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Series and parallel circuits — study guide
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Two ways to wire the same parts
Once you have more than one component in a circuit, there are exactly two basic ways to connect them, and the choice changes everything about how they behave. You can string them one after another so charge must pass through each in turn, or you can hang them side by side so charge can go through one or the other. The first is a series connection; the second is a parallel connection.
The remarkable part is that the same two parts, with the same source, do genuinely different things depending only on which of these two shapes you wire them into. The current that flows, the voltage each part gets, and the total opposition the source feels all come out differently. The wiring is not a detail; it is the design.
Every real circuit is some mixture of these two moves, nested and repeated, but the whole of it is built from just these two. Get the behavior of a pure series string and a pure parallel set clear, and the rest is bookkeeping. What follows is the behavior of each, one rule at a time.
Series: one path, one current
A series string is a single loop with no forks in it. Charge leaving the source has exactly one route: through the first part, then the second, then whatever else is in the line, and back. Because there is nowhere for charge to pile up and nowhere else for it to go, the current through every part in a series string is the same like a single-file line with no side doors: whatever number of people pass one point each minute must pass every point, because there is no other way through.
That single shared current is the defining fact of a series connection, and it has a blunt consequence: the parts are not independent. Whatever limits the flow anywhere limits it everywhere, because it is one flow. Put a part that chokes the current into a series string and every other part in that string feels the same reduced current.
It also means a series circuit is fragile in a specific way. Break the loop at any single point — a switch opens, a wire pulls loose, one part fails open — and the current does not just stop at the break; it stops everywhere, because the one path is gone. A string of old holiday lights where one dead bulb kills the whole strand is series wiring showing its nature.
Series: the voltage is shared out
The source provides a fixed push across the whole series string, and that push gets shared out among the parts. Each part takes a portion of the total voltage as charge moves through it, and the portions add back up to exactly the source voltage like a staircase between two floors: the total drop from top to bottom is fixed, and it is shared out step by step, each step taking part of the whole descent. None of the push goes missing and none appears from nowhere; it is divided, not spent twice.
How the voltage splits depends on the parts. A part that opposes the flow harder takes a larger share of the push, because more of the available voltage is used getting the shared current through it; a part that barely resists takes almost none. Since one current runs through the whole string, the part with the most resistance always claims the biggest slice of the voltage.
This is why a series arrangement is the natural way to divide a voltage down. When you need less than the full source push at some point in a circuit, putting parts in series and tapping between them hands you a fraction of the total, because the total was split across them in the first place.
Series: resistances stack up
Putting resistances in series adds them together. Each part is one more stretch of hard-to-cross path that the same current has to fight through, laid end to end with the others, so the string opposes flow by the sum of the parts. Two hundred ohms follows a hundred-ohm part wired ahead of another hundred-ohm part, and the total is simply 200 ohms.
The rule has a direction worth holding onto: a series combination always resists more than any single part in it. You can only add opposition this way, never subtract it, because every part you put in the line is one more thing the current must get through. Add parts in series and the total current the source can drive goes down.
That makes series the move when you want to hold a current back. A resistor placed in series with a delicate part is the standard way to limit how much current reaches it: the added resistance raises the total opposition of the path, and since one current flows through the whole line, the protected part is spared the excess.
Parallel: many paths, one voltage
Parallel branches all connect to the same two points. Every branch starts at one node and ends at the other, so each branch is wired straight across the source. Because the two ends are shared, each branch sees the same voltage — the full source push — no matter what the other branches are doing.
That is the mirror image of the series story. In series the current was shared and the voltage was split; in parallel the voltage is shared and the current is split. Each branch draws whatever current its own resistance allows at the full voltage, and those branch currents add up at the node to the total the source delivers. A low-resistance branch draws a lot, a high-resistance branch draws a little, and the source supplies the sum.
The branches are also independent in the way a series string is not. Open or break one parallel branch and the others carry on unaffected, because each still has its own complete path across the source. Room lights on their own parallel branches keep working when one bulb burns out, for exactly this reason.
Parallel: more lanes, less opposition
Every parallel branch you add is another route charge can take across the same two points, and more routes mean more total flow for the same push like opening extra checkout lanes for the same crowd: more shoppers get through at once because there are more ways through, even though no single lane got any wider. The source is not pushing any harder, but there are now more ways through at once, so more current leaves it. More current at the same voltage is exactly what less resistance means.
So a parallel combination is less resistive than any of its branches alone — even less than the smallest one. Adding a branch can only open another path; it can never close one, so the total opposition only ever goes down as you add branches in parallel. Two 100-ohm resistors that add to 200 ohms in series combine to just 50 ohms in parallel, below either branch on its own.
The parts do not literally get bigger, and no single lane widened; there are simply more of them carrying the crowd together. That is the whole trick of parallel wiring: it multiplies the paths, and the source feels the many easy routes as one easier one.
Why the wiring is a design choice
Once a build has more than one thing to power, series versus parallel stops being theory and becomes a decision you make on purpose. Most loads want the full supply voltage and their own current, so they go in parallel: each one hangs across the supply, each gets the same push, and one failing does not take the others down. That is why the parts of a working system are usually branches off a common source, not a single chain.
For a build like a small self-driving car, that shape is everywhere. The motors, the sensors, the controller board, and any indicator lights each sit on their own parallel branch off the battery, so every part sees the voltage it expects and draws only the current it needs, and the total the battery must supply is the sum of all the branches. Reach for series instead when the design wants a shared current through a line or a voltage divided down — a resistor in series to hold the current into a delicate part within safe bounds, or two parts in series to tap a fraction of the battery's push.
Getting this right is also how you keep from cooking things. Wire in parallel what needs full voltage, put deliberate resistance in series where a current needs limiting, and add up the branch currents to check the source can supply them. The two shapes and their handful of rules are enough to reason about the electrical skeleton of almost any project you will build.