I’m in electrical school and we’re learning about series circuits and I was just randomly thinking about voltage drop across devices and was wondering how does electricity “know” to not have the voltage drop to zero at the first device in a series circuit and to drop the voltage according to its individual value of resistance compared to the total resistance of the circuit and divide it accordingly across every device on the circuit until it reaches zero.
In: Physics
You can use water pipes as an analog. It’s not the same layout as your circuit but if you have a water pipe connected to a source, and the pipe forks into two, with A going really far to a dead end, and B leading to somewhere else, the water will actually flow into both equally at first. When the water in A reaches the dead end, it backs up all the way to the fork, then new water from the source can’t go down A, so it goes down the path of least resistance which is B.
The big difference with electricity is that this happens at near the speed of light. The point is electricity doesn’t know, it tries all paths and favours ones that are easier to go down with a voltage relative to the resistance.
The “voltage” across pipe B is zero because it’s equally backed up all the way to the end (infinite resistance). If you were an ant in pipe A, the pressure would be the same all around you with no clear direction, which means no voltage differential. You can measure this in a real circuit as a voltage blip that quickly settles to near-zero (reality is messy) in the dead end wire as it “figures out” it doesn’t lead anywhere.
It doesn’t know. In fact, *initially*, it doesn’t nicely divide the voltage drops across every element at all, there are transient effects. They’re just so fast in most circuits that you don’t notice them and we achieve the steady state values “instantly”.
The reason it *ends up* that way is that’s the only solution where everything is stable in time. If the values were anything else there’d be an imbalance between voltage, current, and resistance somewhere in the circuit and (at least) one of them would be changing.
At the initial instant you connect the circuit the current is zero. The voltage on one side of the first device is the supply voltage, the voltage everywhere else is 0. The first device sees the *full* voltage…current spikes…this is “inrush current”. As soon as it starts to flow you start getting a voltage drop across that device. Now the volage on the other side is something less than supply and the next device down the line sees that lower voltage, which starts pushing current through that device, and so on. The voltages and currents keep bounding around in the circuit until everything stabilizes and *that* the value that you calculate using the basic resistance rules. This happens *very* quickly in normal circuits. Once you get to oscillating circuits and inductors this should make way more sense.
It’s not that “it knows”, it’s how the physics works out. Lemme explain.
So let’s say we have a power supply that has a dial to select what voltage it should supply, and assume it is able to provide infinite current. Let’s also just say we set it to 10 volts. If you take your circuit and then connect it in series with the power supply, then the voltage drop across your entire circuit will be 10 volts and the power supply will output whatever amount of current is necessary (or rather, the circuit will restrict the current to be at a certain level). With our theoretical super power supply, this is always true assuming the circuit has no other power supplies connected. If you halve the voltage to 5 volts, the current going through the circuit will also drop by half, since the resistance of the circuit is unchanged.
Since one end of your circuit is at +10/5 volts and the other end is at 0 volts, there’s no point in the middle of the circuit that can possibly be at 0V. (assuming that circuit does not contain any additional power supplies like I mentioned before, and also assuming its only connection to ground is the one it shares with the power supply)
They share a resistance value. If the resistance is the same in both in testing you would find half and half of the voltage needed to power each unit. While the output of the second element you should find 0volts. The outlet or ground side of the first you would see 120v if grounded and working properly.
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