# Question About Current Flow Direction [closed]

I've got a question that has been perplexing me for a while.

(Correct me anytime)

(Please do not give me any textbook definitions.)

Starting with the name of the question itself, I believe current is what powers the components in a circuit, correct? At least that's what they taught me in physics.

This current is a flow of electrons which are motivated by voltage? Voltage doesn't kill you because it's a potential between two points, what kills you is the current that's an output of the voltage. In other words, voltage is required for current, but not vice-versa?

Resistance is the final piece of the bunch to make the "electric trinity" (see what I did there?) The voltage, the current, and the resistance the three amigos that form the basis of this entire field of electrical engineering and contribute to the plethora of mathematics in the subject.

Assuming all I wrote above was correct, or that I at least have the slightest idea what is going on, I still do not understand what path the current will take.

My goal is basically to analyze each line in the circuit and decide where the input of current is and where the output is. Simplified: Where it goes in and where it goes out.

Based on this diagram of a YouTube video I was watching, assist me in understanding the current flow.

(Correct me in any errors I've made during my circuit analysis, please.) This is the path I envision current taking. (Vcc->R1->R2->C1->GND) (R1->DISCHARGE->TRANSISTOR->GND) (C1->THRESHOLD & TRIGGER) (Vcc->Voltage Divider(Unnamed Resistor->Unnamed Resistor->Unnamed Resistor)->GND)

From what I can see, all current needs an input and a ground to flow, in other words the ground provides the sink of potential to motivate the electrons to flow there.

But is there something else I am missing in the analysis?

Also, if you notice C1 has an arrow shape as if it was going up back to R2. If the current just flowed down from VCC->R1->R2->C1->GND, then why does it need to go back up?

I don't get it?

Does the current not go from + (Positive/Abundant Potential) to - (Negative-Lacking Potential)?

Do equipment also have different polarity conventions?

Why does the same current enter both into the positive and negative sides of the components?

I have to be missing some concept or not understanding something because I have no idea how current flows in one way, heads out another, ignores convention goes from the source to ground and back again?

It's as if it has no laws, but abides by a plethora of them?

Thanks again, your assistance in highly appreciated.

## closed as too broad by Andy aka, Elliot Alderson, Marla, RoyC, Dmitry GrigoryevNov 19 '18 at 12:47

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• Your question has assertions that are wrong. Get rid of your incorrect assertions and ask a simple question. Simplify don't try and mystify. – Andy aka Nov 16 '18 at 18:24
• It appears that they are showing you the state of the circuit at the moment that the timer starts discharging. At that point the transistor turns on and the discharge pin is held at ground (more or less -- just say it's at ground). There's some voltage on C1, which drives current through R2 into the discharge pin. At the same time, current is going through R1, also into the discharge pin. – TimWescott Nov 16 '18 at 18:35
• What's wrong with textbook definitions? It sounds like you aren't really interested in learning if it takes any effort. – Elliot Alderson Nov 16 '18 at 21:10
• I think you would be better off using the affirmative "Please explain in layman's terms" rather than the negative "Please don't give me textbook definitions." I could see a number of reasons you may have this preference without being lazy. Clearly there is something about textbook definitions that troubles you and it might be better to define more precisely what you don't want, or simply state what you do want. There is no way to meaningfully discuss these topics without resorting at some level to definitions. – K H Nov 16 '18 at 23:22
• I'm guessing you'd like something less concise than a dictionary definition, more educational(how and why rather than just what). At any rate that line stuck out to me and probably most of the engineers around here, because while you're clearly starting to understand electricity, you do have a lot of misconceptions in that wall of text that you simply will not have anymore once you've read the right introductory level material and learned the associated definitions. – K H Nov 16 '18 at 23:26

Conventional (the standard way to think about it) current flows from positive to negative. In your circuit that is generally from top to bottom.

As the capacitor symbol suggests, there is no electrical continuity from one side to the other. When current flows into a capacitor the capacitor stores charge and the voltage on the capacitor increases. In your circuit the capacitor would be fully charged when its voltage reached Vcc.

The 555 timer is monitoring the voltage on C1, however, and when it reaches 2/3 Vcc the discharge transistor switches on connecting C1 to GND through R2. Current now flows out of C1 through R2 and the transistor to ground. The C1 voltage falls.

When the voltage on C1 gets down to 1/3 Vcc the discharge transistor turns off and the cycle starts again with C1 charging up again.

I've got a question that has been perplexing me for a while.

(Correct me anytime) (Please do not give me any textbook definitions.)

I think I'm just going to work through this line by line, I'm not going to worry about the "textbook definitions" part because it's not clear what you mean, but I've made some speculations about what your request might mean and I will try to write the way I'm guessing you'd like.

Starting with the name of the question itself, I believe current is what powers the components in a circuit, correct? At least that's what they taught me in physics.

Not quite correct. Close enough for barroom conversation, but not close enough for practical purposes. Electrical energy powers the components in a circuit, and the transfer of electrical energy can be accomplished by moving electrons around, but rather than making partially correct statements, Let's just examine the basic electrical properties: (I'm including these as definitions because it's the easiest way to read the information, however these are off the top of my head and should be easier to understand, although they won't be as concise. Power(P) is the product of current(I) and voltage(E), which is to say P=EI, so if either current or voltage decreases by half, power will decrease by half.

Voltage: Also know as electromotive force. For a nice easy to understand oversimplification, you can think of voltage as pressure, so if you imagine conductors as pipes, voltage is the amount of pressure at a given point. Voltage is measured in (V)olts and is often represented in algebra with E

Resistance: The basic property which defines how much a conductor resists the flow of electrons. <- That was probably the kind of statement you'd like me to avoid, but it really is that simple. When you first learn about electricity you learn about resistance first, but just note that once you have the basics figured out, there also exists impedance, which isn't quite the same thing and must be kept separate. "Resistance" refers to reduction in electron flow that results in power dissipation(the conversion of electrical energy to heat). May as well throw impedance in there: When we refer to effects that reduce electron flow by storing energy and returning it to the circuit, we refer to the effect as Impedance instead. Resistance is measured in Ohms($$\\Omega\$$) and is often represented in algebra with R. Impedance is measured in Ohms($$\\varOmega\$$) and is often represented in algebra with Z If we went back to the slightly incorrect "conductors are pipes" analogy, resistance would represent how much the pipe resisted the flow of the liquid, but specifically in ways that waste energy through turbulence and conversion to heat, like forcing the liquid through a smaller pipe, a filter, a nozzle, etc. Impedance would refer to things that resisted the flow while storing that energy, for instance if you have a propeller in the pipe connected to a flywheel, as fluid passes through the pipe it will transfer energy into rotating the propeller, but when you stop actively pumping fluid through the pipe, the flywheel will keep the propeller spinning as it transfers its energy back into the system, keeping the fluid flowing.

Current: Current measures the rate of electron flow, but note that this is a measurement of the quantity of electrons that are flowing, not a measurement of velocity. If you measure the current flowing on two wires and the first measures 1 Amp and the second measures 2 Amps, the second wire has twice as many electrons flowing per second. If we did check the textbook definition for an ampere, we would find that one ampere describes a flow of 1 coulomb of charge (602'214'085'700'000'000'000'000 electrons) per second. You can remember the number(1 mole=6.022*10$$\^{23}\$$) if you want but the important part at introductory level is just that it's a number of electrons per unit of time, converted to a more convenient unit. For the analogy above, current is the flow rate of the liquid, and one more time, just remember that it is quantity, not velocity.

Power: Oh yes, we also have power. This describes the rate at which something converts electrical energy into other forms of energy that do not re enter the system. For a resistor power means the amount of energy the resistor dissipates as heat, for a motor's power rating, it describes the total electrical energy being changed to heat(losses) plus mechanical energy. For basic electrical equations it refers to purely resistive elements. Once you get into more advanced circuits, you will need to deal with True power, Apparent power and Reactive power.

Trying to mix power in with the water in pipes analogy becomes messy, so I'll leave it out.

Power, or True Power, is measured in (W)atts and uses symbol P for algebra

Reactive Power is measured in volt-amps reactive(var) and uses symbol Q for algebra

Apparent power is measured in volt-amps (VA) and uses symbol S for algebra

This current is a flow of electrons which are motivated by voltage? I believe we've covered this

Voltage doesn't kill you because it's a potential between two points, what kills you is the current that's an output of the voltage. In other words, voltage is required for current, but not vice-versa?

Not quite. Ohm's Law describes the relationship between voltage, resistance and current. I = E/R, which is to say current is equal to voltage divided by resistance. This means if voltage doubles, current doubles. If resistance doubles, current halves. If you rearrange the equation to E=IR, or voltage is equal to current times resistance. We do math this way, and if we know any 2 values we can figure out the other, but current is the result of the combination of the voltage(pressure) and the resistance it's applied to. In real life you cannot halve the current through a resistor in order to get it to have twice as much resistance, but you could give the resistor twice as much resistance in order to half the current.

As far as what kills you...

Just FYI, if it kills you, you've been electrocuted, if it doesn't, you've been shocked. There are a number of ways that electricity can kill you. Even if it does not conduct electricity through your body, a high current short circuit can cause a fireball and do any amount of bodily injury or kill you. A low current short circuit can ignite combustibles, light-blind you or damage eyesight, or send shrapnel flying from damaged components. You cannot assume something is safe just because it is "low current" or "current controlled".

For direct exposure, at high voltages and currents, you can expect to become a fireball (grease stain on the wall), and as energy levels get lower, you would be less vaporized and more cooked, down to the point where the energy level is low enough that you're not dying from that concern, and below that point the new concern becomes damage or interference to you're body's electrical systems. AC, in the most broad terms, is worse for this, as it has a greater likelihood of stopping your heart. 50-100mA of current can put a human heart in fibrillation, and varying amounts of current, particularly through the head, can cause nerve or brain damage.

So for the expression, "It's not the voltage, but the current that kills", it's making a bit of an assumption that the listener knows a few other things about electricity, or nothing at all. Voltage, combined with resistance, WILL determine current, however, there are multiple ways of describing both voltage and current, and regardless of which definition is being used, higher current numbers are much more certain to be lethal, whereas there are many devices that apply a high voltage intermittently, and as a result they can be described as safe because although they may cause a high instantaneous current to flow, the average and rms average currents are very low. A taser is an example of this. It applies 100000 volts(or whatever voltage) in tiny pulses a fraction of a second long while the rest of the time it is off. At the moment the current is flowing, current flow is quite high, but that is for such a short period of time that "damage is not done"(IIRC, they are permanently harmful to a small degree, but they fit the legal definitions anyway)

Resistance is the final piece of the bunch to make the "electric trinity" (see what I did there?) The voltage, the current, and the resistance the three amigos that form the basis of this entire field of electrical engineering and contribute to the plethora of mathematics in the subject.

Yes I did see there what you did. E, I and R are not large enough concepts to form the basis of the field, but they are important fundamentals, and they are at the root of much electrical math. However, they are only enough to understand resistive DC circuits, so if you want to understand reactive circuit elements like motors or AC power, you've got to move on down the line a bit and study RLC circuits.

Assuming all I wrote above was correct, or that I at least have the slightest idea what is going on, I still do not understand what path the current will take.

Current will flow simultaneously from high voltage to low along every available path, inversely proportionally to the resistance of that path. If you ever need them, you can learn formulas for total resistance of a set of resistors in parallel (in series they just add up), or for the action of most other circuit elements. Imagine you have a pump pressurizing the fluid in a tank, maintaining a specific pressure. The tank has 3 pipes coming out of it. Water will flow through all three pipes unless there's something blocking them.

My goal is basically to analyze each line in the circuit and decide where the input of current is and where the output is. Simplified: Where it goes in and where it goes out.

Ok I was with you up until this point. At this point you would certainly be better off if you throw some hours at a textbook first and then ask people questions to iron out the bits you don't understand. It's not too tricky for some circuits, but if you get to a point where you want to know the direction current is flowing on a particular node in a resistive network or a network with multiple sources, you'll find it much easier if you learn Ohm's Law, Watt's Law, Kirchhoff's equations, Series and parallel resistance and edison 3 wire circuits. Depends what you mean I guess.

Based on this diagram of a YouTube video I was watching, assist me in understanding the current flow.

(Correct me in any errors I've made during my circuit analysis, please.) Circuit in Question This is the path I envision current taking. (Vcc->R1->R2->C1->GND) (R1->DISCHARGE->TRANSISTOR->GND) (C1->THRESHOLD & TRIGGER) (Vcc->Voltage Divider(Unnamed Resistor->Unnamed Resistor->Unnamed Resistor)->GND)

I'll skip this part

From what I can see, all current needs an input and a ground to flow, in other words the ground provides the sink of potential to motivate the electrons to flow there.

Specifically what current requires to flow are a difference in potential and a complete circuit (for electricity to flow continuously the electrons that "come out" the other end of the load you power must return to the source to replenish the ones leaving on the output). Neither conductor must be grounded, but they could be grounded, and sometimes the earth itself is used as a return conductor rather than a second wire.

But is there something else I am missing in the analysis?

Also, if you notice C1 has an arrow shape as if it was going up back to R2. If the current just flowed down from VCC->R1->R2->C1->GND, then why does it need to go back up?

There are 2 arrows related to C1. The orange arrow beside it indicates that the voltage the capacitor stores is decreasing at that moment, and the smaller blue arrow going upwards from the capacitor on it's schematic line indicates direction of current at that moment. What you're missing here is the definition of a capacitor. It's a device that stores energy in the form of a potential difference between two plates. What that means is two conductors with a large surface area almost touching each other, with an insulator in between.

When a voltage(pressure) is initially applied to such a device, it starts at 0V and electrons on one side of the insulator start packing up on its surface, attracted to the holes(molecules that are missing at least one electron at that moment in time) that are packing up on the other side. As energy is stored this way, the pressure difference(voltage) increases until the capacitor reaches the voltage of the source. If at this point the source is disconnected, the capacitor will release it's stored energy, continuing to power the circuit.
Because one contact of a capacitor is insulated from the other, capacitors don't conduct DC current at all. They can, however, store energy and release it back into a DC circuit and that's what you're seeing there. Because of the way that they store energy, capacitors will conduct current in an AC circuit, depending on the frequency.

I don't get it?

How 'bout now?

Does the current not go from + (Positive/Abundant Potential) to - (Negative-Lacking Potential)?

You can actually look at it either way. One is called electron flow, the other is called conventional flow. The most important thing is to be sure of which one the diagram you're reading has been written in.

Do equipment also have different polarity conventions?

Oh yes. We have all of the conventions you need. Too many to list, and at minimum, the two above should be known when dealing with equipment documentation.

Why does the same current enter both into the positive and negative sides of the components?

A copper wire is just a bunch of copper atoms bonded together. Each of those molecules has a number of electrons to have in its orbits that makes it stable. If an atom has less electron in its cloud than it should, neighbouring electrons will "attempt" to fill the vacancy. A potential(or voltage if you will) refers to a difference between the number of electrons the molecules "want" in that area and the number that are, compared to the same for another area. If you try to keep pulling electrons from an area or pushing them into it without replacing them, you create a capacitor(as mentioned above), so if the electrons didn't return to the voltage source, where would the voltage source get new electrons to replace the ones it was pumping out? The current being input and output must match as a result of this.

I have to be missing some concept or not understanding something because I have no idea how current flows in one way, heads out another, ignores convention goes from the source to ground and back again?

You're definitely missing many core concepts that you will need if you want to eventually turn this knowledge to any practical use. A decent fundamentals of electricity textbook might take some getting used to, but it will take less time to learn in an organized fashion than to try to absorb it all anecdotally.

It's as if it has no laws, but abides by a plethora of them?

That's offensive. I assure you these electrons are fully compliant with all laws of physics. You're going to have to learn more laws, rules and equations than you probably would have thought existed if you want to look at complicated electronic circuits and understand them.

Thanks again, your assistance in highly appreciated.

No problem. If you really want to learn this stuff though, you'll really want to get used to textbooks, as I've barely scratched the surface. This is just intended to give you an idea of what you're missing and where to go next, once you decide how far you want to go with educating yourself.

• Regarding “putting a heart in fibrillation” you are off by two or three orders of magnitude. Off the top of my head IEC60692-1, the medical safety standard, specifies a maximum of 500uA under single fault conditions and <20uA under normal operations. There have been a handful of documented cases of people dying with as little as 40V, although DC voltages below 65V are considered safe. – Edgar Brown Nov 17 '18 at 5:04
• @Edgar Brown That sounds like a rating for a hospital rated GFCI. Hospital grade equipment has better trip ratings, probably because of proximity to the infirm. I tried to phrase it a touch vaguely so as not to provide a feeling of "Well I'm below 50mA so I'm fine". 50-100ma is the supposed "hot range" taught in electrical courses that is most likely to cause fibrillation at 60hz AC. I did not mean to imply that it was impossible to stop your heart with a higher or lower current. I'll work on a rephrase. – K H Nov 17 '18 at 5:21
• @プログラミング哲学者 I would feel irresponsible if I answered your not-particularly-clear question without dealing with all of the fundamental misunderstandings you demonstrated. This would be like me trying to teach BEDMAS(Order of operations) to someone who has not yet heard of addition, multiplication, division and subtraction. It makes far more sense to start by teaching or reviewing the basic operations that the person is obviously unclear on, and that will make approaching the actual goal much easier. – K H Nov 18 '18 at 0:38
• At any rate, the point of the site is to produce a collection of the best possible questions, and provide answers that will not be useful just to the asker, but to every future person who stumbles in from google. You came off as rude in your initial post, so I wrote this as much for myself and those other people as I did for you. I do hope you find it useful if you do decide to learn the basics, but you will probably find your attitude will be a major barrier for you. By the way, it turns out a textbook explanation was exactly what you wanted, plus an analogy. – K H Nov 18 '18 at 0:44
• One word of caution, as I warn in the post, that part of my answer, and the attached analogy will only serve you at the basic levels. Once you get into advanced electrical physics it becomes an unfair generalization, especially when microscale parts, superconductors or very strong insulators are involved. At that point it becomes necessary to understand more what's going on at the molecular level. Best wishes. Let me know if my gift horse has any dental problems ;). – K H Nov 18 '18 at 0:49