Question on Electrics

[Joaquín]

I'm stuck with AC electrics. Any help, or perhaps a web site would be much appreciated.

1. How AC can change polarity if current is a flow of electron (negative by definition).

2. When is said that the voltage is negative, does this mean that somehow the energy is negative? (something impossible, I believe).

3. What's the diference between a capacitor and a battery if both store current?

Thanks

[Squawk 8888]

Finally, a question on a topic I can pretend to understand :)

1. The energy doesn't come from the delivery of the electrons to a destination, it's from the movement of those electrons. You'll get the same power whether they're running around a track (DC) or wiggling back and forth (AC).

2. When we talk about "negative" or "positive" voltage it's a relative term. The voltage is the difference between the charge between two parts of the circuit. If there is a surplus of electrons the charge is negative while a deficit of electrons is a positive charge. Both sides can have a positive (or negative) charge but there is still a voltage between them (and usable energy) if the charges are at different levels. It's important to know whether the voltage is positive or negative because some components are polarity-sensitive. So the answer to your question is that it's not the energy that's negative, it's the charge.

3. A capacitor charges by letting electrons build up on its plates while batteries store and release energy through chemical reactions.

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Per dementia ad astra

[stagger]

Current does involve the flow of electrons, but it also involves the flow of electron-deficient atoms in the opposite direction. Both can be thought of as electrical charge carriers. Conventionally, current is considered to flow from relatively positive points to relatively negative points – i.e. in the direction that the electron-deficient atoms are moving. Of course, the electrons are going the other way – from relatively negative points to relatively positive points - i.e. to where all the electron-deficient atoms are.
Alternating current (AC) involves the flow of charge carriers reversing direction periodically (50 to 60 times per second for most household electrical outlets). One moment the electrons are rushing in one direction and the electron-deficient atoms in the other. Then they switch. And they keep switching – rushing back and forth.
Voltage is potential difference. If there is a conductive path between two points and a potential difference between them then current will flow. The point with a negative potential difference doesn’t have “negative energy”, is has an excess of electrons relative to some other point that will have electron-deficient atoms ready to accommodate these charge carriers.

Does this help?

[Joaquín]

Thank you very much, I getting there (I think). Still, apart from the different process, both batteries and capacitors are basically the same then. On the other hand, a battery is recharged by letting a current flow into it (as capacitors).

I'm sure you can see how confused (and ignorant) I am in thi matter, so more posts are very much welcome.

Thanks again :)

[Tinstaafl]

A couple of points:

1. It's not quite correct to say that atoms lacking an electron flow in the opposite direction to the electron.

The electrons surrounding an atom carry a -ve charge. The nucleous of an atom has a +ve charge. When the correct number of electrons are present to the atom has no charge. Too many electrons in a piled in & around a group of atoms will give that material an overall -ve charge, too few a postive charge.

Because electrons have this -ve charge they are 'drawn' towards areas that have a +ve charge ie too few electrons. Some atoms easily lose one or two of their electrons. These are electrical conductors.

When a conductor is placed in a circuit electrons 'jump' from one atom to the next towards a +ve charge. As each electron 'jumps' it leaves behind a 'hole' where it used be. The atom left behind now has an overall +ve charge.

Meanwhile another electron further back will be drawn to fill this +ve charged 'hole', itself leaving behind a 'hole' in its own atom.

In a sense, it is the 'hole' that is moving away from the +ve side of the circuit & towards the -ve side of the circuit.

In a direct current circuit (DC) eg one from a battery, the movement of the electrons is in one direction only. In an alternating current circuit (AC) the direction is repeatedly reversed eg 50 - 60 times / second for household current, 400 times/sec in aircraft.

The current is made to reverse by rapidly changing which side of the circuit the electrons accumulate in the alternator that caused the +ve / -ve differences in the first place.

2. The diagram below is meant to be a battery or capacitor. The - is the -ve terminal of the battery, the | is the +ve terminal. Imagine a wire loops around to connect the end '-' with the other end's '|'

wire -|-|-| other end of wire

When this circuit is connected, the electrons in the -ve end are drawn towards the +ve side (or the 'holes' in the +ve side are drawn to the -ve side).

How strong the 'draw' is, is measured in 'Volts'. If this is a 12 volt battery, it is capable of causing the electrons/holes to move with a force of 12 volts. This is the 'Potential Difference' between the +ve & -ve sides of the battery.

The +ve side has +12 volts, the -ve side has - 12 volts.

3. A battery causes the excess electrons through a chemical reaction. Allow the reaction to occur & electrons accumulate on the -ve plate within the battery. The chemical reaction starts when the +ve & -ve terminals are connected by a conductor eg a wire.

Once all the chemicals are used up then the battery can no longer produce electricity (the battery is 'dead').

Some chemical reactions can be easily reversed by applying electricity (from another source). This is what happens when you recharge a battery. This other source of electricity is used to force the chemical reaction to reverse, ready to be used again.

A capacitor 'fills up' with electrons as the current tries to flow. An analogy is to think of a capacitor like a dam in a river. The water flows into the dam's lake until the dam is full.

If you discharge the capacitor you then have to wait for it to 'fill up' again before it can be used to supply current.

Hope this helps.

T

[This message has been edited by Tinstaafl (edited 22 May 2001).]

[Joaquín]

Thank you all again but.....

It appears to me that if AC travels backwards and forward, you don't need a return wire, do you? It looks like the electrons go from A to B following a differential of potential, passing through C (a resistance), where some energy is lost/used. At B, then, arrives less energy that the energy departing from A. And when the wizard reduces the potential at A, the electrons flow back to A, again losing some energy at C... Is it like this?

When you say that frequency is, for instance, 60 Hz, does it mean that a bulb connected to an AC circiut afectively turns on and off 60 tiems a second? (I wonder if this is the reason for lights to blink when you look at them from a distance).

Please, be patient, I'm that thick

[This message has been edited by Joaqu'n (edited 22 May 2001).]

[Tinstaafl]

The current will only flow if there is a closed circuit ie a return wire, or something acting as a return wire eg earth.

Without a return wire, what takes away the arriving electrons to allow more electrons 'in'? And, in an AC circuit, when the current reverses, how do the electrons flow in the reverse direction unless the return wire is there. Only now, it's not the return wire, it's changed roles to be the originating wire & the old, originating wire is now the return.

An analogy is a pipe carrying water past say, a water wheel. Unless the water is removed from one end of pipe, no water will flow & no work will be done. It doesn't matter how much pressure there is in the pipe (ie the potential difference or voltage) unless water is removed at one end nothing flows. The principle is similar.

And yes, the bulb is turning off & on 60x/sec if connected to a 60 Hz current.

[stagger]

Tinstaafl is absolutely correct - the electron-deficient atoms themselves don't actually move. It's the holes that move.

I guess I was being lazy and didn't fancy trying to explain how something that is really the absence of something else (e.g a hole) can move around.

[Joaquín]

But Tinstaafl

1. If that is true (and I don't doubt it) it means that from the 2 wires leavin an alternator the current flows equally but in opposing direction. But if you touch one wire you'll feel an electrical shock but you won't if you touch the other one.

2. As I understood your last explanation, if a take one wire with one hand and the other wire with the other hand, I shoul feel a shock alternating between my hands: ie one shock in my right hand one followed by another in my left hand 6o times a second (if the current is 60 Hz). This apparently is not consistant with the experience of 1.

3. Your analogy with the waterpipe looks very similar (if not identical) to my idea of how Direct Current works.

Thanks again for your patience

[This message has been edited by Joaqu'n (edited 23 May 2001).]

[Tinstaafl]

1. The current flow alternates between flowing down one wire then the other. It doesn't flow down each wire at the same time.

Another analogy: Imagine standing at train platform and in front of you is a circular railway track with a train on it that is so long it connects with itself.

If the carriages moved to your left, more carriages would be pushed/pulled around from your right to replace the ones that just moved to the left.

If they reversed direction the same thing would happen but now carriages would appear from your left.

Repeat this oscillation 60 times / sec & that's a 60 Hz cycle. Note that the carriage's movement (ie the electrical current) is only in one direction at a time. In one end, out the other, change around, back into the end just left & out of the end originally entered.

2. You will feel a shock if you complete the circuit and you are also the path of least resistance to for the current to flow through.

Your body isn't able to tell the difference between the current arriving at your left hand / leaving by your right hand or visa versa.

If you connected a meter with a polarity that caused a centred needle to deflect one way or another, depending on the direction of current flow, then you would see it flick from L to R and back again.

3. Repeatedly reverse the water flow & it describes AC. Keep the water flowing in one direction only & it describes DC.

[Joaquín]

And again....

If I understand your point 1. Does it mean that both wires from an alternator are live?

Regarding point 3. In a DC system the current arrives from one wire (the live) then passes through a resistance and continues to where?

(This is getting me obsessed!!!!!!)

[Tinstaafl]

Yes, they are both 'live'.

The difference is that one of them (designated the 'earth' or ground wire) may share a common circuit with the airframe / planet Earth. This means that the airframe is part of the circuit.

Touching this part of the circuit won't cause you to be electrocuted because you not providing the electricity's return path to the other side of the alternator.

This is the same situation as a bird perching on a power line. The current will take the path of least resistance ie via the conductor.

Woe betide the bird or you however if you then touched a conductor that leads back to the other side of the alternator!! You would effectively provide a shorter path for the electricity to follow, bypassing whatever the normal resistance in the circuit is.

In the DC system, the current arrives via a conductor eg a wire, does its bit, then departs through a return conductor back to the battery. This return conductor could be a separate wire OR the design of the circuit could make use of a common, conducting mounting point between the resistor & the battery/generator.

A torch does this. One wire leads from one of the battery terminals to one side of the bulb. The other side of the bulb and the other terminal of the battery are arranged so that they are connected to the torch's casing. This makes the casing the return 'wire'.

Similarly with most vehicles.

[Don D Cake]

Joaquin

First a few words of encouragement....

When I first started to learn about electrickery, I couldn't come to terms that ac current could flow back and forth fifty times a second, it seemed too fast. Now having spent more years than I'd care to mention in the electronics industry I have no problem with ac current flinging itself around at ten thousand million times a second (10GHz). Persevere it will come to you.

Try this analogy....

Forget about electrons, holes and other subatomic particles,they don't matter at the moment.

DC - take a battery, say one from a car. Connect two leads to it. Connect each lead to each side of a bulb. Direct current will flow from the positive terminal of the battery, though the bulb and then to the negative terminal of the battery. The bulb filament will get hot and glow. Bit like a water pump, pumping water around a central heating system. Cut one of the leads and the current flow stops, bit like hammering one of your central heating pipes flat. (Note that I didn't say you cut the pipe, this is why I don't like water analogies because although water would flow out of a broken pipe, electrons wouldn't flow out of a cut cable).

AC - take the DC set up. The bulb is lit. Now very quickly switch the leads to the battery. Current still flows from the positive terminal of the battery to the negative but the other way through the bulb. Because you switched the leads really quickly and you managed to do this fifty times second, the bulb filament doesn't get time to cool down and it doesn't flicker. As far as the bulb is concerned it is connected to an ac supply. Bit like reversing your water pump fifty times a second.

Hope this helps.

[Joaquín]

I'm almost there thanks to your information (and encouragement)

The only thing that I still don't quite understand is: assuming the 2 wires out from a socket are like the 2 wires coming from an alternator: why if I touch one of them I receive an electrical shock, but the same doesn't happen if a touch the other one? I understand why I receive a shok if a touch both (closed circuit). It seems like both wires in AC should be exactely the same.

Thanks one again.


[This message has been edited by Joaqu'n (edited 24 May 2001).]

[cb747]

One wire in the plug is connected to one side of the generator in the power station. The other wire os connected to the ground. At the power station the other side of the generator is connected to the ground. For a curcuit to be complete the power flows from the side of the generator connected to your plug, throught the plug, through the kettle, through the opther pin in the plug, into the ground and back to the other side of the generator.
If you touch the wire directly connected to one side the generator it flows througn you to ground and back to the other side of the generator.
If you touch the other wire which is connected to ground, although you are connected to one side of the generator you have no connection to the other.

Very simplistic but hope it helps.

[Joaquín]

:) I think I got it now thanks to your kindness and patience. :)

Again, thanks Sqwak 8888, Stagger, Tinstaafl, Don D Cake & cb747.

(Now it's time for me to try to crack on Reactance & Impedance :) )

[Whirlybird]

Wow, thanks guys. I just found this thread, and I've been struggling with Electrics too, and this all really helped. Joaquin, thanks for asking the questions I hadn't quite formulated; everyone else, thanks for some really clear answers. If by some miracle I pass the Electrics exam I'll buy you all a drink at the Gatbash!

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Whirly

To fly is human, to hover, divine.

[This message has been edited by Whirlybird (edited 26 May 2001).]