Why when we touch osciloscope probe it shows 50Hz noise?

[op27]

Hello, I have asked this question from different people but never got a
satisfactory answer.
I want to know in terms of electromagnetic physics how 50 Hz frequency of
main is induced in our body and converted to voltage at oscilloscope probe.
Please answer only if you deeply understand this phenomenon. dose it depend
on current? i.e. if we get more currents from main it induces more noise?
how it depends to voltage?

thanks,

 

[Jasen Betts]

On 2009-05-26, op27 <saman75saman@yahoo.com> wrote:

Hello, I have asked this question from different people but never got a
satisfactory answer.
I want to know in terms of electromagnetic physics how 50 Hz frequency of
main is induced in our body and converted to voltage at oscilloscope probe.
Please answer only if you deeply understand this phenomenon. dose it depend
on current? i.e. if we get more currents from main it induces more noise?
how it depends to voltage?

It depends on the degree to which your body is capacitatively coupled
to the mains. (and the degree to which it is earthed, and to a lesser
extent the magnetic flux and the area of the loop formed by the circuit
formed by touchiong the probe.

try touching the probe to a large piece of aluminium foil.

 

[op27]

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:gvgjl2$8vs$2@reversiblemaps.ath.cx...

It depends on the degree to which your body is capacitatively coupled
to the mains. (and the degree to which it is earthed, and to a lesser
extent the magnetic flux and the area of the loop formed by the circuit
formed by touchiong the probe.

try touching the probe to a large piece of aluminium foil.

So magnetic field that is produced when we draw current from mains can be
converted back to electrical current in our body and produces voltage. for
example when large currents are drawn e.g. welding it causes to some tiny
currents produced in our body?

the capacitance of our body should directly depends on our distance to
wirings inside the wall, isn't that?
why oscilloscope amplitude is about a few volts rather than 220V?
this peak to peak voltage depends on what ? can we calculate it?

 

[Sjouke Burry]

op27 wrote:

Hello, I have asked this question from different people but never got a
satisfactory answer.
I want to know in terms of electromagnetic physics how 50 Hz frequency of
main is induced in our body and converted to voltage at oscilloscope probe.
Please answer only if you deeply understand this phenomenon. dose it depend
on current? i.e. if we get more currents from main it induces more noise?
how it depends to voltage?

thanks,

You need to pack your head with aluminium foil.

 

[op27]

"Sjouke Burry" <burrynulnulfour@ppllaanneett.nnll> wrote in message
news:4a1bff4f$0$1644$703f8584@textnews.kpn.nl...

You need to pack your head with aluminium foil.
I said only reply if you understand the phenomenon. surely I don't need

advice from people who don't have anything in their head.

 

[Tim Wescott]

On Tue, 26 May 2009 16:55:37 +0430, op27 wrote:

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:gvgjl2$8vs$2@reversiblemaps.ath.cx...
It depends on the degree to which your body is capacitatively coupled
to the mains. (and the degree to which it is earthed, and to a lesser
extent the magnetic flux and the area of the loop formed by the circuit
formed by touchiong the probe.

try touching the probe to a large piece of aluminium foil.

So magnetic field that is produced when we draw current from mains can
be converted back to electrical current in our body and produces
voltage. for example when large currents are drawn e.g. welding it
causes to some tiny currents produced in our body?

No. Capacitive coupling is entirely an electrostatic phenomenon; when
you stand in an electrical field and get that mains-frequency voltage
impressed on you there is practically no current flow.

Magnetic fields would induce currents, yes, but that's a different issue.

the capacitance of our body should directly depends on our distance to
wirings inside the wall, isn't that?

The capacitance of our bodies to the wiring depends on that distance,
yes, but there are a whole bunch of variables -- just assume that you're
immersed in an electric field at the mains frequency.

why oscilloscope amplitude is about a few volts rather than 220V?

I've seen tens of volts, depending on the environment (florescent
lighting seems to enhance the coupling).

this
peak to peak voltage depends on what ?

The placement and size of the wiring, the existence of other conductors
in the electric field, whether those conductors are grounded, etc.

can we calculate it?

In theory, yes. In practice it works best if you are perfectly
spherical, and floating in the center of a perfectly spherical room with
all classes of conductors (mains wiring, telephone, metal studs, etc.)
all arranged in a spherically symmetric way in the walls, and close
enough together so that you can treat them as being uniformly distributed
within the walls.

--
www.wescottdesign.com

 

[op27]

I think my main mistake was that I was assuming a magnetic field rather than
electrical field produces this phenomenon. regards,

 

[Eeyore]

op27 wrote:

Because there's a lot of it about !

60 Hz in the USA etc.

Graham

 

[Eeyore]

op27 wrote:

"Sjouke Burry" <burrynulnulfour@ppllaanneett.nnll> wrote

You need to pack your head with aluminium foil.
I said only reply if you understand the phenomenon. surely I don't need
advice from people who don't have anything in their head.

http://en.wikipedia.org/wiki/Tin_foil_hat

Graham


--
due to the hugely increased level of spam please make the obvious
adjustment to my email address

 

[John Larkin]

On Tue, 26 May 2009 13:48:28 +0430, "op27" <saman75saman@yahoo.com>
wrote:

Hello, I have asked this question from different people but never got a
satisfactory answer.
I want to know in terms of electromagnetic physics how 50 Hz frequency of
main is induced in our body and converted to voltage at oscilloscope probe.
Please answer only if you deeply understand this phenomenon. dose it depend
on current? i.e. if we get more currents from main it induces more noise?
how it depends to voltage?

thanks,

Quick diode tester:

Grab scope probe tip with your left hand. Observe 50/60 Hz cruddy sine
wave on scope.

Grab one end of diode with right hand, and touch free end to scope
ground.

John

 

[z]

On May 26, 9:51 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 26 May 2009 13:48:28 +0430, "op27" <saman75sa...@yahoo.com
wrote:

Hello, I have asked this question from different people but never got a
satisfactory answer.
I want to know in terms of electromagnetic physics how 50 Hz frequency of
main is induced in our body and converted to voltage at oscilloscope probe.
Please answer only if you deeply understand this phenomenon. dose it depend
on current? i.e. if we get more currents from main it induces more noise?
how it depends to voltage?

thanks,

Quick diode tester:

Grab scope probe tip with your left hand. Observe 50/60 Hz cruddy sine
wave on scope.

Grab one end of diode with right hand, and touch free end to scope
ground.

John

i never thought of that..... thanks

 

[Rich Grise]

On Tue, 26 May 2009 18:51:19 -0700, John Larkin wrote:

On Tue, 26 May 2009 13:48:28 +0430, "op27" <saman75saman@yahoo.com

Hello, I have asked this question from different people but never got a
satisfactory answer.
I want to know in terms of electromagnetic physics how 50 Hz frequency of
main is induced in our body and converted to voltage at oscilloscope probe.
Please answer only if you deeply understand this phenomenon. dose it depend
on current? i.e. if we get more currents from main it induces more noise?
how it depends to voltage?

Quick diode tester:

Grab scope probe tip with your left hand. Observe 50/60 Hz cruddy sine
wave on scope.

Grab one end of diode with right hand, and touch free end to scope
ground.

I did this with a transistor once - I think it might have been a
darlington - but I did see gain.

Cheers!
Rich

 

[Usual Suspect]

Quick diode tester:

Grab scope probe tip with your left hand. Observe 50/60 Hz cruddy sine
wave on scope.

Grab one end of diode with right hand, and touch free end to scope
ground.

John

This didn't work for me.

What did work is:
1. Grab one end of diode with fingers
2. Touch (or clip) scope probe to the other end of the diode.

Rectified waveform shows on scope.

Thanks for the great tip.
--
Al, the usual