# Still confused with basics

Discussion in 'Electronic Basics' started by jaydee, Aug 21, 2004.

1. ### jaydeeGuest

Since an electrical outlet might be used for any size appliance it
must contain a full/max current and then the appliance only uses what
it needs... right?

Does the appliance then reduce it? A simple lamp lightbulb uses very
little but doesn't contain any electronics that would do this. So how
does it use only a few watts and what happens to the rest?

Secondly, the electric meter and bill indicates watts used. How does
this work? This seems to imply that the current is always flowing in
and out like a two-way road and the meter measures the difference
(unlike all the water analogies that flow into a dead-end).

Thanks Jay.

2. ### John FieldsGuest

---
Its resistance keeps it from taking all of it, and what it doesn't use
is there for anything else that plugs into the outlet.
---
---
It measures how much current the appliance uses over time, and it
knows what the voltage supplied to the load is, so it multiplies the
voltage, current, and time and comes up with watt-hours, which is what
you pay for. In the mechanical watt-hour meters, the speed of a
spinning disk increases as more current is drawn, and that spinning
causes a series of dials to move which point to ever-increasing
numbers, so that what happens is that when the readings are taken
month-to-month, the number from last month is subtracted from this
month's number and the difference is the energy you used between the
---

3. ### John LarkinGuest

Think of the power coming into your house as a rotating shaft.
(Factories actually used to have one big rotating shaft from a water
wheel or steam engine, and it ran everything.) If it spins freely with
no load, no energy is taken and the power bill is zero. Any device can
be connected to the shaft (they used to use belts or gears) and it can
use all the horsepower it wants or needs. A mechanical device uses
torque, and the power it gobbles is torque times speed.

An electrical system has wires (== shaft), voltage (== rotation of
shaft), and loads take as much current (==torque) as they need. The
price you pay is voltage * current * time * some_dollar_factor,
because that quantity represents how much fuel the utility had to burn
to make that much energy.

A low resistance load is like a high-torque load. It may even pull the
line voltage down a bit, just like a big load might slow down the
drive shaft some.

I like that better than the water pressure analogy.

John

4. ### Michael A. CovingtonGuest

Repeat after me...

VOLTAGE IS THE FORCE THAT CAUSES CURRENT TO FLOW.

That is, voltage is the cause and current is the effect.

When you apply voltage to anything, the amount of current that flows depends
on the resistance of that thing. That's why a big light bulb consumes more
current than a little one. It has a lower resistance and lets the electrons
flow through it more easily (thus grabbing more of them per second and
giving off more heat and light).

Voltage = the pressure that makes electrons moved.

Current (amperage) = a measure of the number of electrons actually going by
per unit of time.
The same thing that happens to the water that isn't coming out of your
faucet when the faucet is turned on just a little bit. Those other
electrons aren't flowing. Pressure is available to make them flow, but
pressure is just pressure -- not flow -- if you don't let many of them move.
It indicates watt-hours. That is voltage times current times time. I.e.,
pressure times number of electrons that were actually moved through your
house at that pressure.

It's as if the water company were charging you for the pressure of the water
as well as for the quantity of it. That would be a reasonable way for them
to charge you if you were using to turn waterwheels (rather than just
consuming the water). The power company is charging you for the energy of
the moving electrons, rather than for the electrons themselves, because you
give them all back, exactly as if you were using water to turn waterwheels
and then giving it back. They'd charge you for pressure and for the volume
that flows by, because the product of these is the amount of energy you've
received. (Not the amount of water.)

5. ### John FieldsGuest

---
Yeah, me too.

The only thing that isn't taken care of is the oscillatory nature of
the AC, which would correspond to an oscillating shaft coming into
your house. For heaters and lamps it wouldn't matter, you just grab
the shaft and friction does the trick, no matter which way it's going,
but to get the unidirectional rotation you need to run compressors and
simulate motors you'd need some kind of a mechanical converter. Maybe
like those little manually propelled railroad cars with the push-pull
bars used?

6. ### John LarkinGuest

Right, it's not a great mathematical analogy to AC. And a shaft has
"inductance" (torsional flex) and "capacitance" (mass) but no
equivalent of resistance, unless you make it out of taffy.

If you apply a sudden twist to a long shaft, it propagates just like a
step on a transmission line.

I did design a mechanical full-wave rectifier once, so a ship shaft
rotation indicator would count revs in both directions. It used a
couple of shafts and some belts, and some really cool one-way
mechanical couplings that look just like bearings and act like diodes.
The MEs were impressed.

John

7. ### John FieldsGuest

---
Don't you have it backwards?

That is, since mass fights anything trying to make it, or stop it,
from moving, isn't that like inductance, which fights tooth and nail
to keep any change of current through it from happening?

And isn't torsional flex like capacitance, which will absorb as much
energy as you can push into it until it starts to fill up and fight
back?

Also, doesn't it have resistance? No matter what you make it out of
it'll still get hot if you flex it, I think. Just like a cap has ESR
and a choke has winding resistance to contend with, a shaft has grain
boundaries which rub against each other and and spoil what would
otherwise be a perfect Q.
---
---
Yes, nice.

And the shaft will have a characteristic impedance, just like a
transmission line, but I think those guys call it its "moment of
inertia"
---

8. ### jaydeeGuest

Thanks guys, I'm seeing the light (a little).

So an appliance built with low resistance takes more current and an
appliance like a light bulb contains some material causing high
resistance.

Regarding the meter... it's not measuring the difference between what
came in and what's going out, but simply the flow into the house
caused by some applicance usage.

If yes, then why do we need the neutral wire? Whats the extra wire
leaving the house for? I thought I understood the loop concept.

Thanks again,
Jim

9. ### John LarkinGuest

Right. A light bulb with a fat filament will conduct a lot of amps
(let a lot of electrons rush through) so use a lot of power and get
very bright. A skinny filament throttles down the current to a
trickle, so uses less power. If you weld ten skinny filaments together
side-by-side, you make a fat filament that conducts 10 times as well.
That's "putting resistors in parallel." Weld them end-to-end, and the
current really trickles down, resistors in series.
Right. Actually it integrates the product of voltage * current. The
voltage isn't supposed to vary much from 120, but the meter accounts
for it anyhow, just to be accurate.
Current needs a loop, just as a shaft needs a foundation connecting
the motor and the load to return the torque. Without a shared
mechanical anchor for both ends, the motor or the load housing would
just spin and no power would be delivered. We use the neutral wire as
the reference or foundation to conduct the return current. We could
(electrically and mechanically!) use the earth (ground) as the return
path, but it's safer to use a neutral wire, and use the third (ground)
wire as a safety shield.

John

10. ### John MillerGuest

It's because your home service has *two* 110-volt circuits, which can be
used individually for 110V, or combined for 220V. The neutral is the
return for either/both of the 110V legs, and is not necessary for 220V.

--
John Miller

Life is like an onion: you peel it off one layer at a time, and sometimes
you weep.
-Carl Sandburg

11. ### John FieldsGuest

---
It's like this:

HOT--> +------------------+----------------+------>TO MORE HOUSES
| | |
| | |
+------|-------+ +-----|-----+ +-----|-----+
| | | | | | | | |
| [GENERATOR] | | [TOASTER | | [ANY LOAD]|
| | | | | | | | |
+------|-------+ +-----|-----+ +-----|-----+
| UTILITY | YOUR | ANY
| COMPANY | HOUSE | HOUSE
NEUTRAL--> +------------------+----------------+------>TO MORE HOUSES

12. ### Rich GriseGuest

The electricity doesn't pile up at your house like water, or go
into the light bulb and dissipate as light or anything like that.
(not the electricity itself - its _energy_ comes out as light.)
For the water model, it still comes in one pipe and goes out the
other.

The energy gets transferred by the movement of the water. How
hard does the power station have to push it (the pressure, or
voltage) times how much it has to push (the flow == current).
That force times motion is the work done, or energy transferred,
and the power is the rate at which that energy is getting
transferred. The water, or electric charge, is just the medium
for moving that energy from one point to another.

Hope This Helps!
Rich

13. ### jaydeeGuest

The light is getting brighter!

I just thought of another analogy... If you had a well connected to
your water heater... you would be paying to heat the water, but not
for the water itself.

My real objective was to understand enough to hook up a generator to
the breaker box and reduce my bill. Transfer switches do the opposite
of what I need... they work when the utility power is off and
disconnect when the power comes back on.

I've also read someplace that home generated power isn't pure (?)
enough to blend with the power company's. If so my only alternatives
would be to use extension cords or a sub-panel for the few circuts I
could feed (yes/no).

Thanks for the education,
Jay

14. ### John FieldsGuest

---
Actually, it's backwards from that in that when the utility power is
ON, that power actuates the switch and causes it to connect power to
the loads in your house. When the utility power goes off, the
transfer switch reverts to its de-energized position and a set of
normally closed contacts connect the alternate power source to the
---
---
I don't understand what you mean. Are you talking about selling
energy back to the power company or running some things on your house
on your own power at the same time you're running some others on power
company electricity, or connecting your power source in parallel with
the power company's so that you can run your house on both at the
same time?
---

15. ### Jonathan KirwanGuest

What that probably meant is in connection with those systems that actually send
power back into their system from your remote location and that in these cases
the power company needs your generation facility to be "compatible" with theirs.
Some power companies will make the meaning of "compatible" to be a nearly
impossible goal to reach, just to be a pain. Some will encourage you and help
you understand the details. But you can't just hook a generator up and start
feeding power back into the lines without synchronizing up and without the right
pieces in place. If all you are doing is to use your locally generated power to
defray your own use and are NOT trying to push power back into the system of
your provider, these issues don't arise.

At least, that's my guess about it.

Jon

16. ### Michael A. CovingtonGuest

My real objective was to understand enough to hook up a generator to
Can you generate power more cheaply than the power company?

When dealing with AC, you have to synchronize your oscillations with
theirs...

17. ### Peter BennettGuest

You MUST NOT connect your local generator in parallel with the power
company!!!

You will need a manual transfer switch to transfer those circuits you
want to power from your generator between the generator and power
company.
"purity" isn't the problem. AC power is continually varying in
voltage, so, if you want to connect two AC sources in parallel, you
must ensure that both sources are "in phase" (varying exactly in step
with each other) and of the same voltage.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter

18. ### Rich GriseGuest

I was working at a temp job as a document coder for big litigations.
One of the litigations was about some company who was going to make
a big profit that year that the government mandated that the power
companies have to buy power from the customers. They had all kinds
of schemes for excuses to generate power, like gas turbine generators
with the exhaust used to heat a greenhouse, and crap like that.

They went broke.

Good Luck!
Rich