help understanding volts and amps

[lavastudios]

hey guys, i'm currently in the middle of my first pedal build (electra style distortion), and while i'm waiting to receive some of the parts i've ordered, i've been trying to get a basic handle on the physics of electricity. i've known for many years that too many volts can damage a pedal, but you can't damage a pedal with too many amps. i've read that the power source pushes out the voltage, while the circuit pulls as many amps as it needs, but from what i can work out, this isn't a particularly accurate description of what is really going on. the old electricity/water analogy confuses me. if volts are like water pressure, and amps are like the quantity of water, then as far as i can see, this would mean that voltage and amps (along with resistance) determine the speed of the flow of electrons. but if this was the case, then increasing either would have the same effect (increasing the speed of the the flow of water/electrons), and therefore, increasing either voltage or amps should be able to damage a circuit. even more confusing - i keep reading that it's the combination of voltage and amperage that is dangerous to humans. why is high amperage damaging to us, but not to a circuit?

any help with this would be greatly appreciated!

[Sanguinicus]

Are you familiar with Ohm's law? This is the key.

The reason voltage is dangerous are numerous. Higher voltage exceeds components' rated limits. You'll probably find some pedals will happily run on between 7-11 volts.

High current is dangerous to a circuit. If you have a short circuit somewhere, that is, the current takes a shorter path to ground, that is, bypassing a load, things are going to go wrong pretty quickly. This is demonstrable using Ohm's law. A short circuit, for all intents and purposes very low resistance, no load. Let's use 9V for example. Like a stomp box. Ohm's law: V=IR, voltage=current.resistance. Rearrange to solve for current: I=V/R. I=9/0.00001 (very low resistance). If you solve I, you can see the current goes really high. You've killed the circuit. There's a power component to this, P=I^2R, but we can leave that for now. But it explains why short-circuiting circuits get really hot.

Also, you underestimate how much current is required to kill someone. We're talking 10's of mA. To get that across your body you need thousands of volts, because the body is a very good insulator; resistance is in the order of megohms. This is why you probably won't get a jolt from your DIY project because the electromotive force, that is, voltage, is very low and can't push the current through your body. Let's test this using Ohm's law. Say the body's resistance is 1MOhm. An insulator for all intents and purposes. And 10mA across the heart will kill you. V=IR. V=.01 x 1,000,000. V=10,000V. Ten Thousand Volts. That's going to ruin your day.

So when people say it's not voltage that kills you, it's current, they're half correct. There's no current without voltage. While it's the current that kills you, it doesn't give you a license to go touching high tension power lines because "voltage doesn't kill".

Let's take a static shock.  A static shock could be millions of volts. It's like lightening emitting from your finger. Air is an insulator so to get the electricity to conduct, you need huge voltage. Why doesn't it kill you? The current is so negligibly low. That spark you see could only be one electron. The current from a single electron is beyond the scope of this thread.

Tread with caution using the water analogy, it's pretty poor. I prefer the jug analogy: A jug is filled with water. The higher you raise the jug, the more potential energy the water has. This is voltage. Voltage is a potential energy. Start pouring the water. The water is current. The spout is resistance. Keeping in mind the height of the jug is the potential energy. Let's simulate the human body. Very small spout, the jug is hundreds of metres high. That trickle of water is gonna hit you pretty hard.

Maths is king in this town.

[R.G.]

Hmmm. OK, try this.

Let's go back to the water analogy. Volts is like pressure, amps is like gallons per minute (or liters per second, or shotglasses per fortnight, whatever). If you have a hose filled with water at, um, 200 psi, but there is an adjustable nozzle on the end of the hose and the nozzle is dialed closed, then ... nothing happens. The hose interior is a "conductor" of water, and the outside of the hose is an "insulator". The nozzle is a closed off-valve, and no water flow happens.

If you dial the nozzle open just a -tiny- bit, a drizzle of water drips out the end, enough to get your shoes wet.  The water that hits your shoes is low pressure and low flow rate, so it's low voltage and low "current", so no damage is done.

The "power" of a water flow is the product of pressure and flow rate, just like electrical power is the product of voltage and current.  The nozzle is a high "impedance" to water flow, so there isn't much flow and you can touch the end of the hose nozzle without getting soaked.

However, if you open the nozzle fully, you're going to get a fair blast of water, lots of it flowing, and at high pressure. The high pressure, no longer impeded by the dialed-down nozzle has the ability to get you thoroughly wet in short order, sting on your skin, and might damage your eye or burst an eardrum if it hits them. There is a high pressure *and* and high flow rate, so a lot of power is transferred to whatever the water hits.

At quite high pressures, you can be damaged by the pressure-voltage alone. 200psi is pretty nasty for hose work, but in some hydraulic systems, the pressures get to several thousand psi.  In such cases, a pinhole leak lets a needle-fine jet of fluid out, and it can cut right through your flesh to the bone. High electrical voltage can puncture things electrically like this, so high voltages can be dangerous on their own, even at low flow-rate/current.

At the other end, there's low pressure/voltage and high flow-rate/current. Imagine a river; not particularly rampaging, just flowing. Not much pressure involved, but it's flowing lots of water. This is a very high current scenario, and the only thing that keeps this from being dangerous by itself is that the river is confined to its bed. If the river is diverted into your back yard, just flowing along, it drowns your back yard and floods the house in a few seconds. Lots of damage, but from high flow rate, not high pressure.

In the middle is a fire hose. You can break up riots with fire hoses because they have medium pressure and high flow rate; the resulting power is enough to knock people off their feet and move them bodily. The product of pressure/voltage and flow-rate/current is a high power.

Stay with me, we're getting there.

Things vary in what they're sensitive to. Hoses, like wire insulation, are sensitive to pressure. Put too much pressure in a hose or a pipe, and it bursts. In a circuit, you are looking for the components to let through just the right amount of current at the right time, and they mostly do this by having a ready source of voltage/pressure (the power supply) and letting through the right amount of current. The Brits call vacuum tubes "valves" precisely for this reason - they let through just the right amount. But everything has a pressur/voltage limit, where it can no longer hold off the pressure, and it breaks. So a circuit may have a maximum voltage it can work from before being damaged, and a lower voltage it needs so that everything inside runs just right. Below the point where things start breaking, it takes just enough current to do what it's designed to do.

And some things are sensitive to current/flow-rate.  Fuses are the classic example. They are **designed** to conduct right up to their rating, then to open up. They are designed as a "weak link" so that the current damage happens there, where it's easy to fix, before other things get hurt.

And then there's resistance. Resistance is literally the ratio of how much voltage it takes to push an amp of current through. Back at the hose/nozzle analogy, imagine that the nozzle is not adjustable. Instead, the amount of water that flows is proportional to the pressure you put across the nozzle. The nozzle is acting like a resistance.

It is true that for resistances - things that have a linear relationship of current and voltage - rising either current or voltage raises the other. But we use a lot of things that aren't resistors. We use capacitors. Caps don't care what current you try to push into them. They will take in any amount of current - until the voltage exceeds what they can stand, and they break, like a pressure tank bursting. Transistors are heavily NON linear, and we value them for that. They can turn on or off or just the right amount of electricity based on a tiny control signal on their base. But they can't hold off infinite voltage, breaking like the capacitor, and they can't conduct infinite current - the bonding wires inside actually melt like fuses.

And then there's what is dangerous to humans. Humans are delicate, sensitive - and stupid.  If you zap a human with high voltage, what happens? Scuffling your feet on dry carpet in the winter can build up a static electricity charge in the thousands of volts. But you can zap someone with your finger and neither you nor they die. There was not enough *energy* to do much damage. Energy is power times time. So the voltage you zap them with is large; the power you zap them with is also large, but it doesn't last long enough to do anything more than kill a few skin cells. Same idea with electric fences for livestock. The voltage is high, the current is high, and the power is high, but the total energy delivered is low because the zaps are short. You jerk suddenly, but it's over.

You can get shocked by much smaller voltages. The AC power line is only maybe 170V peak (in the USA). But that's enough to overcome the resistance of dry skin, and start current flowing through you. In this case,though, the time is not limited. Unless you can let go somehow, you just sit there with medium voltage and lots of current flowing through and it eventually cooks you or stops your heart. There used to be a hot dog cooker that worked just like this - there were prongs for each end of the hot dog, and it passed AC wall current through and cooked them in under a minute.

High current is primarily dangerous because it can generate lots of heat. Car batteries can dump lots of current, and it used to be that beginning mechanics would sometimes lose a finger, having it burned off by the huge currents running through a ring that happened to short two places connected to opposite sides of a car battery. This generates high power in the ring and the heat does the damage.

Finally, high current is not damaging to a circuit only in the case that the circuit is working properly and only sucking down as much current as it wants. The circuit is in control and only "eats" as much current as it wants. If you use enough voltage to force too much current into a circuit by breaking something down with voltage, then too much current flows because the circuit can no longer self-limit the current to what it needs, and things start overheating.

This really all does make sense eventually, but it's not trivially simple.

Questions?

[Sanguinicus]

Hmmm. OK, try this.

That was good. And puts my explanation to shame.

[lavastudios]

hey Sanguinicus and R.G., thanks for the replies! this is extremely helpful - i'd never realised that the design of the circuit is what allows it to take on a high amount of amps - that makes a lot more sense to me now. so in Sanguinicus's jug analogy, the height of the jug causes the water to fall faster (as it builts up velocity), and likewise, in R.G.'s hose analogy, a narrow nozzle would make the water flow out faster - does this mean i'm correct in assuming current refers to the speed of the electron flow, as opposed to how many electrons are travelling alongside each other in the wire? to go with another analogy, it's like there are the same amount of lanes on the road but the cars are going faster, rather than the cars always obey the speed limit, but there are more lanes?

cheers guys, really appreciate the help with this!

[Sanguinicus]

does this mean i'm correct in assuming current refers to the speed of the electron flow, as opposed to how many electrons are travelling alongside each other in the wire? to go with another analogy, it's like there are the same amount of lanes on the road but the cars are going faster, rather than the cars always obey the speed limit, but there are more lanes?

cheers guys, really appreciate the help with this!

How to put this simply. It's not speed, it's the number of electrons. More electrons, more current. This is very simplistic, but it'll do for now. Gotta be careful with analogies. Speed isn't really accurate here. Sure speed in my analogy and flow rate in RG's are a factor, but that's because they're physical systems. You need to marry the equivalent physical properties in an electrical system witht he physical systems. You need to consider each system's potential energy. Arguably, my analogy is better and demonstrating this (maybe I didn't understand RG's properly. Highly likely). The higher the jug, the more voltage, the more potential energy. The water speeds up yes (until it reaches terminal velocity, but I digress), but the analog to voltage here is height of the jug. Think of lifting a ball over your head and releasing it. It has more stored potential energy under gravity than releasing a ball from your hip.

Some might argue "speed" is good enough, but I believe this to be inaccurate. If we consider momentum, the faster the water or ball is, the more momentum or energy it has, but it's the act of raising it that gives it that energy. We can get into change in velocity to determine with what force the water or ball strikes the ground but this is going way off.

i'd never realised that the design of the circuit is what allows it to take on a high amount of amps

This is also inaccurate. Or rather an inaccurate description of what's happening. A circuits current demand is only what it needs to function. Exceeding this demand means something is wrong.

Hope this helps.

[Transmogrifox]

Thinking in terms of electron flow particularly in terms of velocity and direction was one of the most crippling things for me when I was trying to learn this.

What you want to focus on with understanding current is volume per unit time.  With things like cars on a highway with limited number of lanes, this implies velocity, but with electricity this generally means you have a lot more lanes.  But where the electron flow gets less useful is because you have lanes going in both directions and you have a lot of exits and round-abouts, cars entering and exiting all the time, and generally like a confusing relay race where there are more players on the highway than the ones registered for the race.  One car at the starting point may not actually end up at the ending point, but then again it might.

When you get done adding up the average number of cars that pass the finish line per second vs the average number of cars that start the race, they are equal, but you will go dizzy if you try to account for what happens in between, what directions they are going, how many lanes they are traveling at any given time, who enters the racing lanes, who exits....

Back to the water analogy, the best metric for current by analogy is gallons per minute in a closed system and a constraint that the velocity is a constant.  The velocity of electricity in copper is very nearly the speed of light.  No matter what the current or the voltage, if a squirrel becomes part of an arc on a power line in Chicago the system monitoring equipment won't see a disturbance on the other end 100 miles away until about 536 microseconds later.

So since we see that the velocity of electricity is a constant, we see that current is not stuff going faster but only more units crossing the finish line.

We don't consider velocity in electrical circuits at audio frequencies because time for electricity to travel 4 inches is measured in picoseconds, which is effectively zero to an audio circuit.

Imagine trying to track a single water molecule through a water system in which there are many pockets and side-pipes that flow circularly in eddies, push water into mixing tanks and pull water out of mixing tanks and so on.  You would probably find some water molecules stay in the system forever just circulating around in one eddy somewhere.  This is by analogy about how useful it is to think about electrons in an electrical system.

I hope I didn't make it more confusing.  This is really the bottom line:

How to put this simply. It's not speed, it's the number of electrons. More electrons, more current. This is very simplistic, but it'll do for now.

A simplistic understanding is what makes it possible to do something practical with electricity.

[R.G.]

Yes - as they both said, current is the number of electrons per unit time moving, not the speed at which they move.

Back at the water analogy, rivers are high current, low speed. Fire hoses are much lower current, but higher speed. Current flow is how many electrons move past some point. This is actually a quantifiable number - huge, but quantifiable - as they taught us in my first few physics courses. And water flow, which I alluded to as gallons per second, etc., is actually discrete water molecules as well, just huge numbers of them.

And here is a real break with the water analogy: electron speed in a conductor is not terribly variable in general, and this quantity doesn't come up in most electronics work. There are so very many electrons in even a tiny wire that you can make huge numbers flow without changing their speed much.

So current flow is HOW MANY electrons pass a point in a certain amount of time.

Moving each electron takes a certain amount of force/pressure. The amount of pressure/voltage/force to make X number move depends on the material it's moving through, and we label materials by how much pressure/voltage is needed to move how many electrons as "resistance".

[digi2t]

Hmmm. OK, try this.

That was good. And puts my explanation to shame.

R.G. is flat out the best at dumbing stuff down, especially for noobs like me. If I hadn't discovered this forum, and Geofex, I believe pedal building would have been just too overwhelming a proposition for my brain.

What I can't get is how he explains the same shit over and over again, but every time it sounds like it's the first time. Patience of a saint that man. 

[ashcat_lt]

Yeah, RG must have a pretty boring day job! 

None of these analogies ever really made sense to me.  I mean, I get it to a point but only kind of if I squint.  But it also doesn't really seem to matter.  Maybe it's just me, but all that really matters is the math.  For our purposes it doesn't matter how or why it works in a physical sense as long as it's predictable and repeatable.  In a very real way we are doing math to our signals, each component or group of components is a function that transforms our input number to some output number and I think we need to understand those functions more than we need to understand about rivers and hoses and tanks.

On top of that, in most audio work, we don't much care about the current.  It's the voltage that encodes our signal and it's the voltage that we need to transform via our circuit snippet functions and eventually read as our output.  We build our circuits in general to do what we need to do voltage-wise and let the currents fall where the may/must.  You have to check currents at some point in order to spec physical parts, but it's not usually necessary in order to understand the circuit.

Luckily, there really is a very small number of functions to deal with.  Everything useful is a voltage divider, and that one function [Vout = Vin * Rbot/(Rtop + Rbot)] is ultimately the one that will describe anything you've done in analog electronics.  All of the other useful functions are about figuring out exactly what Rtop and Rbot actually are at a given point in the circuit.

There's nothing wrong with wanting a better understanding of how the world around us works, but at a certain point it's the practice that matters.  Use proper safety precautions around electricity and you'll never need to know whether it's volts or amps that'll kill you.  Don't, and it won't really matter.

[Cozybuilder]

But I like reading about Paul's tractor adventures and some of the best circuit analogies ever!

[lavastudios]

thanks everyone! i think i've come a fair way with my understanding today thanks to you guys. so there are components which aren't damaged by high amperages (at the ideal voltage), and other components which are (such as leds),  but a properly designed circuit will account for this by using resistors to lower the amperage at those spots, thereby protecting any vulnerable components. is that correct?

cheers!

[Sanguinicus]

High vs. low current is all relative. An LED might happily run at 10mA, but will die at 50mA. It's high for an LED. 10mA is high for the human body. It's all relative. If a component is pulling a high current relative to it's rated operating current, something is going wrong. Even if the voltage is correct.

You are right about resistors. They limit current, for example in a simple battery-resistor-led circuit. It's important to understand what the resistors is doing though. It's limiting the current by way of dividing voltage. Refer to Ohm's law to test this.

I recommend you search for an electronics basics sort of thing on the internet. You certainly seem to be interested. We could spend pages and pages on here going through all the basics. It's a rabbit hole.

[PRR]

> confusing - i keep reading that it's the combination of voltage and amperage that is dangerous to humans

Electrocution is VERY unpredictable, mostly because we can't do Controlled Experiments, so the data is slim.

Your body is a dry sack of wet salt water. (Maybe you think you are more than that, but electricity don't care.)

Resistance through the healthy skin is 10K to 100K.

Resistance through the chest, inside the skin, is more like 10 Ohms.

And "historically", power voltages have been selected to be "less often fatal". Edison thought that voltages over 100V led to more dead workers around his generators; one thing and another, we have 115V/230V systems. Not too low, because we want POWER for our lamps and motors, power at low voltage is more current, which means fatter wires, which cost more. Not too high, because we would need better and more fool-proof insulation which costs more.

So another part of the confusion is that many exposures are to "barely fatal" voltage/insulation. If things go a little one way or another, you live or die, and won't know why.

Electrocution comes in stages. Tingle, can't-let-go, heart trouble, and outright burnt flesh.

"Healthy" skin does not stay healthy with prolonged current flow. Cell walls break down, salt-juice leaks out, skin resistance may drop from 100K to under 1K in some seconds.

A fair number of accidental electrocutions follow this path. At 100V and 100K-10K skin resistance, the current is 1mA-10mA, a tingle, but not obviously dangerous. If the victim does not soon let go and get out, the current flow opens the skin cells. Current flow increases. When it overwhelms muscular nerve potentials, the victim can't let go or move away. This current also flows through the chest, but the heart and diaphragm are small in the chest and don't get the full flow. But as current increases, skin damage gets worse, resistance drops to a hundred ohms, current rises more. At some point 100mA through the arm and chest becomes 10mA through the heart, the heart nerves are overwhelmed, heart either quits or flips-out (each small area beating madly without coordination). Breathing diaphragm may also be paralyzed. Death follows in 5 or 15 minutes.

Aside from just dying, large electric flows will damage your nerves. Some people who didn't die from massive shock report many different long-term after effects.

Because the resistance of chest and heart is very low, an ammeter on the heart "may" be the most convenient experimental number.

If you are not sticking probes through your skin, your initial skin resistance means you must be exposed to significant voltage before any dangerous current flows. The voltage IS important. 12V hardly hurts humans. 25V is generally allowed very flimsy or no protection from contact. In US tradition, 70V is allowed on bare screws where untrained contact is less likely (PA amplifiers). Over that we like totally enclosed devices or technician-only junction panels.

(But note that cats are somewhat more sensitive, and use their mouths where we might use hands. Cats are severely burned and sometimes killed chewing on 12V wall-wart cords. Dogs too: the early history of the Electric Chair is horrific because Edison experimented on stray dogs, and apparently some people are tougher-- sizzled without quick death.)

[induction]

Fun fact I remember from an article posted on the wall next to the accelerator in the lab I worked in as an undergrad: DC currents across the chest are fatal between 100 and 200 mA. Above 200 mA, the victim has a strong chance of survival with immediate medical attention (severe burns and unconsciousness are still likely, and breathing will probably have to be kick-started) because the heart clamps into a severe contraction. The article in the lab said that this clamping has the effect of increasing the heart's resistivity, which causes the current to travel around it instead of through it. But this page says that the clamping prevents the heart from going into defibrillation. The linked article also contains this bit of common sense:

From a practical viewpoint, after a person is knocked out by an electrical shock it is impossible to tell how much current has passed through the vital organs of his body. Artificial respiration must be applied immediately if breathing has stopped.

[lavastudios]

I recommend you search for an electronics basics sort of thing on the internet. You certainly seem to be interested. We could spend pages and pages on here going through all the basics. It's a rabbit hole.

i read a bunch of different things, flicked through my housemate's physics textbook and watched about 10 different beginners electronics vids on youtube, but each time they had pretty much the same info and each time i had the same questions at the end. i did forum searches as well, but sometimes there's no substitute for putting a question in your own words and considering the responses. i do feel like my questions have been answered really well here though, and i'm now getting further along in a couple of different youtube series - now it seems more about the equations, more practical stuff, which i'm fine with. i just didn't want to accept the fundamentals and move on without asking why and how. i'm sure it was a very basic question to you guys, but you gotta start somewhere right! anyways, i really appreciate all the responses.

[Transmogrifox]

i did forum searches as well, but sometimes there's no substitute for putting a question in your own words and considering the responses.

I agree.  The forum is a living place where you can get help right at the place where you're hung up

It's sort of like building a skyscraper.  You have to build a crane that attaches to the structure so you always have something high enough to attach your crane to get the next pieces into the next place where you're building.

[LightSoundGeometry]

What I need help with is the combo circuits with more than one voltage source. Is there any tricks, advice or secrets to working a circuit like this? or does it really come down to either you are great at algebra or not?  the method we are doing is a linear equation substitution with two loops, one has to be the outside loop and then you have to algebraically set it up to factor out the missing value ..thats where I get confused..you have to be able to know what will equal what to set up the problem in the first few steps or else the whole thing is screwed up. Once you have all the voltage drops in place you can follow the polarity and it finally becomes clear getting there is a mofo though for me..if I am not mistake, do tubes work with a 3 part energy source in one circuit ?

I am doing okay with basic RC, RL and RCL circuits so far. When we start looking at phase and resonance it gets a little confusing with the trig and calculus graphs ..Its very difficult for someone like me who is not that good at math haha..Only two weeks left in the semester and it looks like I will pass this semester. I hope I am strong enough to make it all the way to the end of the degree.

[PRR]

> combo circuits with more than one voltage source

Is this for school homework?

We don't do your homework.

Also the textbook problems with two batteries are almost never seen in the real world. Or not as neat problems. We may have multiple sources (particularly audio sources) but there is almost always a close-enough simplification and approximation which lets us move-on.

[LightSoundGeometry]

> combo circuits with more than one voltage source

Is this for school homework?

We don't do your homework.

Also the textbook problems with two batteries are almost never seen in the real world. Or not as neat problems. We may have multiple sources (particularly audio sources) but there is almost always a close-enough simplification and approximation which lets us move-on.

although I am only a B student, I don't need your advice or help..thanks though, I will do just fine without you and was not asking for anyone "to do my homework."

87% on math final and 87.66% overall in EE 101 - High B all around with a 3.67 GPA and two associates. Clearly not as intelligent, or smart as you Paul, but that's okay..I will figure it all out.