What the hell is current? (And other electrical conundrums)

[Raj Ahten]

Well I just started a new job with the railroad and as the main part of that I'm having to learn about electrical circuits. I'm having a hell of a time figuring out what exactly current is. Voltage and resistance make sense in my mind but overall electricity is staying damn mysterious to me. Meanwhile I'm using Ohm's Law and Watt's law to figure out values in circuits but I'm floundering badly as I'm plugging numbers into formula's with no idea of what the true meaning is.

So can anyone here point me to some sources that can explain this to a complete neophyte? The video's and such I've found online have been supremely unhelpful so far.

[Grandmaster Jogurt]

Current is the rate of flow of charges, either of electrons or ions. It's in units of charge (Coulombs) per second. A rough picture you could have, I think, is it's how many electrons are flowing past a point every second.

Are you familiar with hydraulics at all? A decent analogy can be made to hydraulic systems: voltage is like the height difference between two points, resistance is like the hydraulic roughness/friction, and current is thus the amount of water flowing past a point over a certain timespan.

[Simon_Jester]

Current is the amount of electrons going past a point on the wire in a given second.

Resistance measures how hard a substance makes it to push current through it.

Voltage measures how hard current is being pushed.

Good enough?

[Raj Ahten]

Thanks for the replies. They should help me get started on it. For some reason all the explanations I found were all extra obtuse.

[Cykeisme]

Eh, already been explained better, but I just thought I'd put my layman's two cents in.

If you look at it like flowing water, these would be the equivalent (and unit):

charge (coulombs) is anologous to "volume of water" (liters)
current (coulombs per sec AKA amperes) is analogous to "flow rate" (liters per second)
voltage (volts) is analogous to "pressure" (newtons per square meter AKA Pascals)
resistance (ohms) is analogous to "flow restriction" (?)


And crap, I just realized I have a question for the board. Is there an SI unit for flow restriction other than simply "Pascals per liter/sec"?


Edit: I was searching the internet to satisfy my random curiosity about a unit for restriction in flow (which I did not find), but then I found this, which may be helpful.. it's a wikipedia page about exactly what you might need:
http://en.wikipedia.org/wiki/Hydraulic_analogy

[Magis]

And crap, I just realized I have a question for the board. Is there an SI unit for flow restriction other than simply "Pascals per liter/sec"?

The most appropriate fluid mechanics analogue to electrical resistance is pressure drop / head loss, and the most appropriate analogue to electrical resistivity is friction factor. http://en.wikipedia.org/wiki/Darcy_friction_factor

Not that the fluids/electrical analogy is very good, because it's not.

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A bit more technical definition of the electrical terms follow.

Current is the net rate of charge flowing past a plane that is oriented perpendicular to the flow. Note that 1 electron has a charge of 1.062x10^(-19) Coulombs. Current, I, has units Coulombs per second.

Voltage difference, or potential difference between two points in a circuit is the energy that is done by 1 Coulomb of charge as they move between points (assuming the starting point has higher voltage/potential than the destination point). Hence, voltage, V, has units Joules per Coulomb (also called Volts).

In a conductor/resistor, as voltage increases between two points, so does the current flowing between them. V is proportional to I. Resistance, R, is the constant of proportionality between V and I. Hence, V = I R. Different materials have different values of R. Resistance can also be thought of as determining the amount of power (energy/sec) done in the resistor given a voltage, V, across it. Power = (V^2)/R.