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# Direct current "frequency"?

## Direct current "frequency"? - Physics Forum

### Direct current "frequency"? - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.

#1
11-13-2007, 01:21 PM
 Jeff Guest Posts: n/a
Direct current "frequency"?

I was reading some alternate physics information recently and came
across something that didn't quite sound right. The author was
quoting
from an article where the author of that article talked about
applying
a direct current of "high frequency but low pulses, in the auditory
range."

Correct me if I'm wrong, but you can only talk about frequence of
current when referring to alternating current, correct? I know you
can
pulse DC current but is there something I'm missing when the author
talked about the frequence of the direct current??

#2
11-13-2007, 01:57 PM
 PD Guest Posts: n/a
Direct current "frequency"?

On Nov 13, 8:21 am, Jeff <[Only registered users see links. ]> wrote:

Well, it's poor language, to be sure.
But on the other hand, AC specifically refers to a case where the
voltage and the current *change sign* during a cycle. But a pulse
which is always of one polarity can be considered DC, and so a
sequence of pulses can be DC and still have a frequency.

#3
11-13-2007, 01:57 PM
 PD Guest Posts: n/a
Direct current "frequency"?

On Nov 13, 8:21 am, Jeff <[Only registered users see links. ]> wrote:

Well, it's poor language, to be sure.
But on the other hand, AC specifically refers to a case where the
voltage and the current *change sign* during a cycle. But a pulse
which is always of one polarity can be considered DC, and so a
sequence of pulses can be DC and still have a frequency.

#4
11-13-2007, 06:48 PM
 dlzc Guest Posts: n/a
Direct current "frequency"?

Dear Jeff:

On Nov 13, 7:21 am, Jeff <[Only registered users see links. ]> wrote:

Like to your PC speaker, for example.

PD has the right response. Your entire computer runs on pulsed DC,
and all frequency means in "cycles per second". It does not have to
reverse (the definition of AC) current flow direction to have a
frequency. You could have an AC sine wave impressed on a DC bias, or
just switched DC. Describing how often the waveform repeats is all
that is required to describe it with a frequency.

You can even describe how often you change your oil with
"frequency"... ;>)

David A. Smith

#5
11-13-2007, 06:48 PM
 dlzc Guest Posts: n/a
Direct current "frequency"?

Dear Jeff:

On Nov 13, 7:21 am, Jeff <[Only registered users see links. ]> wrote:

Like to your PC speaker, for example.

PD has the right response. Your entire computer runs on pulsed DC,
and all frequency means in "cycles per second". It does not have to
reverse (the definition of AC) current flow direction to have a
frequency. You could have an AC sine wave impressed on a DC bias, or
just switched DC. Describing how often the waveform repeats is all
that is required to describe it with a frequency.

You can even describe how often you change your oil with
"frequency"... ;>)

David A. Smith

#6
11-13-2007, 07:04 PM
 Jeff Guest Posts: n/a
Direct current "frequency"?

On Nov 13, 1:48 pm, dlzc <[Only registered users see links. ]> wrote:

Thanks David and PD. I *think* that clears it up. What confused me was
his description of DC having a frequency AND a separate "pulse" rate
(which I assumed was the frequency when defined in terms of pulses per
second).

But if you can have an AC sine wave impressed on the DC bias, that
might be what he was describing, in addition to the pulse rate.

Thanks,
Jeff

#7
11-13-2007, 07:04 PM
 Jeff Guest Posts: n/a
Direct current "frequency"?

On Nov 13, 1:48 pm, dlzc <[Only registered users see links. ]> wrote:

Thanks David and PD. I *think* that clears it up. What confused me was
his description of DC having a frequency AND a separate "pulse" rate
(which I assumed was the frequency when defined in terms of pulses per
second).

But if you can have an AC sine wave impressed on the DC bias, that
might be what he was describing, in addition to the pulse rate.

Thanks,
Jeff

#8
11-13-2007, 09:31 PM
 dlzc Guest Posts: n/a
Direct current "frequency"?

On Nov 13, 1:04 pm, Jeff <[Only registered users see links. ]> wrote:
....
....

Yes, that is a problem when a reporter says "this is what this guy
said".

Actually, my guess is that only selected pulses from a high frequency
pulse train were "duplicated" on this other circuit, giving actually
an audible frequency with a very short rise and drop time.

In other words, I think the quote is pure technobabble, and you were
right to be confused. But I am just a MechE.

David A. Smith

#9
11-13-2007, 09:31 PM
 dlzc Guest Posts: n/a
Direct current "frequency"?

On Nov 13, 1:04 pm, Jeff <[Only registered users see links. ]> wrote:
....
....

Yes, that is a problem when a reporter says "this is what this guy
said".

Actually, my guess is that only selected pulses from a high frequency
pulse train were "duplicated" on this other circuit, giving actually
an audible frequency with a very short rise and drop time.

In other words, I think the quote is pure technobabble, and you were
right to be confused. But I am just a MechE.

David A. Smith

 Tags current , direct , frequency

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