Newbie Question - 1A at low voltage vs 1A at high voltage

[ekeppel]

Hi Folks!

I've googled this and haven't really found anyone asking the same question, so I wanted to ask here.

I do know what voltage, resistance, and current are, familiar with Ohm's Law and how to calculate Watts, but just had a nagging question and figured that someone here might have a good answer for me.

As I understand it, 1 Amp is approx. 6.2x10e18 electrons passing a given point in one second.

So given that definition, why is 1 Amp current draw at 12 volts any different than 1 Amp current draw at 120 volts? If electrons were counted passing through either circuit, both should have approximately 6.2x10e18 electrons moving by a chosen point in one second, right?

Obviously, one circuit draws 12W and the other 120W, but I would love it if someone could reconcile this with the above definition of an Amp.

I must be overlooking something very simple. Thanks for any insight you all can provide, and thank you for taking the time to check out my first post.

--Eric

 

[kpatz]

Power (watts) is a product of current and voltage. W = V * I.

To use a water analogy, think of voltage as water pressure and current as volume of water over time, and a water wheel as a resistance (load). The water pressure and the volume (amount) of water passing over the wheel determines the amount of power the wheel produces. If you increase the pressure but keep the volume the same, the wheel will spin faster. If you keep the pressure the same but increase the volume, the wheel will spin faster, up to the point where the resistance of the wheel reaches equilibrium with the pressure (like with Ohm's Law, the amount of current that flows through a resistance depends on the voltage).

 

[BobK]

Why is 1 gallon of water per second falling 12 feet more power than 1 gallon of water per second falling 1 foot?

Bob

 

[ekeppel]

Thanks guys,

Yeah, I have heard the water analogy many times in the past, but I guess I was overthinking it.

If I correct my thinking, I should be picturing one gallon of water per second (1A) coming out of a smaller pipe (higher voltage) vs one gallon of water per second (1A) coming out of a larger pipe (lower voltage). That makes sense to me, since I know that to pass the same amount of water as a large pipe, a smaller one would need higher pressure.

I think I got it, and I feel kinda dumb for asking, now, lol.

Thanks!
Eric

 

[ekeppel]

Why is 1 gallon of water per second falling 12 feet more power than 1 gallon of water per second falling 1 foot?

Bob

Ok, this get's me more confused now that I stop to think of it, hehe. The water falling from higher has more speed, so it has a greater impact on what it hits than the water that accellerated from a lower height.

I know it's only an analogy, but if electrons move at a finite speed (c) and neither slow down nor speed up, what exactly happens to them when we increase the voltage in a circuit drawing 1A?

They can't move any faster, since they are limited by the speed of light, so what else is occurring at the atomic level to allow them to do more 'work' or have a higher 'pressure' despite the same number of them passing a given point in one second? I guess this is at the root of my amp question.

Edit: Ok, from what I could find googling, I think I am seeing somewhat what happens...If I understand correctly, higher voltages are from higher electron energy levels, as opposed to the speed the electrons are traveling. I'm making up this analogy, so it may be way off, but I am picturing each electron as a tiny spring flying through the conductor, as the voltage is increased, the springs are compressed more and more under pressure, gaining potential. Then when the electrons get to the load, the springs are released to do work. The electrons travel at the same speed through the conductor in high voltage circuits as they do in low voltage circuits, but the potential for work is higher at the atomic level. If that is way off, please help me with a better analogy so that I can come to grips with it. I probably should have posted to a physics forum, rather than here - sorry!


-Eric

 

[davenn]

I know it's only an analogy, but if electrons move at a finite speed (e) and neither slow down nor speed up, what exactly happens to them when we increase the voltage in a circuit drawing 1A?

They can't move any faster, since they are limited by the speed of light, so what else is occurring at the atomic level to allow them to do more 'work' or have a higher 'pressure' despite the same number of them passing a given point in one second?

You have to be careful with the water analogy as it will get you into trouble, as it breaks down under all but the most simplified description of current flow

Electrons DONT move ant anything near the speed of light in a circuit

Electrons move VERY SLOWLY through a circuit, its called "Electron Drift"
google it for more info. You walk faster than the speed electrons move along a wire

its the electric and magnetic fields around the wire that move at near the speed of light

cheers
Dave

 

[ekeppel]

You have to be careful with the water analogy as it will get you into trouble, as it breaks down under all but the most simplified description of current flow

Electrons DONT move ant anything near the speed of light in a circuit

Electrons move VERY SLOWLY through a circuit, its called "Electron Drift"
google it for more info. You walk faster than the speed electrons move along a wire

its the electric and magnetic fields around the wire that move at near the speed of light

cheers
Dave

Ok, THANK YOU! I can see that I need to totally rethink how I view electricity. I always assumed that the electrons moved at e, but it's actually the EM fields that move at that speed? Very helpful to know!

So, does the spring analogy in my previous post hold any water, or should I think of it in a different way?

 

[Arouse1973]

Watts is joules per second just like current is coulombs per second. 1W is the rate of energy needed to move a coulomb of charge through an electrical potential of 1Volt. The higher voltage requires more energy to move the coulomb of charge because of the larger electric potential. So for the same amount of charge to be moved more energy per second is needed thus a higher wattage is produced.

Think of the voltage as some ones hand holding on to yours and think of the energy as people around you (the electron) trying to push you away from this hand. The higher the voltage the stronger the grip of the persons hand and thus the more people or energy needed to free you from the grip of this person and send you round the circuit.
This energy goes with you and is what is used by the circuit and in a simple case of just a resistor then some of this energy is turned into heat as it tries to pass through the resistor. The higher the voltage the more energy involved and the hotter the resistor will get. In this case you could forget about current altogether.
Adam

 

[davenn]

Ok, THANK YOU! I can see that I need to totally rethink how I view electricity. I always assumed that the electrons moved at e, but it's actually the EM fields that move at that speed? Very helpful to know!

So, does the spring analogy in my previous post hold any water, or should I think of it in a different way?

yes the E/M field moves at the speed of light minus the velocity factor of the wire
( we wont go into VF at this time )

The drift velocity of electrons does vary with current flow. But it's still relatively slow
you will find on some of the www sites in google how they calculate the drift velocity
for a given amount of current and conductor diameter.

forget the spring thing learn the real way, then you wont get caught up in analogies that don't work too well.

For a good basic understanding of electronics and what is happening in a circuit, you don't really need to know about electron drift etc

Dave

 

[Arouse1973]

Yes I agree Dave. I always like to use the term flow of charge and not really worry about what the electrons are doing. Because in a.c circuits they don't do anything really just sit there vibrating and don't move around the circuit.
Adam

 

[ekeppel]

Great explanations all around. Thanks everyone.

I know that I don't need to understand what is happening at the atomic level to predict how a circuit will behave when I build it. Thank goodness, or I would never get anything built! That's what circuit modelling apps are for.

It's just my curiosity, and lack of physics knowledge that gets me wondering what is going on behind the scenes sometimes when I start trying to break down the analogies.

Everyone's replies were all quite helpful and should stop my lying awake at night pondering these particular questions.

Unfortunately, I come up with new questions on an alarmingly regular basis.

 

[shumifan50]

A simple analogy:

Consider a pipe filled with balls and the one end open and the other closed by a rubber cap with a hole. The smaller the hole the more difficult it is to push the ball out through the rubber cap; the cap gives resistance(R) to allowing the ball through.
If we want to push another ball into the open end of the pipe, one has to pop out the rubber cap. The higher the rubber cap resistance(smaller hole) the harder we have to push on the input ball to get the ball to pop out the other end. This increase in push can be considered potential(V). If we keep pushing balls in at a fixed rate, say 1 ball per second, they will pop out the other side at 1 ball per second and this rate(A) can be considered the current. So we can make the hole smaller and keep pushing balls in the open end at 1 ball per second, but this will require more force to push each ball into the open end - higher potential. So by making the hole smaller we increase the resistance and we have to push harder to push another ball into the open end=increased potential. All this can happen at a constant 1 ball per second. This means by making the hole smaller and keeping a rate of 1 ball per second, we have to work harder to keep this up(W). The same reasoning can be followed when increasing the rate to 2 balls per second with a constant hole size.

After writing the analogy I realised that it would work equally well for a piece of wire, electrons and a resistor at the end.

 

[Arouse1973]

Great explanations all around. Thanks everyone.

I know that I don't need to understand what is happening at the atomic level to predict how a circuit will behave when I build it. Thank goodness, or I would never get anything built! That's what circuit modelling apps are for.

It's just my curiosity, and lack of physics knowledge that gets me wondering what is going on behind the scenes sometimes when I start trying to break down the analogies.

Everyone's replies were all quite helpful and should stop my lying awake at night pondering these particular questions.

Unfortunately, I come up with new questions on an alarmingly regular basis.

I have been doing this for over twenty years and I still have unanswered questions because you don't stop learning. That's what is great about sites like this, you get stuck in a rut at work doing the same old thing. It's very refreshing to find people who enjoy it as much as you do.

And yes I still have sleepless nights about electronics so welcome to a long long road and enjoy it, it's worth it in the end.

Adam