# Ac & dc

1. Feb 28, 2009

### swty todd

while travelling in the electric locals(trains) that we have our here, i read a signboard said "do not travel on rooftops because the lines above have 25000v AC or 1500 DC"

i was wondering how electricity could be both AC and DC.

2. Feb 28, 2009

### HallsofIvy

The sign didn't say "and", it said "or". It may be referring to more than one line.

3. Feb 28, 2009

### triden

Probably not the case for these wires, but you can have an AC signal with a "DC offset", which means that the AC waveform is translated up or down to a different potential.

4. Feb 28, 2009

### KnowPhysics

Yes, what you are asking is reasonable. but it is not like how you were thinking (both AC and DC in same wire at the same time). Actually AGC technology enables the train to run from any available source - (the dual voltage (1500 or 25000V)) it can run in DC (1500v) or in AC (2500v) with respect to the available source at that time.

5. Mar 1, 2009

### Pumblechook

Not at the same time as people have said.

There are trains which switch from running on AC to DC at different volages..maybe within the same journey.

6. Mar 1, 2009

### Niles

Is this equivalent of defining a new zero-point for the potential energy in classical mechanics? Or what is this offset used for?

7. Mar 1, 2009

### Staff: Mentor

Don't think it's equivalent. You can have an AC signal "riding on top of" a DC voltage. It's quite common in circuits -- like in the middle of an audio amplifier, you will have the AC audio signal going through amplifiers and filters, generally between a power supply and ground (like 12V and GND). The signal may be AC or DC coupled between stages, just depending on what you are doing. At the output of the amp, you use capacitors to make the output of the amp centered at 0V, so you don't put any DC through your speakers.

Another reason to have AC superimposed on DC is to transfer power as well as information. The POTS telephone lines that run to your home carry the AC audio signals (dial tone, voice, DTMF Touch-Tones, etc), and also have a DC voltage (GND and -48V) that is used to power the old-style telephones.

In the context of the original post (OP), it is two different ways to transfer power, so the AC and DC would not be used together, I wouldn't think.

8. Mar 1, 2009

### swty todd

my dad says that there are AC lines here up to a particular station and after that there are DC lines and these trains run through all these stations.
that's why the sign.
thnx...

9. Mar 1, 2009

### Phrak

Trains run both ways.

Would a sign that reads

{ ** DANGER **
{ 20 MEGAOHMS
{ .ELECTRICITY.

be more intimidating?

10. Mar 2, 2009

no.......