Difference between AC and DC current


by QuantumTheory
Tags: current, difference
QuantumTheory
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#1
May21-05, 01:22 AM
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What's the difference?
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exequor
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#2
May21-05, 04:59 PM
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alternating current varies with time, sinusoidally... where as DC remains steady_ hence the names.
brewnog
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#3
May21-05, 05:40 PM
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I'll elaborate here.

Exequor was referring to the flow of electrons. In AC, the electrons change direction sinusoidally (in the UK, for mains electricity this frequency will be 50Hz, in the US it's 60Hz I believe). In DC the electrons will carry on flowing around the circuit without changing direction.

In terms of practical differences, AC is used to transmit power over long distances for a few reasons. Firstly, electricity comes out of the generator at the power station as AC. Secondly, it's easier to 'step' up and down the voltage of AC electricity to convert high voltage/low current to high curren/low voltage. To transmit over long distances, AC offers the advantage because resistive losses are minimised with the use of high voltages.

woohaa!
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#4
May22-05, 01:37 AM
P: 1

Difference between AC and DC current


Well said!!!
Colt22
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#5
May22-05, 10:47 AM
P: 14
ok, but how does the current change direction in AC, if it were to change direction as you say the 2 wires would alternate between pos and neg polarity and the direction changes, so if i had a loop made of some AC cable, the electrons would flow clockwise for half of the 60hz, and counter clockwise for the other half of the 60hz. But if this was true then touching the pos or neg wire and a ground will result in a shock pulsing at 30hz, because ur only gettin one of the direcions, and even if it wasent pulsing at 30hzm you just plain dont get a shock if you grab the ground and the white lead (the non hot one i think), but ur fine if u touch the ground and the black(hot lead) u get a shock, what gives?
brewnog
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#6
May22-05, 11:30 AM
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Not sure what you mean.

AC does mean that the electrons change direction constantly. With AC, we don't think of a wire being positive or negative, because of this constant change. If you touch either of the wires and ground, then you will probably get a shock. You won't just be 'getting' one of the directions, - the flow of current is constantly alternating.

I'm not being rude, but can you try and rephrase your question? I don't understand you.
exequor
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#7
May22-05, 12:10 PM
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In DC there is a constant polarity over time, where as in AC the polarity changes with time. I don't know how this relates to shocking in AC because at any give point there will be current flowing in one of two directions. If we go back to a simple AC generator it would be seen that if a magnet is spining and the coil is stationary (or vice versa) current would be induced in one direction for a 1/2 cycle and then in the other direction for another 1/2 cycle because the poles of the magnet would have swapped positions. So at 50Hz if you touch the live wire and you are grounded you should get a shock.
cosmik_debris
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#8
May22-05, 12:37 PM
P: 5
The only alternating wire is the live (hot) wire. It alternates between 240v and -240v 50 times a second (50Hz) here in the UK. The neutral (non-hot) wire is effectively tied to ground (0v) which is why you won't get a shock if you hold the neutral (non-hot) and groud wires (they're both, theoretically, at the same potential).

When the live (hot) wire is at a positive voltage, electrons will flow from the neutral (non-hot) to the live (hot), but when the live (hot) is at a negative voltage electrons will flow from the live (hot) to the neutral (non-hot).
Colt22
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#9
May22-05, 05:36 PM
P: 14
im still u clear why you only get shocked when you touch the live(hot wire) and other the ground or the neutral. If u touch the neutral and the ground nothing will happen. I asked my grandfather this(he was an electrician) and he cant really explain AC to me, but he did verify that you do not get a shock from the neutral wire and the ground wire
cosmik_debris
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#10
May22-05, 06:09 PM
P: 5
Because both the neutral and the ground wire are at the same voltage, 0v, so electrons (current) have no need to flow. No shock.

The hot wire on the other hand is not at the same voltage, it is either higher or lower than the ground (changes 60 times a second). Therefore the electrons will flow from the wire which is at the lowest voltage, to the one which is at the highest voltage, and if they have to flow through you in order to do that, then they will!
Dngrsone
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#11
May23-05, 09:03 AM
P: 35
Colt,

In America, if one looks into the Power panel, one would most likely see all the ground wires (generally bare, sometimes green) tied into a buss on one side, all the white (neutral) wires tied into a nearby bus and the black wires (hot) come out of the fuses/breakers. There will be a connection between the white wires/bus and the ground bus, if it isn't a single bus.

This is the single-point ground reference for the neutral line in the building. The reason why one can touch the white wire and not get shocked is, as cosmik has said, because the neutral wire is electrically grounded.

However, do not count on the white wire always being neutral... the average homeowner doesn't necessarily know which wire is which and there are miswired electrical outlets around, so one should treat all wires as live until proven otherwise.
Xodar
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#12
Jun5-05, 08:32 AM
P: 13
Regardless of country, a three-wire AC system consists of an active wire, a return wire, and a neutral wire. The active wire is "live" at all times, carrying a potential that varies by 240volts or 110volts (rms) above and below the reference level (ground). This reversal in potential means that the electrons will flow in alternate directions during each half of the cycle, hence the name "alternating current". Work is done by the passage of the electrons, regardless of the direction of flow, so this point is largely irrelevant. As to when you get "shocked" ...

Electrons will only flow if there is a return circuit to the source, and this is the purpose of the return wire. When you close the switch in your circuit (light bulb, freezer, whatever) the return wire will also be "live" as it will be connected to the active wire through the energised device.

The neutral wire is normally connected to earth (usually at one point in the wiring system) and provides a reference level for those devices that need it. (Note that those devices with only two wires connecting to them are referred to as "double insulated" and do not have a neutral connection). The neutral should not carry current - if it does, this indicates a fault and will normally cause your earth leakage protection devices to trip. This is what happens when you cut through your cabling with a power saw or put your knife into the toaster while it is on ... IF you have earth leakage devices fitted ... DO NOT try this at home ... it is supposed to save your life but...

If you have the switch on, then both active and return wires are "hot" or "live" - if you touch either of these at the same time as the neutral or some other grounded surface then you will get a shock (and hopefully cause the earth leakage to trip in some fraction of a heartbeat and hence save your life). This obviously also applies if the switch is not on and you touch active and neutral simultaneously.

If the switch is not on then the return wire is not live. You can touch both return and neutral and live ... but why tempt fate? If you touch active and return then you bypass the switch and become the electrical device ... in this case the earth leakage devices in your house may not trip as there is no excessive current on the neutral line - you are now relying on the fuse or circuit breaker in the active line to trip but these are slower than earth leakage trips and there is a greater chance that they will not save your life.

Summary ... switch on -> 240v, 240v, 0v ... switch off -> 240v, 0v, 0v ... touch two wires of different potentials and life gets electric.
Averagesupernova
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#13
Jun5-05, 11:50 AM
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Xodar, I live in the U.S. so I am not 100% sure of how things are in other countries electrical-wise but I think you have posted quite a bit of mis-information.

You talk about the neutral not carrying any current. In the U.S. the neutral carries the same current as the hot wire in 120 volt appliances. In 240 volt appliances touching any wire except the neutral will give you a shock regardless of how the switch is thrown. I could go on...
Xodar
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#14
Jun5-05, 02:04 PM
P: 13
I am appropriately chastised ... although I think it is miscommunication rather than "misinformation".

It does appear that North America is different to the rest of the world (again) in having two live conductors (http://en.wikipedia.org/wiki/Split_phase). This is based on a split phase system where the 240v supplied to the house is split with a centre tap on the transformer, giving a central connection connected to ground (and called neutral) and two 120 volt lines. As you say, a connection for a 240volt appliance then requires both live wires (and is a very dangerous beast). I expect the 120 volt devices are far more common, and these I expect conform to the description that I suggested of one live wire and a return (neutral).

Those parts of the world that use 240volts for everything (UK, EU, Aus, etc.) have only one live wire (240v) to the socket, and use the "neutral" (what I originally called the "return") to complete the circuit once the appliance is plugged in and switched on. Only then are both lines "hot" or live. (http://en.wikipedia.org/wiki/Domesti...lugs_&_sockets). In these plugs the third wire is the earth, which is distinct from the neutral at this end of the wiring system. Because the earth wire does not normally carry any current in these systems, it is possible to earth appliances by connecting their frame to this third wird (no current -> no potential drop -> ground potential throughout the house).

Other potential for terminology confusion: the Earth Leakage Trip that I mentioned is apparently called a GFCI in the US (Ground Fault Circuit Interruptor).

I hope this satisfies any translation concerns...
Averagesupernova
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#15
Jun5-05, 07:03 PM
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I wouldn't say that a 240 volt appliance in the U.S. is any more dangerous than anywhere else.

Can you please explain in detail why you think that touching a wire on a 240 volt system will give you a shock when the appliance switch is on but not shock you when the switch is off?

Calling the return wire 'neutral' in parts of the world where only 240 is used gets the point across to those of us on the split phase system, but it is actually incorrect. It is so called neutral because it is in the center of the transformer. It is 'in the middle'. It is correctly called the return on these systems.
GENIERE
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#16
Jun5-05, 09:11 PM
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Xodar You do have some misconceptions. The power distribution system in Australia (ignore frequency and voltage standards) is very similar to that of the US. Both are slightly different than England, all three different than France, and all four different from Japan. The difference lies in the means of referencing the neutral to ground (Earthing); it is variations of isolated, impedance grounded, single point grounded, or multi-point grounded. The differences arise from opinions on the means of efficient and uninterrupted power transmission, not safety. It is at the point of use wherein safety concerns are prioritized. For purposes of safety, in Australia a person touching a live wire and ground would get a 240vac shock, in the US the person would get a 120vac shock. If the person in the US touched both live wires he would receive (as will the Australian) a 240vac shock. Both countries require all exposed metal to be earthed or the internals to be doubly insolated. How the neutral is derived, whether it is connected to an appliance or not, has no bearing on safety.

In my opinion, the 240vac system as in Australia is slightly superior in the sense it is less confusing to a layperson when attempting to do his own electrical work at home. We have single and double pole breakers, shared neutral systems wherein the GFCI or your Earth Leak Trigger cannot function, and the 120vac branches may require larger conductors.


...
Dngrsone
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#17
Jun6-05, 08:56 AM
P: 35
Actually, in the US if one touched both the hot wire and neutral on a 120V circuit, then that individual will recieve a 120v shock. Only within the power box or 440V circuits will they recieve a 440VAC shock.

I am familiar with American electrical systems, so I will talk specifically about our electrical distribution. The power coming in form the service (generally a pole, but more and more often in urban settings it's an underground service) comes as three wires and a ground. The wires are Black, Red and White, generally, where black and red are "hot" legs of a 240VAC service, the white is a "neutral" point taken off a center-tap on the power transformer to provide a reference between the two hot wires. The ground is a cable that provides an earth connection for the entire mess.

As someone mentioned earlier, code requires all exposed metal components in an electrical system to be tied directly to ground, including metal boxes and conduits.

In the main circuit box (circuit breaker box, fuse box, etc.), the white neutral wire will be tied to the ground. The red cable will go to one side of the breaker mains and the black to the other. Each breaker/fuse will tie into one or the other hot main (a 240v breaker will tie one leg into each) and provide overcurrent protection for the circuit wired into it. One can also get Ground Fault Circuit Interrupt (GFCI) breakers for the main box, though at cost.

All circuit wires will have their white wires tied into the neutral buss, all green or bare ground wires will be tied into the ground buss, each red or black hot wire will go into its respective circuit breaker.

When wiring switches, the switching will be done on the hot wire... neutral and ground wires should not be interrupted.

The tie point between neutral and ground in the main box should be the only placed in the house where these two wires are connected (sub-breaker boxes will have seperate neutral, ground connections)... this is to protect the user. Any device which has a three prong plug attached will ground its case. That way, if there is a short from the hot or neutral to case, the case will not become "hot" and create a potential hazard to the user. Even with circuit breaker protection, a short circuit will allow current to flow in the amount of time it takes for the breaker to trip, and with a properly grounded case, the current will go directly to earth and the user would be protected from shock.

If there are any questions, read this and if that doesn't adequately cover it, ask away.
exequor
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#18
Jun6-05, 12:00 PM
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Sometime ago I asked about phases in this forum and I think the whole thing was mis-understood. When I was talking about a 2 phase system I was not referring to this split phase that you guys mentioned earlier. I live in the Caribbean and or a residential connection you get 240V from the pole, a live (240V) and a neutral which go to you circuit breakers (I'm omitting the electricity meter). You also have to setup an earthing pole outside of your house with the cable going to you main box. For every outlet in your house you have three connections, the live, the neutral (return) and the ground (earth). If you hold the live and any other one, or the live alone you get shocked, since your feet will be grounded.

Now back to phases, I think that prior to this I was told that there was nothing as 2 phase and 3 phase, but for commercial applications here the buildings get 3 wires from the pole (2 live and a neutral). I don't know for sure if they just spit one phase and send it as two separate live connections (split phase) but I was taught in Physics:electricity that in an electric (AC) generator if you have 3 coils place 60 degrees apart with the magnet spinning at the middle then the angle at which the induction takes place would result in the electricity having 3 phases (and this can be shown on a graph). So could there be more than one phase?

Btw, I wanted to clarify this. If you hold the live and the neutral but you body is in the air (you are suspended with no strings, etc._ its hypothetical), would you receive a shock? Or would the current just flow from live to neutral?


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