a grip on what is actually happening in any electrical circuit is to understand the relationships between Volts, Amps, and Ohms in Ohm's law. Let's explore how these relationships change depending on the layout of the circuit and its components.

Series

A circuit is considered to be "in series" when all components are connected in such a way that there is only one possible path for current to flow. This means that each voltage source, switch, load, or other component is in succession with the other components of the circuit. Electrons flowing through such a circuit flow through every component in turn. Figure #1 shows a pictorial representation of a simple series circuit with a battery voltage source and two light bulbs as loads. Figure #2 shows a schematic representation of the same circuit. If you trace the path of an electron as it flows from the negative terminal of the battery through the circuit to the positive terminal you will notice that it passes through one lightbulb (R1) before it passes through the other lightbulb (R2). There is no way for an electron to pass through bulb 2 (R2) first, nor is there any way for an electron to pass through one bulb but not the other. The electron flow (current) follows the one and only path through the circuit.

You may remember Christmas tree lights of the past. When one bulb burned out (creating an open circuit) all of the bulbs would turn off. This is because the burned out bulb had interrupted the current flow in the only current path. With no current in the circuit no bulbs will light.

Resistor (in this case a light bulb)

Current in a Series Circuit

In 1847 a German physicist named Gustav Kirchhoff made a statement about the behavior of electrons in a circuit. "Kirchhoff's law" says that for every electron that enters a circuit another electron leaves the circuit.

We can model this concept if we imagine a pipe with a diameter just big enough to accept a golf ball (see figure #3). If we fill this pipe with golf balls we have a model of a copper wire where golf balls represent electrons of copper atoms. If we push a new golf ball into one end of the pipe a golf ball will fall out of the other end of the pipe. If we push ten golf balls into the pipe in one minute, then ten golf balls will fall out of the other end in that minute. If we drilled a little hole anywhere

10 balls in

Fig. #3 Pipe is full of balls in the pipe, just big enough to peek in, we would count ten golf balls going by that point in one minute. Ten golf balls a minute is a rate in the same way that 6.28 X 1018 electrons in a second (one amp of current) is a rate.

In a series circuit, just like in the pipe, current flow is the same throughout the circuit. No matter how many loads, in any order, current will be consistent everywhere. Knowing the amperage of a series circuit makes it easier to use Ohm's law to analyse that circuit.

Fig. #3 Pipe is full of balls

10 balls past any point

10 balls out rj

Resistance in a Series Circuit

The total resistance of a series circuit is merely the sum of all the individual resistances.

This is easy, the total resistance (Rt) of the circuit in figure#2 is the sum of R1 + R2 or 5Q. The formula holds true for resistors of any value as long as they are in a series string. RT is a valuable component for applying Ohm's law to the circuit as a whole.

Voltage in a Series Circuit

The Voltage of a series circuit is a bit trickier to determine than Amperage or resistance. The total Voltage (ET) of a circuit can be figured using Ohm's law.

Unlike amperage however, voltage is not the same throughout the circuit. ET is the voltage as it is measured across the two terminals of the voltage source, including the whole circuit and all resistors. The voltage measured across any single resistor varies with the size of the resistance. Look again at Figure #2. If we use Ohm's law to solve for voltage across R1 we get 4.8V (ER1 = 2.4A X 2Q). This 4.8V is called a voltage drop because it reduces the voltage available in the rest of the circuit. Lets continue by solving for voltage across R2. ER2 = 2.4A X 3Q; the voltage drop across R2 is 7.2V. This makes sense, as you may have noticed, the sum of the voltage drops equals the total applied voltage.

'Total = 10 AmPs (main Lines only)

fuses are wired in series with the circuit that they protect. But loads themselves are usually wired in parallel for the same reasons that Christmas lights are no longer made in series. See the side bar on voltage sources for an example of a use for series wiring in renewable energy systems.

Negative main line (between - terminal & junction)

'Total = 10 AmPs (main Lines only)

Negative main line (between - terminal & junction)

12volts-

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