hey everyone, im new to the board and have many questions.first one, what makes a magnet attract?
thanks, scott

[This message has been edited by electricman03 (edited 06-08-2003).]

Welcome to ECN!

Got quite a few good questions there! I'll try to answer for you with some concise and simple explanations.

1:

In the 3 magnetic elements (Iron, Nickel and Cobalt), the atomic structure (lattice) is arranged (or becomes arranged) in straight lines. The atoms line up kind of in a "Hand-in-Hand" fashion, which results in a circulating flow - hence the magnetic attraction.

2:
When electrons flow in a conductor, there is a resulting Magnetic field produced (at a right angle to the flow).
In the core of a transformer, or a solenoid's core, the winding produces a magnetic flux in the core - via Induction. The magnetic flow acts similar to the electrical flow, and changes are made correspondingly.

3:
Simplest answer: A difference in Potential.
Simple example: Pick up a rock off the ground. Hold it above the ground. The rock can fall to the ground - hence there is a Potential Difference between the rock and the ground.

4:
For DC circuits (like a car battery or a drycell), the current flows from Negative Polarity (Cathode), through the load device (like an Incandescent Lamp), then to the Positive Polarity (Anode).
The current flowing is in the form of Electrons, which are "-" charges.
In actuality, there are "+" charged "Holes" flowing out of the Anode towards the Cathode, and the Electrons "Fall" into the holes - but that's a little advanced for now!
DC is Direct Current, meaning the current flows in a single direction with a nearly fixed amplitude.

For AC, there is no fixed Polarity, so it is safe to say that current flows in BOTH directions.
On a simple 2 wire circuit, current flows in one direction for 1/2 of a cycle, then that same current reverses direction and flows the other way for the remaining 1/2 of the cycle.

Think of how a See-Saw reciprocates back and forth, and you can invision the current flowing in an AC circuit.
Another visual method is the way a Sawsall blade recips back and forth, also a piston / connecting rod and crankshaft for an internal combustion engine.

AC stands for Alternating Current, and has no fixed Polarity.


5:
Briefly described above. This is an indepth subject, so best for you to study this one on your own with some very good manuals!

AC and DC are two completely different Animals! Even though they are in basic sense very similar and can use similar basic equations / formulas / circuit elements, AC has SOOOOOOOOOO many more advanced items which makes it very different - and extremely complex!
Much easier to grasp the basics on your own via books, then we can go into more detail on line.

I could try to toss out some AC basics, but it is very difficult to use text in describing AC basics.

Check out some of the items posted in the Technical Reference section, as they may be a little help or inspiration.

Scott35

p.s. needed to edit bad spelling!

[This message has been edited by Scott35 (edited 06-08-2003).]

i have one more question for now, what makes electricity flow through a circuit only when you flip a switch ?

A switch is a physical break in the pathway taken by the current. An open switch is tantamount to an infinite resistance in the circuit. Think of a hose feeding water to a sprinkler. If you step on the hose the constriction in the pipe is now that infinite resistance and current flow stops. Remove the resistance and current flows again.

Your other questions about why do magnets attract are very deep and fundamental. There are four fundamental forces in nature. They are, in order of strength: gravitational force, weak force, electromagnetic force, and strong force. You can easily test the first. Drop an apple. The apple is attracted to the earth, and the earth to the apple. Why? How can there be this action at a distance? Even Newton himself was disturbed by this. The electromagnetic force is another 'action at a distance' force that governs the interaction forces between electric charges. It is one of the rules that make the universe work the way it does. By the way, magnetism is a manifestation of moving magnetic charges. If there was no electric charge there'd be no magnetism. Iron, cobalt etc. just happen to be materials that display their magnetic properties at common temperatures because of there microscopic crystalline structure. Anything with electric charge is in someway magnetic. Even the neutron which has no net electric charge has a so called magnetic moment.

The simplest explantion is that a magnetic field is associated with a moving electron. It is a "force" in the way if affects its surrounding. That is its ability to attract or repel other "particals" in its space.

Whether and electron is moving through a medium, such as a conductor, or around the nucleus of an atom, it affects the space around it. By, itself, the elctron's effect is weak, but when coupled with the effects of other electrons in the same space, the effect is stronger. But, that is only true if the electrons are alligned the same way, or else their effects will cancel.

Iron atoms, unlike others, have the ability of their elctrons in orbit to align them selves in the same direction. This is called polarisation. The total effect of this can be felt far outside the iron mass.

Electrons moving together in unison in a conductor also produce a magent field. If that conductor is wound in the form of a coil, the magnetic field produced in each turn is magnified, and the effect is stronger. A force is felt far from the coil, but drops off in strength per the square of the distance from the coil.

The force produced in a conductor is caculatable and is the defining point for the unit ampere.

"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per metre of length."

I have a question to this statment by Scott35: "For AC, there is no fixed Polarity, so it is safe to say that current flows in BOTH directions."

For 120 Volt circuits it seems to me that the current has only one direction. Although the AC generator creates a nominal voltage of +120 to -120, I only see current flow when the voltage is positive. Therefore, when the voltage is negative, there is a lull in current flow.

If you can imagine a 120/240 Volt transformer with a center-tap to ground. As the AC generator cycles, the direction of the current changes from A leg to B leg. However, Whether the current is flowing from A or B, the potiential to ground will never be overcome, therefore it will always flow through the neutral to ground.

I have to appologize if that isn't entirely clear, I'm still pretty new to electricity but desire to know as much as I can.

Thanks,
Byron

Byron,

current will flow if the potential is positive or negative. Thus, the current has two directions also for an ordinary 120VAC circuit.

Imagine a DC system with +120V and -120V to ground. A light bulb will work both between the +120V and the grounded conductor and between the -120V and the grounded conductor. Clearly, there is a current flow in both cases. This type of system was in fact in fairly extensive use in the first half of the 20th century.

Byron;

C-H answered the question very concisely!
Good job, C-H!!!

AC current flows in both directions in the circuit - regardless if the circuit is Line-To-Line or Line-To-Common (AKA "Neutral").

Current flows in one direction for ½ the cycle, then flows in the opposite direction for the last ½ of the cycle.
This happens whether or not the circuit has a Grounded Conductor. Grounding a circuit Conductor only drives it + the System to a "Ground Reference" - it doesn't create a Half-Wave situation between an Ungrounded Conductor and the Grounded Conductor.
(BTW: "Neutrals" are commonly Grounded Conductors)

Let us know if you have additional questions regarding this topic! Feel free to ask away!

Scott35

p.s. the DC 3 wire system C-H is referring to can be found in the Technical Reference section.
Also, under normal situations, there will be almost no current flowing in the Earth ground its self.

Thanks for the response. These concepts are sinking in slowly. I have purchased Delmar's Textbook of Electricity which is helping. Are there any other books or sites you would recommend for a beginner?

Thanks,
Byron

Byron,

Just to add a further point which might make it easier to visualize.

If you connect a load across two conductors, current will flow if there is any difference in potential between them.

It doesn't matter if one wire is grounded and the other at some positive or negative voltage with respect to ground, or whether one wire is positive to ground and the other negative.

In fact you will still get current to flow if both wires are positive to respect to ground or both are negative with respect to ground, so long as there is a difference in potential between them.

Electrons will flow from whichever wire is the more negative to the other wire. It doesn't matter where the ground connection is applied.

With AC, the polarity is changing (120 times per second in the case of a standard U.S. supply), so the current does indeed flow in both directions alternately -- Hence the name alternating current.


[This message has been edited by pauluk (edited 11-08-2003).]

Byron;

Thanks for the response.
These concepts are sinking in slowly.

No problem; this stuff does take a while to grasp (especially discussing it in non-real time + with text!), so just keep asking away when something is unclear.

I have purchased Delmar's Textbook of Electricity which is helping.
Are there any other books or sites you would recommend for a beginner?

For books, get one which covers advanced AC theory in some detail, along with another book covering basic Electronics.
Additional books would be something covering applied power system designs and calculations - like the "Designing Residential / Commercial / Industrial Projects, using the NEC" series of text books.
Others would be circuit composition manuals.

As far as sites, just pop questions at us here!
There are many reference sites - and a few nice link sites, but I don't have any suggested ones.

Toss some questions our way per the subjects you are studying in the book you have now (or from others you get in the future), and see how your progressing.

Scott35