5.2. Simple series circuitsLet's start with a series circuit consisting of three resistors and a single battery: The first principle to understand about series circuits is that the amount of current is the same through any component in the circuit. This is because there is only one path for electrons to flow in a series circuit, and because free electrons flow through conductors like marbles in a tube, the rate of flow (marble speed) at any point in the circuit (tube) at any specific point in time must be equal. From the way that the 9 volt battery is arranged, we can tell that the electrons in this circuit will flow in a counterclockwise direction, from point 4 to 3 to 2 to 1 and back to 4. However, we have one source of voltage and three resistances. How do we use Ohm's Law here? An important caveat to Ohm's Law is that all quantities (voltage, current, resistance, and power) must relate to each other in terms of the same two points in a circuit. For instance, with a singlebattery, singleresistor circuit, we could easily calculate any quantity because they all applied to the same two points in the circuit:
Since points 1 and 2 are connected together with wire of negligible resistance, as are points 3 and 4, we can say that point 1 is electrically common to point 2, and that point 3 is electrically common to point 4. Since we know we have 9 volts of electromotive force between points 1 and 4 (directly across the battery), and since point 2 is common to point 1 and point 3 common to point 4, we must also have 9 volts between points 2 and 3 (directly across the resistor). Therefore, we can apply Ohm's Law (I = E/R) to the current through the resistor, because we know the voltage (E) across the resistor and the resistance (R) of that resistor. All terms (E, I, R) apply to the same two points in the circuit, to that same resistor, so we can use the Ohm's Law formula with no reservation. However, in circuits containing more than one resistor, we must be careful in how we apply Ohm's Law. In the threeresistor example circuit below, we know that we have 9 volts between points 1 and 4, which is the amount of electromotive force trying to push electrons through the series combination of R_{1}, R_{2}, and R_{3}. However, we cannot take the value of 9 volts and divide it by 3k, 10k or 5k Ω to try to find a current value, because we don't know how much voltage is across any one of those resistors, individually. The figure of 9 volts is a total quantity for the whole circuit, whereas the figures of 3k, 10k, and 5k Ω are individual quantities for individual resistors. If we were to plug a figure for total voltage into an Ohm's Law equation with a figure for individual resistance, the result would not relate accurately to any quantity in the real circuit. For R_{1}, Ohm's Law will relate the amount of voltage across R_{1} with the current through R_{1}, given R_{1}'s resistance, 3kΩ: But, since we don't know the voltage across R_{1} (only the total voltage supplied by the battery across the threeresistor series combination) and we don't know the current through R_{1}, we can't do any calculations with either formula. The same goes for R_{2} and R_{3}: we can apply the Ohm's Law equations if and only if all terms are representative of their respective quantities between the same two points in the circuit. So what can we do? We know the voltage of the source (9 volts) applied across the series combination of R_{1}, R_{2}, and R_{3}, and we know the resistances of each resistor, but since those quantities aren't in the same context, we can't use Ohm's Law to determine the circuit current. If only we knew what the total resistance was for the circuit: then we could calculate total current with our figure for total voltage (i="E"/R). This brings us to the second principle of series circuits: the total resistance of any series circuit is equal to the sum of the individual resistances. This should make intuitive sense: the more resistors in series that the electrons must flow through, the more difficult it will be for those electrons to flow. In the example problem, we had a 3 kΩ, 10 kΩ, and 5 kΩ resistor in series, giving us a total resistance of 18 kΩ: In essence, we've calculated the equivalent resistance of R_{1}, R_{2}, and R_{3} combined. Knowing this, we could redraw the circuit with a single equivalent resistor representing the series combination of R_{1}, R_{2}, and R_{3}: Now we have all the necessary information to calculate circuit current, because we have the voltage between points 1 and 4 (9 volts) and the resistance between points 1 and 4 (18 kΩ): Knowing that current is equal through all components of a series circuit (and we just determined the current through the battery), we can go back to our original circuit schematic and note the current through each component: Now that we know the amount of current through each resistor, we can use Ohm's Law to determine the voltage drop across each one (applying Ohm's Law in its proper context): Notice the voltage drops across each resistor, and how the sum of the voltage drops (1.5 + 5 + 2.5) is equal to the battery (supply) voltage: 9 volts. This is the third principle of series circuits: that the supply voltage is equal to the sum of the individual voltage drops. However, the method we just used to analyze this simple series circuit can be streamlined for better understanding. By using a table to list all voltages, currents, and resistances in the circuit, it becomes very easy to see which of those quantities can be properly related in any Ohm's Law equation: The rule with such a table is to apply Ohm's Law only to the values within each vertical column. For instance, E_{R1} only with I_{R1} and R_{1}; E_{R2} only with I_{R2} and R_{2}; etc. You begin your analysis by filling in those elements of the table that are given to you from the beginning: As you can see from the arrangement of the data, we can't apply the 9 volts of E_{T} (total voltage) to any of the resistances (R_{1}, R_{2}, or R_{3}) in any Ohm's Law formula because they're in different columns. The 9 volts of battery voltage is not applied directly across R_{1}, R_{2}, or R_{3}. However, we can use our "rules" of series circuits to fill in blank spots on a horizontal row. In this case, we can use the series rule of resistances to determine a total resistance from the sum of individual resistances: Now, with a value for total resistance inserted into the rightmost ("Total") column, we can apply Ohm's Law of i="E"/R to total voltage and total resistance to arrive at a total current of 500 µA: Then, knowing that the current is shared equally by all components of a series circuit (another "rule" of series circuits), we can fill in the currents for each resistor from the current figure just calculated: Finally, we can use Ohm's Law to determine the voltage drop across each resistor, one column at a time: Just for fun, we can use a computer to analyze this very same circuit automatically. It will be a good way to verify our calculations and also become more familiar with computer analysis. First, we have to describe the circuit to the computer in a format recognizable by the software. The SPICE program we'll be using requires that all electrically unique points in a circuit be numbered, and component placement is understood by which of those numbered points, or "nodes," they share. For clarity, I numbered the four corners of our example circuit 1 through 4. SPICE, however, demands that there be a node zero somewhere in the circuit, so I'll redraw the circuit, changing the numbering scheme slightly: All I've done here is renumbered the lowerleft corner of the circuit 0 instead of 4. Now, I can enter several lines of text into a computer file describing the circuit in terms SPICE will understand, complete with a couple of extra lines of code directing the program to display voltage and current data for our viewing pleasure. This computer file is known as the netlist in SPICE terminology:
series circuit v1 1 0 r1 1 2 3k r2 2 3 10k r3 3 0 5k .dc v1 9 9 1 .print dc v(1,2) v(2,3) v(3,0) .end
Now, all I have to do is run the SPICE program to process the netlist and output the results:
v1 v(1,2) v(2,3) v(3) i(v1) 9.000E+00 1.500E+00 5.000E+00 2.500E+00 5.000E04
This printout is telling us the battery voltage is 9 volts, and the voltage drops across R_{1}, R_{2}, and R_{3} are 1.5 volts, 5 volts, and 2.5 volts, respectively. Voltage drops across any component in SPICE are referenced by the node numbers the component lies between, so v(1,2) is referencing the voltage between nodes 1 and 2 in the circuit, which are the points between which R_{1} is located. The order of node numbers is important: when SPICE outputs a figure for v(1,2), it regards the polarity the same way as if we were holding a voltmeter with the red test lead on node 1 and the black test lead on node 2. We also have a display showing current (albeit with a negative value) at 0.5 milliamps, or 500 microamps. So our mathematical analysis has been vindicated by the computer. This figure appears as a negative number in the SPICE analysis, due to a quirk in the way SPICE handles current calculations. In summary, a series circuit is defined as having only one path for electrons to flow. From this definition, three rules of series circuits follow: all components share the same current; resistances add to equal a larger, total resistance; and voltage drops add to equal a larger, total voltage. All of these rules find root in the definition of a series circuit. If you understand that definition fully, then the rules are nothing more than footnotes to the definition.
