What exactly is voltage and current?

Bimmerboy

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Been reading through some threads (still have to tackle Darell's EE course) and looking around the net, and still don't have more than some vague ideas as to what voltage and current are, and how do they function (i.e. what do they do?).
Seen and heard analogies made, like "imagine water through a pipe... voltage is how much water is flowing and current is like the water pressure", and on flashlightreviews.com, voltage and current are compared to marbles, etc.
So, what makes the light in an incandescent bulb or performs work in an electric motor, voltage or current? Why? And how are they intertwined?
One would think the analogies would make things easy, but the whole picture hasn't totally clicked yet. Soon, I hope the proverbial light bulb will turn on over my head (that would make a cool graemlin).
In the meantime, I'm trying not to be too lazy and am looking up more of this stuff.
Thanks in advance.
 

Monolith

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I'll start the thread off and someone else can join in.

[ QUOTE ]
Bimmerboy said:what voltage and current are, and how do they function (i.e. what do they do?).

[/ QUOTE ]

Voltage is "potential" that is measured relative to something (typically "ground"). Voltage is basically the difference in charge levels between these two references. People often associate "ground" with zero voltage, but that is not necessary.

Current is the flow of electrons (or holes - I won't debate this) from a higher potential to a lower potential. Current also produces electromagnetic fields as they flow. This is what makes an electric motor work (the fields interact with iron cores and electrically changes the iron core magnetic polarity to help spin the motor). "Resistance" (e.g., resistor, etc.) impedes current flow (current follows in the path of least resistance - that's why people get electrocuted). A light bulb has a resistive element in it that glows when current is passed through it due to its resistance. Resistors are also used to create "potential" drops to reduce voltage levels, but at the cost of reducing efficiency because power is given off as heat (due to the resistance).
 

David_Campen

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[ QUOTE ]
Seen and heard analogies made, like "imagine water through a pipe... voltage is how much water is flowing and current is like the water pressure", and on flashlightreviews.com, voltage and current are compared to marbles, etc.

[/ QUOTE ]
Yeah, except the other way around - voltage equates with pressure and current with volume of flow.
 

Monolith

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Not sure if this helps "get it," but everything tends to want to be at the same level or potential. This desire to be at the same potential creates a current between the differing potentials in an attempt to make them equal. We (collectively) make this work for us by forcing the potentials (voltages) to remain at a constant level (hopefully) despite the force trying to equalize the two potentials. We have learned that we can make this current work for us by controlling what the current flows through and how much current is flowing through it.

The relationship between voltage and current is:

P = I x V (power equals current x voltage)

To transmit power, it is more efficient to use high voltage and low current. A high current, low voltage power line would need very thick cables.
 

turbodog

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A good way to visualize this is:

Imagine you're standing on a beach and looking at waves coming in from the ocean. The height of the wave would be voltage..... it's more or less a potential for work. A higher wave can do more work/damage than a smaller one.

Now imagine there's a wall between you and the wave and that this wall has an vertical slot in it so that some of the wave will make it through. That's roughly resistance.... it resists the incoming water/voltage.

As the slot increases in width, more water is able to make it through. That would be current. The height of the wave is the same, but you're getting a wider chunk coming toward you.


These are good tools to try and understand the concepts. Be aware that they do not accurately describe the relationship between voltage, resistance, and current.

To try and directly answer some of your questions:

incandescent light:
The filament has a high resistance (compared to ordinary copper wire). As electricity passes through this filament wire it heats up. Heat loss is described by (i^2)*r.

That means that the heat lost in the filament wire is the square of the current times the resistance of the wire.

Example: 100w light bulb runs at 120 volts. We know that amps are volts/resistance. That puts the amps of the filament at 100/120 or .83 a.

So the resistance of the filament is: v/r=a or 120/.83 or 144 ohms.

So the bulb specs are:
100w (rated at 120v)
.83 amps (at 120v)
144 ohm resistance

If you increase the voltage to 240v, the current increases:
v/r=i (i is typically the variable for amps): 240/144=1.7 amps.

Put 1.7a into the above wattage equation: w=(i^2)*r, w=1.7*1.7*144, this is 416 watts. This is too much power for that bulb and it will "blow". It "blows" because the filament actually gets so hot the wire melts.

But the bigger relationship you need to see is this: why did doubling the voltage practically quadruple the power output of the bulb (100w vs 416w)? You're feeding it twice the voltage (that's twice the power right there). The doubled voltage results in a doubled amperage. This doulbes the output/power/watts of the bulb. That's why a doubling of the voltage results in a 4 fold increase in power.

some notes on this:
1) is I had kept up with my decimals better it would have been exactly 400 watts instead of 416
2) filaments (or most any wire that gets THAT hot) will change their resistance as they heat up. For our purposes we're ignoring that.
3) these are basic resistance/wattage/voltage/amperage calculations. electric motors are more complex, electronics are complex, but you wanted to get a handle on the basics.

Quick and dirty comments on an electric motor:
A wire that carries an electric current produces a magnetic field. The inside part of a motor (the part that spins) has windings of wire, so it has essentially magnets in there. These magnets repel against magnets embedded in the outside (the "can" or "housing") of the motor (the part that doesn't move). This causes the inside to spin HALFWAY around.

At this point a "switch" of sorts reverses the power to the inside "magnets" (actually electromagnets) and they repel against the outside magnets, the motor spins halfway around, the fields reverse again, the motor spins, and so on.

note: this is a really simple description of a dc motor with permanent magnets in the motor's can. There are many variation (ac and dc) on this. But the concept is the same.


whew!
 

greenLED

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turbodog, the beach/wave analogy is awzum(possum) /ubbthreads/images/graemlins/smile.gif This was exactly what I needed to make Quickbeam's "marbles in a bucket" analogy click.
 

prescottrecorder

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All of the equations you're seeing are just different ways of stating Ohms Law which is that current (I)= voltage (E, or in the above V) divided by resistance (R) , I=V/R or I=E/R depending on what naming convention you're comfortable with for voltage. So the more voltage or pressure with a given resistance, the more current flows. Power has to do with how much you're accomplishing or how much work is getting done. Pushing an amp of current through a low resistance isn't as much work as pushing the same current through a high resistance. Power (P)= Current (I)times Voltage (V or E) which is exactly the same as P=I(squared)times R after a little algebraic manipulation.

Keep in mind that things are not so simple with non resistive loads like inductors (coils like motor windings) or capacitors (or combinations of both) where the result is not a fixed value but one that varies with time. Terms like time constant,phase, and resonant frequency creep into the conversation.

Maybe it's obvious from our shared experience with hot thing s glowing, but everything above absolute zero temperature radiates electromagnetic energy. If it's hot enough, part of the radiation is pushed into the visible spectrum and we see it as light. Just walking around town, however, your body is mainly radiating infrared, microwave and other such lower energy, non visable radiation. Unless you've got your flashlight on /ubbthreads/images/graemlins/grin.gif
 

prescottrecorder

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Oh, and being hot is only one of the ways things generate electromagnetic radiation (called blackbody radiation). Synchrotron radiation is a much more powerful mechanism seen in broadband radiation viewed by radio astronomers when they look at the universe, for example. Coherent radiation(single frequency as opposed to broadband) also occurs in nature as elements and molecules fall from higher enery states to lower ones and radiate at a frequency which depends on the difference in energy between the states. Hydrogen radiates at 1420Mhz, for example, as a spin state spontaneously changes to a slightly lower energy spin state. LEDs exploit a higher energy difference in states to generate light. Laser (Light amplification by the stimulated emission of radiation) and Maser (same but with microwave frequencies) radiation is familiar to us as a technology but happened first in nature. The natural ones are really, really big though.
 

Bimmerboy

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Thanks for your response, Monolith.
OK, some of this is starting to come together. Between your reply, recent posts by evan9162 and others, and some research last month on fan speed controls, I may be starting to understand at least something.
Now I'm going to use quotes excessively...

[ QUOTE ]

Voltage is "potential" that is measured relative to something (typically "ground"). Voltage is basically the difference in charge levels between these two references.


[/ QUOTE ]

Would it be a good analogy to say voltage is like the potential energy of a bowling ball up on a shelf in relation to the floor, and the higher the ball is could be seen as higher voltage?
And more to what you said (I hope I'm using the right terms here), does electrical potential have to do with how many of electrons a negatively charged atom has to give up to a positively charged one (or is it the other way around)?

[ QUOTE ]

People often associate "ground" with zero voltage, but that is not necessary.


[/ QUOTE ]

Using the bowling ball analogy again, this would mean that the kitchen floor can be "ground" since it is lower than the shelf, but not necessarily having zero potential energy (there's still the basement)?

[ QUOTE ]

Current is the flow of electrons (or holes - I won't debate this) from a higher potential to a lower potential.


[/ QUOTE ]

Does this mean that current is a product of voltage, sort of like how kinetic energy is created when potential energy is released?

[ QUOTE ]

Current also produces electromagnetic fields as they flow. This is what makes an electric motor work (the fields interact with iron cores and electrically changes the iron core magnetic polarity to help spin the motor).


[/ QUOTE ]

Is this called inductance?

[ QUOTE ]

"Resistance" (e.g., resistor, etc.) impedes current flow (current follows in the path of least resistance - that's why people get electrocuted).


[/ QUOTE ]

I guess resistance is like shoving a lot of water through a little pipe. When this happens with water, the result is high pressure (sort of like a loss of efficiency... takes a lot of energy to move water under high pressure as opposed to low pressure). With electricity, the result is heat.
Another water comparison is that it also takes the path of least resistance.

[ QUOTE ]

A light bulb has a resistive element in it that glows when current is passed through it due to its resistance.


[/ QUOTE ]

This I knew... hey I'm good /ubbthreads/images/graemlins/wink.gif
So current is what produces physical work in an incan bulb, LED, electric motor, etc., not voltage?

[ QUOTE ]

Resistors are also used to create "potential" drops to reduce voltage levels, but at the cost of reducing efficiency because power is given off as heat (due to the resistance).

[/ QUOTE ]

Does a resistor work by resisting current, or lowering voltage?
Is current limited only by way of reducing voltage, or not neccessarily?

And last question for now (I'll be amazed if people suffer through my rambling)...
I've heard it said that it's not voltage that's dangerous, but current. That's why a stun gun can be 50,000 V, but doesn't shove a lot of current so it doesn't kill ya'.
So back to the bowling ball. The potential energy (voltage) of it sitting on the shelf is not dangerous, it's the kinetic energy (current) of it falling on your head that's dangerous. Is this sort of the idea?

So, on a scale of 1 to 10, how am I doing?
I hate to learn everything by analogy here, but electricity is a mysterious thing to the newbie. I also don't expect someone to sit there and answer every single question I just asked. Hopefully though, this thread will gather enough questions and answers to benefit anyone who is confused by some of these concepts.
 

Bimmerboy

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Wow... all these responses to read through! Thanks, guys. I was busy with making my last post so I'd better get readin'.
 

turbodog

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I'll try to touch on yoour points.

Bowling ball height is good and accurate.

Electrical potential does have to do with electrons available for transfer. Please note that mose chemical cells (a battery is actually a collection of cells) top out at 3-4 volts per cell. If you want more volts you have to 1)stack multiple cells together in a battery 2)use a non-chemical means (generator/alternator) to produce the electricity

Ground does not always equal zero. Example: I have a dc power supply for a computer. It has 3 output leads: ground, 5v, 12v. A measurement taken between 5v OR 12 and ground would read 5 or 12. A measuremenu between 5v and 12v would read 7v.

Voltage priduces current flow. Current flow is simple an measurement of the electrons moving. You cannot separate the two.

Inductance is when a magnetic field causes voltage to be created in an otherwise "dead" wire. Example: power pole transformer. Voltage enters the transformer from the power lines at say...... 100,000 volts.

The transformer is comprised on a iron core with wire from the incoming (known as primary) side wrapped around it. The voltage/current in the primary side emits a magnetic field that is picked up by the iron core. This is in turn picked up by a secondary wrapping of wire known as the secondary winding. This voltage is much lower and then feeds into your home. The difference in voltage is directly related to the ratio of the # of turn/wraps of each wire. In this case, it would be a stepDOWN transformer in a ratio of maybe 1000:1.

Inductance can be a nasty thing with long runs of wire. Lightning produces a magnetic pulse, and can induce voltage in the wire. I have seen plenty of network wiring pick up this pulse and then fry network cards and network switches.
 

prescottrecorder

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I'm thinking that your bowling ball analogy might work better like this:

The potential energy of the ball on the high shelf is more like the total power delivering capacity of a battery. It's amp-hour rating would be sorta like the height of the shelf. Voltage would be the force of gravity (which depends on the mass of the bowling ball). Things get a little dicey from here on because the laws of inertia don't really apply to current flows through resistors. As I think about it though,there are probably some analogies to current through a coil or inductor. Initial current after a fixed voltage is applied is small because the expanding magnetic field is essentially pushing back. As the field builds to it's maximum and eventually stops moving, current is also at it's max and limited only by the resistance in the coil wire and other series resistance in the circuit. But at that point there is energy stored in the magnetic field. Sorta like things have reached terminal velocity. If you suddenly want the current to stop (suddenly open a switch, let's say), the magnetic field will rapidly collapse and generate a high voltage and possibly an arc across the switch. Just like the bowling ball at terminal velocity has converted all of the potential energy it's going to into kinetic enery and that energy can be converted into some other form or perform work if the ball is slowed down (in a hurry if it hits your head).

So let me see if this makes any sense at all. Bowling ball on shelf is total power capacity available.

Mass related to voltage.

Current is velocity which is changing over time because of inertia. Equivalent to having an inductive load in a D.C. circuit.

When you push the ball off the shelf, the potential energy is converted to kinetic energy with a little loss due to air resistance.

When you apply a voltage across an inductor (coil), the energy is converted into a magnetic field with a little loss due to the coil resistance.

At this point, not much work is being done other than heating the air a little and the coil wire a little.

You could do a lot of work in a small amount of time by suddenly stopping the ball or current. In the case of the suddenly opening switch or suddenly increasing resistance, the collapsing fields are going to try to keep the same current going by generating a much higher voltage. The ball is going to want to keep going too due to inertia and is going to suddenly generate a lot more force than the pull of gravity in crushing your skull.

Or in both cases, you could forget converting one energy form into another and do some work right away, maybe by putting the ball on a teeter totter and making something on the other end move against resistance. Or by having the magnetic field cut through another coil and induce current in it to do some work (like in a motor or transformer).

OK, I'm finished for now. I'll probably read this after I post and wonder what the heck I was talking about.

It's fun to try to make analogies



/ubbthreads/images/graemlins/grin.gif
 

Leeoniya

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voltage and resistance and current flow are like this:

so u come home from a party with your relatively sober (high resistance) girlfriend, and despite the pressure you apply on her to get into bed asap (voltage), she still refuses. so you pull out a bottle of fine wine, or champagne in attempt to lower her inhibitions. you convince her that it'll help her sleep and she grabs a glass of alki - (current). as the flow increases, the resistance drops. (rather vice versa, but stay with me here). once she's had a few drinks, her resistance is much lower, and the flow of alcohol is much less restricted. allowing you to lower your pressure (voltage) to get the same amount of alki through. eventually she'll break down like a zener diode and consume so much alcohol, she'll wish she was a TVS chip.

...and then we all morph into the Tron movie.

hope that helps.
Leon.
 

andrewwynn

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LOL.. thanks for the laugh leon..

I was in EE in college and was going to make my thesis on a complete water-based analogy system of all the important electrical concepts like switches (valves).. motors (pumps).. wires (pipes).. i never got that far, so i've only done them 'piecemeal'... but there is a very simple analogy of voltage and current that can be represented with water.

'pressure' in water is the direct equivalent of 'voltage' in electricity.. it is the magic that 'pushes' the liquid.. a child understands that if there is 'more pressure'.. that 'more water' will come out of a given supply system (i.e. hose). The rate of flow of the water is measured also as 'current' coincidentally... so the water analog of 'current' is also 'current'.

in the water analogy.. the 'total amount' of water represents the energy.. just like the total amount of electrons in the electrical system represent the energy... the amount of energy that is flowing past a given point represents the current.. but you can't tell 'power' by current alone.. power is dependent on both the current and the voltage.. or current and pressure... in the water analogy.. opening a faucet will increase the current, and since in most cases the pressure at the outlet is not affected much you will by in large increase the power being released..

in the electrical equivalent of this circuit.. the valve becomes a 'variable switch' or rheostat (similar to a potentiometer).. variable resistance to flow... the 'pressure' of the water behind the valve becomes the 'voltage' of the circuit.. the 'valve' becomes a variable resistor .. as the resistance is lowered.. the current will increase. if the voltage source has the capability to hold the voltage level, than power to the circuit will increase proportionally.

current is the energy moving through the 'pipe/wire'... there is a direct relationship between energy, time and power.... energy = power x time... and power is also current x voltage ... so energy is current x voltage x time...

in the water system.. current would be measured in the likes of gallons per minute or liters per minute.. in the electrical system the unit used is ampere which basically is a measurement of how many electrons move past a point per second (and it's a hellovalottofem).. 6.241506 x 10 ^ 18 electrons to be fairly precise.. that magic number represents the charge of a 'coulomb'.. one coulomb of charge per second is one definition (of many) for an ampere... 'current' in electricity is measuring how many coulombs of charge pass through a given point in a circuit per second.. 1 coulomb per second is exactly 1 ampere.

in and of itself current is not a very useful measurement..... if there is a lot of pressure and no current there is no effective 'work' or 'power'.... to know what is going on both current and pressure (voltage) is required.. voltage x current represents power or the ability to do work... pressure with no current is a static system (like your pipes in your house being pressurized.. this is 'potential energy'.. another good analogy is a balloon or your car tire).. if there is a lot of current and no pressure... that's a tougher analogy since current is directly related to differences in pressure acting on a fluid... but... think of trying to blow on something distant from you... you can affect a candle 20' away from you with a lot of breath.. but since the pressure is so distributed you won't be able to blow it out.. this 'system' has a lot of 'power' and a lot of 'current' but no 'pressure'.... now take that same 'system' and change it around.. use a 20' long hose.. and put it next to a candle.. you can blow on the hose and blow out the candle because the 'power' (pressure x current) is focused.. the pressure is maintained by keeping the area small.. now the power is concentrated on a small area.. both systems have the same power (say 4 liters of air per second... x .05ATM pressure).. at the source.. but w/o the focus of the hose.. the first example.. the power is spread so thin there is no 'force' on the flame.

Light works this same way.. the units of lux vs lumen are directly related by area and distance.. lux is a unit of brightness.equivalent to 1 lumen of light spread out over 1 square meter of area at a distance of 1 meter... it dims at the rate of the SQUARE of the distance so the intensity drops of very quickly.

So.. summary.. voltage is the 'pressure' that pushes electrons around.. current is the motion of those electrons. Moving the electrons under pressure represents moving energy and when you move energy over time that is power... power is the useful thing, since it represents work getting done in a known amount of time... the amount of energy represented in the motion of a continent is unimaginable... but when a tiny bit of 'continental motion' happens in a short time vs the slow progress usually happening.. the 'power' is unimaginable and that's the likes of a big earthquake.

hope that wasn't too confusing.

-awr
 

VidPro

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no no no :)
its like ants, ya see .
you have a line of ants, the more ants per second are the voltage. the more ants per second the more irritating they are.
the amperage is when you have Army ants, with the same number of ants per second down each line.
but 150 lines of them.
its the ant current that gets you :)
 

Bimmerboy

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LOL @ "it's the ant current that gets you".
A huge debt of gratitude to everyone for the detailed explanations and analogies. I'm finally getting it! This makes my day actually, partly because it's now so much easier to understand how eletricity works when applied, and makes the bits and pieces of knowledge randomly floating around come together. As with most things, when you know a few things here and there about a topic, but don't really understand the basics or foundation, the whole subject just doesn't make a lot of sense.
This is cool stuff, so now it's time to start memorizing formulas, etc... *curl left hand fingers, stick out thumb... stare at it for a while*
 

MrAl

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Hi there,

Some interesting (and funny) posts here /ubbthreads/images/graemlins/smile.gif

Another way of looking at these two (current and voltage)
is to look at the way they appear to us humans rather than
try to explain what's actually going on in the quantum
world.
Luckily, there is a VERY simple way to do this...

Voltage is a quantity that is measured ACROSS something
simply because it shows up in the physical world that way,
while current is a quantity that is measured THROUGH
something because whatever makes up the current flows
through something.

It's easy to remember this, and you'd be very surprised
at what you can calculate knowing these two simple facts.

VOLTAGE <==> ACROSS
CURRENT <==> THROUGH

These are the most important facts about current and
voltage.

We are limited as to what we can detect with our bodies
(eyes, ears, etc.) so to make the leap from the quantum
world up to our level we have to use instrumentation to
help us.
We use a voltmeter to measure voltage and an ammeter to
measure current (or a multimeter that does both set to
the correct quantity to measure).
To measure a voltage we place the meter leads
ACROSS an element, and to measure a current we connect
the meter so that the current flows THROUGH the meter as
well as the circuit.


Take care,
Al
 

lymph

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I like the water and pipes analogy. I'm no EE, but this is how I started to think of it. All the wires or traces in your circuit are pipes, and they're always filled with water (they have electrons that can easily be moved when pushed). Connecting a battery is like connecting a pump - it provides pressure (voltage) on the water in the pipes which makes it move (current). Smaller pipes, or pipes with lots of bends (resistors) will allow less water through (current) with a given amount of pressure (voltage).

Power is the product of the pressure and the current. Your bigger pipes can handle more power. Voltage is kind of like the speed of the water molecules, and current is like the actual amount of water molecules moving past a point.

This is really simplistic, and probably incomplete and/or wrong, but it works for me. It gets kind of screwed up when we talk about capacitors. /ubbthreads/images/graemlins/smile.gif Feel free to take this apart, smarty-pantses.
 

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