Stupid question of the day (or year)

[curtispdx]

Okay readers, prepare your pupils for some rolling...

Here's my dumb question: When a flashlight is on do the coils in the tailcap create a magnetic field?

Stupid, right? BTW my previous dumb question thread was "Hey, have you seen the 500 LED flashlight?" :shakehead

 

[Ragiska]

no, they do not create a magnetic field.

 

[Fichtenelch]

Only AC can create a magnetic field, correct?

 

[Ragiska]

Only AC can create a magnetic field, correct?

no, DC can.

 

[HKJ]

Okay readers, prepare your pupils for some rolling...

Here's my dumb question: When a flashlight is on do the coils in the tailcap create a magnetic field?

Yes it does create a magnetic field, but not very strong.

Even a straight wire with current in will create a magnetic field.

Only AC can create a magnetic field, correct?

No, any current will create a magnetic field. AC creates an alternating field that among other things, can be used in transformers.

 

[fisk-king]

Yes it does create a magnetic field, but not very strong.

Even a straight wire with current in will create a magnetic field.



No, any current will create a magnetic field. AC creates an alternating field that among other things, can be used in transformers.

absolutely correct:twothumbs




I imagine the field created is nothing much to get worked over.

 

[somename]

With DC you would need an iron core to create the magnetic field otherwise you basically just made an inductor.
For instance take a nail wrap it with wire and connect the two ends of wire to a battery and you have a magnet.

If your worried about voltage coupling on nearby conductors, the only way to get the voltage to couple to a nearby wire would be pulsing current through the coils with the wire in close proximity to the coil.

Even then you would need that nearby conductor to be wrapped in a coil as well to get a decent amount of voltage.

 

[HKJ]

With DC you would need an iron core to create the magnetic field otherwise you basically just made an inductor.
For instance take a nail wrap it with wire and connect the two ends of wire to a battery and you have a magnet.

No, as I wrote above: any current creates a magnetic field.
Coiling the wire will intensify the field and adding a iron core will intensify it even more.

If your worried about voltage coupling on nearby conductors, the only way to get the voltage to couple to a nearby wire would be pulsing current through the coils with the wire in close proximity to the coil.

Any change in current will induce a current in nearby wires.

 

[somename]

No, as I wrote above: any current creates a magnetic field.
Coiling the wire will intensify the field and adding a iron core will intensify it even more.




Any change in current will induce a current in nearby wires.

Yes, you are correct. I'm just taking about field levels that he may or may not be concerned about.

I'm not talking about small levels where a person is measuring newton metres squared per coulomb and looking at electric flux and field lines.
I'm just trying to be general without going into too many details that not everyone what to hear.

Back to the question though, the field through the coils in the tailcap will be very small.

 

[Max_Power]

DC current flowing through a coil stores energy in an electric field surrounding the the coil. If you suddenly turn off the current, the electric field collapses and induces a current of opposite polarity in the inductor. On a coil with a strong magnetic field, the voltage of this reverse current can briefly be many times higher than the original DC voltage since the field collapses rapidly (a rapidly changing electromagnetic field will induce more voltage in the coil.) This is why you will see reverse-biased diodes connected across relay coils and solenoids, to short out this inductive kick and prevent it from damaging components downstream on the supply rails.

Flashlights generally don't have much inductance or current so this sort of thing isn't normally an issue.

 

[Moonshadow]

Even the very tiny currents in the neurons in your brain generate magnetic fields which can be detected with suitable equipment, but the fields are so small that the detector needs to be made of superconductor and dunked in liquid helium.

So all electric currents generate magnetic fields, but although the ones in the tailcap of your torch will be a bit stronger than your brianwaves, they are still too small to notice on an everyday basis.

(Goes off to switch his D10 on and off next to a compass just to make sure. If Mrs MS asks what on Earth I'm doing, I'll blame you lot . . .)

 

[curtispdx]

Thanks for the replies everyone. Pretty interesting, I wish I was smarter...

I wasn't worried about any magnetic field (if one existed) but was wondering if it existed could it be used somehow. I assumed that it would be small in normal lights but I was wondering what it'd be like in a 3C Mag running 3 AW IMR cells powering a 1185. That's probably running about three amps, right?

A bunch of smart people here. "Scuse me while I wipe the drool off of my shirt and shave the hair off of my knuckles...

 

[Moonshadow]

Actually, it's not such a daft question as it first seems. You're right to point out that the current draw is substantial in these lights.

I've just done a quick calculation, based on the tailcap spring having a diameter of a couple of centimetres or so and a current draw of one amp, and you do in fact get a magnetic field of the same order of magnitude as the Earth's magnetic field, perhaps even a bit higher.

That's right in the centre of the coil, mind you, and it will drop off very rapidly outside so probably still too small to deflect the compass needle (OK, yes I did try it . . . ).

And the Earth's field is very small to begin with of course. Interesting though.

 

[JulianP]

I wonder what happens when the LED is on strobe, or better, on pulse-width modulated dimming. Will the magnetic field be strong enough to interfere with something like a pacemaker? (Please don't test this at home, as a positive answer might not make it to this cpf thread) :sick2:

 

[bstrickler]

It will have to be a VERY powerful magnetic field emanating from the light, as the reed switch in pacemakers require a magnetic field of 90 gauss to be within 1.5 inches of a pacemaker, to close the reed switch (which causes the pacemaker to kick in).

The magnet from a NightStar flashlight has a surface gauss of 5200, and when its 2 inches away, it has a gauss of 30. That means you will need one BEEFY coil in the light to affect a pacemaker. in a flashlight, the magnetic field is too weak to really be able to penetrate the body of the light. Probably a max of 1 gauss will be able to be read right at the body of the light, if you're lucky (this is just an educated guess).

The data I just explained here was extrapolated from:

http://www.appliedinnotech.com/support/questions-answers.php (second to last question)

~Brian

 

[bogeymachine]

yes, current generates magnetism

no, you won't toast a pacemaker with tailspring EMI

check out the criteria for FCC Class B certifications...fascinating stuff:tired:

bogey out

 

[Grytpype]

LOL. If the current is strong enough and you hold on tight, the magnetic effect in your light will pull you smack into the nearest large metal object, such as a battleship.

 

[curtispdx]

Years ago I heard of a story involving a MRI machine and a cop's gun either ending up magnetized or stuck to the machine itself.

I heard it second or third hand so who knows how accurate the story was...

Off topic, sorry.


Edit: This isn't the one I heard about (mine was local) but like this:

http://www.thefirearmblog.com/blog/2010/01/03/dont-bring-a-gun-near-an-mri/

 

[Foxfyre]

You got my curiosity up so out came an led modded mag light and my Trifield Natural meter (measures changes in static or DC electromagnetic and magnetic fields).
I hooked bell wire to the center and outside ends of the battery spring, ran them to the battery and casing at the end of the light and with it lit waved the spring as close as I could past the back of the meter. The readings for ten passes averaged between 35 and 50 milligauss with a couple spiking just slightly higher.
The chart that came with the meter said a multiplication factor of 5.8 for a one hertz field so 45 (a rough average of the field measured) times that came out to 261 milligauss. How accurate that is I won't hazard to guess but it was fun trying it.