Real basic voltage/crrent questions...

[T-Dogg]

Can someone explain voltage and current in a simple fashion to me? From what I understand, voltage is the difference in potential energy between two points, and current is the "power" of the electrical flow between those two points - kinda like the whole water analogy where a it will flow faster when there's more pressure behind it. I don't even know if this much is accurate.

But what really confused me is, I've been reading a beginner electronics book someone recommended ("The Art of Electronics" by Horowitz & Hill) and they started talking about circuits where the current and voltage are out of phase with one another. This made me realize I don't understand any of this. I thought voltage and current were just different quantitative measurements for the some electrical signal - but this makes it sound like they are two seperate signals that can be out of phase. If anyone can point to a good reference to start understanding even basic stuff I'd really appreciate it. (But I really don't want to spend another $75 on another book thats too over my head

Thanks!
Tim

 

[SSLtech]

Current is like the flow of water past a given point in a river... that's why it's called "current". It is measured at a single point.

Voltage is like the 'force' of the water pressure, even if nothing is actually flowing. -A 9V battery with no circuit to allow water to flow is like a dam holding water, with no pipes to allow water to flow. Voltage is measured as the difference in potential between two points.

Resistance is how "thin the pipe is" when you make a circuit... a wide pipe has low resistance and allows much current to flow for a given pressure. A narrow 'bottleneck' pipe allows only a small amount of current to flow for the same pressure, and has a high resistance.

For any given resistance, more pressure (voltage) will also cause more current to flow.

Thus in electrical terms, Ohm's famous law:

V (volts) = I (current) x R (resistance).

10 volts through a 10 ohm resistor causes 1 amp to flow (10 = 10 x 1)
10 volts through a 1 ohm resistor causes 10 Amps to flow (10 = 1 x 10)
10 volts through a 2 ohm resistor causes 5 Amps to flow (10 = 2 x 5)

...and so on.

A way of drawing it out is as a triangle with V on top, and I and R on the bottom. THen you can see by glancing at it that:

V = I x R,
I = V over R,
R = V over I

Does this help?

Keith

 

[SmG]

[quote author="T-Dogg"] I thought voltage and current were just different quantitative measurements for the some electrical signal - but this makes it sound like they are two seperate signals that can be out of phase. If anyone can point to a good reference to start understanding even basic stuff I'd really appreciate it. [/quote]
Try this - or even the beginning of this...

 

[T-Dogg]

Keith -- great metaphor, thats exactly what I was looking for! Pretty sure I get it now, I need to put my bib on and be spoon fed this stuff in simple terms ya know!

The second example with the capacitor charging helped with the phase thing too... (I think -- seeing it made sense enough to get the basic idea'r...)

Ok -- one more question:

When they talk about the oscillation of current and voltage in an AC circuit, what does the amplitude of the oscillation mean for current? For voltage I'm assuming it's swinging more positive or negative... Is the current moving back and forth through the circuit -- like water flowing through a pipe in one direction and than abrubtly changing direction as you head from peak to trough on the sine wave?

Sorry guys, but I promise I'll do my homework outside of class too!

Thanks so much,
Tim

 

[PRR]

To "see" current, make a mark on the wire, look real close, and count the number of electrons that pass this mark in one second. One zillion electrons per second is one Ampere.

We can't actually see electrons, but the indirect measurements and the Standard Ampere are conceptually the same as "one zillion electrons past a point in one second".

Same as if you were inventing a way to measure the flow of water in a river. Build a dam. Use a 1-gallon bucket to throw water over the dam as fast as it comes in. Count how many buckets per second. This isn't practical, so you use an indirect method (dam it, but then cut a notch in the dam, and see how high the water rises in the notch.) The "buckets per second" for a major river is an awkwardly high number, so you pick a convenient reference. Say you figure the Nile is a nice average river. Measure it, and call that flow "One Nile". Then you can look at other rivers and rate them. The Mississippi might be "7 Niles". Deadwater Creek might be "0.002 Niles".

Voltage is water fall. Say you have one river that drops 100 feet from source to sea, in a wide straight channel; another that drops 1,000 feet in a narrow twisty channel. They might both have the same gallons per second. But if you set up water-wheels right, you can get a lot more POWER from the same current with a higher drop.

Power is then Flow times Drop. Take two rivers: one has 1 Nile of flow with 100 foot drop, the other has 0.1 Niles of flow with 1,000 foot drop. You can get the same Power out of either river, though you will have to use different proportions on your water wheel. The 1 Nile 100 foot river would use a wheel that is very wide but not very high. The 0.1 Nile 1,000 foot river could use a narrow wheel, but it has to be very high. The ratio of height to width in the water wheel is similar to the resistance in electrical circuits.

 

[PRR]

> Is the current moving back and forth through the circuit -- like water flowing through a pipe in one direction and then abrubtly changing direction as you head from peak to trough on the sine wave?

Yes and no. If you look real close, the electrons in an AC current do move back and forth. But with a sine wave, there isn't an "abrupt" change of direction. More like daily rush-hour on the highway. In the morning the cars go into the city, in the evening they go back home, but the flow gradually increases 7AM to 9 AM, then falls off, and around 4PM starts increasing in the other direction to a peak, and then falls off again. (Rush-hour traffic isn't quite sine, but you know it fades up and down, no abrupt change of flow or direction.)

 

[Scenaria]

I used to hate the electron flow vs hole flow....

 

[crazydoc]

[quote author="PRR"]But with a sine wave, there isn't an "abrupt" change of direction.[/quote]
And a sawtooth (with a little slope to the negative-going leg) would be people driving leisurely to work, then turning around and speeding home when they get there. A square wave, of course, would be Scotty beaming them back and forth.

 

[PRR]

> A square wave, of course, would be Scotty beaming them back and forth.

Well, no. It isn't about how fast, it is how many per second.

The analogy would be: from 8AM to 9AM everybody drives west. From 9AM to 10AM, everybody drives east. And so on.

 

[crazydoc]

Duh! You're right, of course.
I was thinking of the slope of the curve as the current, whereas actually it's the change in current.

 

[CJ]

In a capacitor, the current has to flow into the plates to build up voltage.
So the current gets there before the voltage.
Usually 90 degrees quicker.

In an inductor, the voltage leads the current by 90 degrees.
I don't have a simple anaolgy for this.