A lot of electronics questions

[David_Simmons]

@panamajack

I’d still say electricity is primarily about the flow of energy, not necessarily electrons or even charge. Current is defined as the flow of charge. The energy propagates at c, the speed of light in a vacuum [And while this is technically always true, it’s almost always less confusing to refer to the signal traveling at less than c].

This is true but it’s still convenient to not complicate the thing. If one coulomb/sec of electrons starts at one end of a conductor, one coulomb/sec of electrons will flow out the other end at a later time equal to the length of the conductor divided by the speed of light in the medium. Since electrons are indistiguishable we might as well say they are the same electrons.

[panamajack]

@David Simmons

Since electrons are indistiguishable we might as well say they are the same electrons.

Fair enough.

I did say I was overcomplicating with respect to electromagnetic energy. All photons always travel at c, but it’s almost needlessly quibbling to point that out. An EM wave traveling through a medium interacts with the medium in such a way that we may as well say it’s traveling at less than c. (Nonetheless someone here is always prepared to quibble needlessly.)

edit: In my opinion, the situation sometimes gets confusing enough that we need to distinguish, and people do actually think electrons travel all the way down a wire (or wonder how AC can work if the electrons ‘just sit there’).

[Q.E.D]

@panamajack

An EM wave traveling through a medium interacts with the medium in such a way that we may as well say it’s traveling at less than c. (Nonetheless someone here is always prepared to quibble needlessly.)

It is not “needless quibbling.” It’s a necessary design consideration to know at what rate the energy (not the individual photons) propagate through a particular medium, especially when designing RF transmission systems. While it’s true that photons always travel at c, it’s their average velocity that’s important for things like antenna and feedline design.

[panamajack]

@Q.E.D.

It is not “needless quibbling.”

I wasn’t saying you were. It was the opposite point, the one I stated (all EM energy travels at c), which I felt would be needless quibbling. I’d simply put it that way from the start to avoid someone nitpicking on that point.

I apologize for not making it all that clear.

[David_Simmons]

@panamajack

In my opinion, the situation sometimes gets confusing enough that we need to distinguish, and people do actually think electrons travel all the way down a wire (or wonder how AC can work if the electrons ‘just sit there’).

And I don’t see anything wrong with thinking that. By the time you get to lines that are long enough to make any difference you will be thoroughly familiar with the theory. In the case of AC I would use the model of an oscillating pulley driving another pulley via a flexible belt and having its output connected to the load by a ratchet.

That’s a problem with models. All of our forumae apply to models and in the last analysis, at the extremes, they all fail and we have to rely on numerical analysis.

One of the more ironic statements I’ve seen in a math book preceded the section on numerical analysis in the book Mathematics for Physicists and Engineers by Sokolnikoff and Redheffer. It said that in previous chapters we had covered many analytical methods for solving physical problem. However in most cases they were suitable for a first estimate but to get a more accurate answer you needed numerical analysis.

[Q.E.D]

@panamajack

I apologize for not making it all that clear.

Gotcha. No problem.

[dgrdfd]

Sorry for taking so long to reply. Real life caught up with me recently.

@Arjuna34

If you only get one, I’d get the ARRL handbook, which may be in your local library (if you’re in the US). especially since most of your questions deal with communications, modulation, antennas and transmission lines, etc.

I will definitely take a look at those books. I have read part of the Art of Electronics book, but the copy I had was older and didn’t have a lot of new information in it.

@engineer_comp_geek

RF stuff is kinda voodoo. It’s difficult to really understand, and you can’t develop a good hands on idea of how it works unless you really play around with it a bit. The ARRL handbook at your local library is a good place to start if you want to do some reading.

Figuring out what part to use where just takes experience. When I first started out designing things, I did a lot of “how the heck am I going to do that” :eek: but after several years, that feeling starts to go away. You can read all you want, but you can’t develop design skills without designing. Start with simple things. Play around with things in a simulator. It’s a lot cheaper than buying parts. On the other hand, you do occasionally need to buy parts so you can figure out what you did wrong when they don’t work. Troubleshooting is also a learned skill.

If you don’t have it already, run out to ye ol local bookstore and buy a copy of “The Art of Electronics” by Horowitz and Hill. It’s a darn good book.

Thanks for the reply. I think that’s a big part of my problem. I have worked with these circuits some in school, but I really lack the real world experience with them to become comfortable.

@Jurph

For the fourth question, you could probably get a study guide for a Ham Radio license and find most of the basic antenna theory you’d need.

Thanks for the reply and links. That’s something that I definitely think would be worthwhile to look into. Something “real world” that could help me understand a lot of this would be great.

@panamajack

This line right here makes me wonder if you’re slipping into a common misconception about electricity - that it is the flow of electrons. If you are, disabuse yourself of this notion quickly.

You caught me. I was thinking of electricity as a flow of electrons instead of as a flow of electric charge. In fact, one of my next questions was how much an amp of current weighs when going through a wire. You know, since 1 amp is a coulomb/sec, and a coulomb is amount of charge, and each electron has charge, and each electron has mass. I thought of it as “shooting” electrons down a wire instead of as electric charge flowing through a wire. I will look into Howard Johnson’s books.

@CJJ*

Again, Maxwell’s equations explain this far more elegantly that the lumbering post above, and I’ve completely omitted detail about how the electric field couples antennas (I’ve always though the “changing magnetic field” explanation is more intuitive, even if it leaves out a lot of detail). Still, I hope it helps…

Thanks. That definitely helped. I have used Maxwell’s equations a couple years ago in an electromagnetic fields class I took. I will have to go and get refreshed again and see if I can make sense of it. Thanks again!

@David Simmons

Antennas are mysterious. It just plain turns out to be a fact of life that if you have alternating current in a wire you can detect alternating electric and magnetic fields in the space surrounding the wire. The interaction of the two fields results in an electromagnetic wave that travels away from the wire at the speed of light. And here again, that’s just a fact of life. Each field falls off as the distance and the power, which is their product, falls off as the square of the distance.
. . .

Further explanations are available if there are questions.

That actually made a lot of sense. Thanks!

@CJJ*

For AC, the properties of capacitors and inductors are such that an engineer can treat them like resistors whose resistance varies with the applied frequency (yes, these devices also affect the phase of the AC signal, but let’s not get too far ahead of ourselves). To summarize briefly, inductors provide more impedance to a signal at higher frequencies, a capacitor provides less (and, counterintuitively, larger capacitance actually introduces a smaller impedance. A basic capacitor is two very-close-but-not-touching plates; increase the size of the plates or move them even closer together, the capacitance goes up, and intuitively it’s easy to see how either of these changes would make it “easier” for an AC signal to cross the separating dielectric).

All of that stuff sounds very familiar to our phasors we talked about at school. For instance, for AC signals the impedance of an inductor is jwl (j is imaginary i, w = 2* PI* frequency, and l is the inductance in Henry’s) and of course the impedance of a capacitor for AC signals is 1/(jwc). So this goes along directly with what you mentioned above.

I think I have a much better understanding now and plenty of resources to look into for further information. Thanks again and I apologize for the long hiatus from the thread!

[Musicat]

@dgrdfd

2. My next question relates to bandwidth. We all have heard about how fiber optics have allowed for much higher data transmission rates.

Like this one: World’s fastest (consumer) broadband, 40 Gb/sec.

[Q.E.D]

@dgrdfd

For instance, for AC signals the impedance of an inductor is jwl (j is imaginary i, w = 2* PI* frequency, and l is the inductance in Henry’s) and of course the impedance of a capacitor for AC signals is 1/(jwc).

That’s almost, but not quite, right. You’re mixing up impedance and reactance a bit. Inductive reactance is wL and capacitive reactance is -1/wC. Impedance is then calculated by R + jX, where R is the circuit DC resistance and X is the reactance.

[Crafter_Man]

@Q.E.D.

That’s almost, but not quite, right.

**dgrdfd ** is correct if we assume he’s talking about ideal components:

The impedance of an ideal inductor - all by itself - is jωL.
The reactance of an ideal inductor - all by itself - is ωL.
The impedance of an ideal capacitor - all by itself - is 1/(jωC).
The reactance of an ideal capacitor - all by itself - is -1/(ωC).

If you include an ideal series resistor,

The series impedance of an ideal inductor + ideal resistor is R + jωL.
The series reactance of an ideal inductor + ideal resistor is ωL.
The series impedance of an ideal capacitor + ideal resistor is R + 1/(jωC).
The series reactance of an ideal capacitor + ideal resistor is -1/(ωC).

[Crafter_Man]

@panamajack

electric current is not the flow of electrons. It is the flow of energy.

Well, it looks like we have a number of candidates for the definition of electric current:

1. Electric current is the flow of electrons.
2. Electric current is the flow of charge.
3. Electric current is the flow of energy.

I don’t like #1, since electrical current can be made up of charged particles other than electrons. Like ions. Electrons do not make up the current inside a battery, for example. And electrons do not flow in a human body when it gets electrocuted.

I don’t like #3, since you can have current without the flow of energy. A current induced in a superconductive loop, for example.

I like #2.