# If the earth is one big magnet, why there is not current induced in our selfs?

if the earth is one big magnet, why there is not current induced in our selfs? I mean, why there is not electromagnetic induction in the human body?

## Answers and Replies

Borek
Mentor
Have you checked? I bet there is some induction, just the current is very low.

HallsofIvy
Homework Helper
How strong is that "big magnet"?

Alfi
If you have a volt meter handy, put it on the mv scale and grab the probes in each hand.

I run about 80mv on an ungrounded surface.

Actually if you have an oscilliscope hand you might want to test the current across your body. I think you'll find that there's either a 50 or 60Hz AC current (probably in the milliVolt range) passing through your body at all times. This comes from the various power outlets in the room around you. You have to remember that the resistance of skin is quite high

human body 'battery'

If you have a volt meter handy, put it on the mv scale and grab the probes in each hand.

I run about 80mv on an ungrounded surface.

Yes, good point; I've measured that too....but that's not caused by induction from earth's field; it is simply a fact that different locations in our human bodies are at different potentials; i.e., our bodies act like a weak 'battery'. If I'm not mistaken, there have even been electronic devices that are designed to run off that potential difference.

Creator

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if the earth is one big magnet, why there is not current induced in our selfs? I mean, why there is not electromagnetic induction in the human body?

Why is there not a current induced in our bodies?
Very simple....there must be a time variation in the magnetic field to produce an induced EMF.
At any particular location on earth the B field is constant in time (to good approx.)....if it were not so then every piece of wire (and metal) on the planet would contain an induced EMF!

Creator

P.S. Having said that you should be aware that just about any ferromagnetic material, due to its position in the earth's magnetic field, DOES acquire a slight magnetic N - S polarity. It gives the Navy's Submarine Dept. fits. Break out your compass and check it out on iron objects that have been sitting around long enough to 'acquire' magnetization from earth's fields.
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Very simple....there must be a time variation in the magnetic field to produce an induced EMF.

no.. to produce an induced EMF in a body, there must be a time variation in the magnetic flux, which can be brought about by a time variation in either the area of the body through which the magnetic field permeates or the magnetic field or both, not necessarily the magnetic field alone.

even though the magnetic field is constant at any point on the earth [as you said, to a good approximation], the value of $dA / dt$ can be very high. However, as has been mentioned earlier, combined with the very weak magnetic field at the surface of the earth alongwith the high resistance of the human body, the current constituted in our bodies is extremely small.

Ok, I will conclude, that there is some very low current, while we are moving, since the magnetic field is constant. So to have EM induction either we should move and the magnet should be constant or the magnet should move and we will stay, am I right?

Ok, I will conclude, that there is some very low current, while we are moving, since the magnetic field is constant. So to have EM induction either we should move and the magnet should be constant or the magnet should move and we will stay, am I right?

Not exactly...
Let me be more precise so the nit-pickers don't start on me.

The field at any point on earth is constant....so standing still (which is what you originally seemed to ask) there would be NO EMF.
Moving in a constant magnetic field will not necessarily produced the EMF either.....you must be moving in such a way that the B flux (through your body) changes .....(as Rohan has said).
Staying at one location this would generally require you to be spinning, (which I don't think you had in mind).

If you are moving linearly, you would have to go through areas of different B field strength....to get an EMF.

... makes sense?

..

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Borek
Mentor
[nitpicking]Earth magnetic field is not constant[/nitpicking]. So even standing still doesn't guarantee you are induced current free

Andy Resnick
Why hasn't anyone done a simple calculation?

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

Estimating the Earth's field at 50 mictoTesla, a 6 foot tall person moving at 5 mph generates 0.000204 Volts. Given a resisivity of saline (69 Ohm*cm), I get an induced current of 15 nA, which is measurable.

Not exactly...
Let me be more precise so the nit-pickers don't start on me.

The field at any point on earth is constant....so standing still (which is what you originally seemed to ask) there would be NO EMF.
Moving in a constant magnetic field will not necessarily produced the EMF either.....you must be moving in such a way that the B flux (through your body) changes .....(as Rohan has said).
Staying at one location this would generally require you to be spinning, (which I don't think you had in mind).

If you are moving linearly, you would have to go through areas of different B field strength....to get an EMF.

... makes sense?

..

Yes, that's correct. Also thanks for the calculation, I'll have it in mind.

You might look at the magnetization of ships to give you some idea of how this works with large, good conductors moving at fairly high speeds.