Earth magnetic field for dynamo?

[*Aqua*]

Hi,

I learned in school that magnetic field can induce an electric current in a coil. Can't a satellite which travels through Earth's magnetic field as a coil and use the induced current?

Or isn't it strong enough?

[TechnicalK3rbal]

It's not strong enough.

[*Aqua*]

How weak is it? What current can you expect?

[DaveofDefeat]

Earths magnetic field 0.00005 tesla (surface)

Small bar magnet 0.01 Tesla

Yea.

[Rakaydos]

Technically, you can. The problem happens when you realize the electrical power is generated by creating "magnetic drag", slowing the satelite. It would rapidly decay from orbit.

Conversely, there's been some work on a orbital maintanance system that pumps power from solar panels INTO the earths magnetic field, to counter orbital decay.

[-Velocity-]

Earths magnetic field 0.00005 tesla (surface)

Small bar magnet 0.01 Tesla

Yea.

That would be a very demagnetized bar magnet! Actually permanent magnet materials typically are around like 0.3-1.2 Tesla.

[jwenting]

That would be a very demagnetized bar magnet! Actually permanent magnet materials typically are around like 0.3-1.2 Tesla.

nope. 1 Tesla is incredibly strong. Like in the range of a mid-range electromagnet. If you have one of those and want to turn it on you are required to notify aviation authorities so they can set up a temporary no-fly zone as it disrupts compasses. A magnet that strong when running will over time magnetise the steel reinforcement inside the building's concrete shell.

Your watch is going to take damage over time from that.

Watchmakers are very happy with university physics departments because of that. Increases their business quite a bit.

[-Velocity-]

How weak is it? What current can you expect?

It's not hard to calculate (at least, not if you're an EE). The magnitude voltage induced on a coil is equal to rate of change of the magnetic flux passing through the coil, times the number of turns of the coil. Let's say you were in a low polar orbit, so you were orbiting every 90 minutes, and you managed to keep your coil oriented in one direction. You might see something approaching a sinusoidally-varying magnetic field of strength ~40 uT with a period of ~90 minutes. The equation for this field would be-

B(t) = 4X10^-5*sin(2*pi*(1/(90*60))*t).

(2*pi*1/(90*60) = w (omega) )

Say your coil had N = 100 turns and an area A = 1 m^2. The magnitude of the voltage is given by:

V(t) = -dPhi/dt*N (Phi - magnetic flux)

V(t) = -A*N*dB(t)/dt

V(t) = -w*4x10^-5*A*N*cos(w*t)

Let's throw out the negative sign (it's just a 180 degree phase change).

So the magnitude of the sinusoidally varying voltage is:

2*pi*(1/(90*60))*4x10^-5*1*100

= 4.7 uV (4.7 X10^-6 volts).

Remember, that's the peak voltage of a sinusoidally-varying voltage with a period of 90 minutes.

If your coil has a resistance of 0.1 ohms/turn, then you have a total resistance of 10 ohms, and a peak current of 4.7 uV / 10 ohms = 0.47 uA (4.7x10^-7 A). That's not a lot of current. Additionally, in case you're curious, you're generating (4.7 x 10^-7)^2*10/2 = 1.1 pW (1.1x10^-12 W) of power. The divide by two in there is because this is an AC signal, so you need to use its RMS value in power calculations.

So... even if this power wasn't being directly subtracted from your satellite's orbital energy, you're still talking about power levels that would be far below any useful values.

Now... if you got a *REALLY* big coil... then you'd be talking about useful amounts of power. But you'd still be falling out of orbit by using this power. Alternatively though, you might apply power to the coil and boost your orbit.

Edited April 15, 2014 by |Velocity|

[-Velocity-]

nope. 1 Tesla is incredibly strong. Like in the range of a mid-range electromagnet. If you have one of those and want to turn it on you are required to notify aviation authorities so they can set up a temporary no-fly zone as it disrupts compasses. A magnet that strong when running will over time magnetise the steel reinforcement inside the building's concrete shell.

Your watch is going to take damage over time from that.

Watchmakers are very happy with university physics departments because of that. Increases their business quite a bit.

Incorrect. While 1 T is very strong, it well within the range of remnant flux densities offered by NdFeB and SmCo rare-earth magnets. More common and cheap hard magnetic materials have much lower remnant flux densities, like 0.3 or 0.5 T.

BTW, I will be graduating with a Ph.D. in electrical engineering in around a year, and my research specialties are electromagnetics and MEMS. I did my master's thesis on soft magnetic materials. Last month I presented a paper at a conference about a silicon MEMS device I created that utilizes a 1.5 mm X 0.5 mm X 0.5 mm NdFeB rare-earth magnet. The remnant flux density of the rare-earth magnet we used is around 1 T, probably a bit greater (like 1.1 T). We never measured the exact value, which isn't exactly an easy thing to do... we just trusted our magnet manufacturer to deliver the quoted magnetization.

It's hard to get much above 1.2 T in permanent magnets.

So please, don't lecture me on electromagnetics

Anyway, it's really cool to play with those little tiny NdFeB magnets. They are so strongly magnetized, and small enough, that they automatically point north if you set them on just about any surface- the torque induced upon by them by the angular difference between their magnetization direction and the direction of Earth's magnetic field is strong enough to overcome static friction!!!

Also cool: Running a (larger) NdFeB magnet over a block of non-ferromagnetic metal rapidly. It induces eddy currents in the block of metal which oppose the motion of the magnet. One thing I did was to drop a NdFeB magnet down a 1 m long aluminum tube. 5 SECONDS LATER it came out the other end!

Edited April 15, 2014 by |Velocity|

[Tommygun]

NASA tried to do something similar on two shuttle missions. I can't remember the experiment's name.

They basically spooled out a few kilometers of electrical cable out of the shuttle.

If I remember right, the first attempt ended when the cable got stuck and the second try ended with the equipment being shorted out.

I don't recall if the electrical short was caused by internal failure or the induced current.

[ZetaX]

nope. 1 Tesla is incredibly strong. Like in the range of a mid-range electromagnet. If you have one of those and want to turn it on you are required to notify aviation authorities so they can set up a temporary no-fly zone as it disrupts compasses. A magnet that strong when running will over time magnetise the steel reinforcement inside the building's concrete shell.

Adding onto what |Velocity| said: you also seem to mistake flux density, which is measured in Tesla, with total energy density. In layman's terms and inaccurately, the former is how strong the magnet is and the latter how far it reaches (with a given strength). That's also why flux density is often given "at the surface" as it decreases rappidly outward (for your usual dipole magnet it is falling with the cube of the distance if I remember correctly).

Thus even if I could create a small 100T unobtainium permanent magnet in my house, it would not be much of a concern for anyone else. I am pretty sure you will have serious problems detecting a neodymium magnet weighing about 1g from several meters distance by compass. For the same reason, magnetation of objects, even only with distance some tenths of a meter, is completely neglegible.

And the above is not only from a theoretical perspective, but as owner of several kilogramms of neodymium magnets I can testify for all of that, too.

[-Velocity-]

Adding onto what |Velocity| said: you also seem to mistake flux density, which is measured in Tesla, with total energy density. In layman's terms and inaccurately, the former is how strong the magnet is and the latter how far it reaches (with a given strength). That's also why flux density is often given "at the surface" as it decreases rappidly outward (for your usual dipole magnet it is falling with the cube of the distance if I remember correctly).

Thus even if I could create a small 100T unobtainium permanent magnet in my house, it would not be much of a concern for anyone else. I am pretty sure you will have serious problems detecting a neodymium magnet weighing about 1g from several meters distance by compass. For the same reason, magnetation of objects, even only with distance some tenths of a meter, is completely neglegible.

And the above is not only from a theoretical perspective, but as owner of several kilogramms of neodymium magnets I can testify for all of that, too.

You (accidentally, I think) used the word "density" in "total energy density". You really mean the total magnetic energy stored in the magnet, which is proportional to B*H (or B squared) times the volume.

Another useful way (in fact, a more useful way, IMO) to express the overall strength of a magnet is with the magnetic moment. For a permanent magnet, the magnetic moment is approximately given by the magnetization of the magnet times the volume. For a high permeability (high mu relative) magnetic material, the magnetization is approximately B over the free space permeability mu naught (4*pi*10^-7).

Anyway, yea, with a small magnet, the magnetic fields drop off so quickly as to be negligible after only a short distance. While the tiny magnets we used had a very high flux density (around 1.1 T) their magnetic field drops off so quickly I can set them on my computer without damaging the hard drive. As long as I don't directly contact them to my credit card's magnetic strip, they won't damage it either. I would attach an image of one of our magnets, but strangely enough, there is no option to add an attachment.

[*Aqua*]

Technically, you can. The problem happens when you realize the electrical power is generated by creating "magnetic drag", slowing the satelite. It would rapidly decay from orbit.

You are right. I didn't think about that.

It's not hard to calculate (at least, not if you're an EE).

I don't know what you mean with EE but thank you very much for your calculation.

So the the voltage is in the µ-region. Yeah, that's really not enough.

Thanks to everyone for your comments!

[-Velocity-]

I don't know what you mean with EE but thank you very much for your calculation.

So the the voltage is in the µ-region. Yeah, that's really not enough.

EE - electrical engineer

ME - mechanical engineer

Chem-E - chemical engineer

etc.

[*Aqua*]

Are these abbreviations common in the English speaking world?

[Othuyeg]

You would propably want to go to Jupiter to have some real fun with magnetic fields.

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

You could for example utilize Lorentz force.

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

Basically, Jupiter could be used to launch probes.

[PlonioFludrasco]

As it was said, the so-called tethered satellite was tested in two Shuttle missions in the 90s, on STS-46 and 75, as a joint project between NASA and ASI (Italian Space Agency, that built the satellite).

[Justy]

EE - electrical engineer

ME - mechanical engineer

Chem-E - chemical engineer

etc.

Are these abbreviations common in the English speaking world?

"EE" is common for an Electrical Engineer, yes. But as for the others? "ME," in my experience, more commonly refers to either a Medical Examiner (also known as a Coroner, a doctor who investigates deaths for the police) or a particularly horrible version of Microsoft Windows. I've never seen anyone abbreviate Chemical Engineer to anything before now; "Chem-E" may be a common abbreviation among engineers, but not among the public.

Edited April 16, 2014 by Justy

[Death by Reentry]

"EE" is common for an Electrical Engineer, yes. But as for the others? "ME," in my experience, more commonly refers to either a Medical Examiner (also known as a Coroner, a doctor who investigates deaths for the police) or a particularly horrible version of Microsoft Windows. I've never seen anyone abbreviate Chemical Engineer to anything before now; "Chem-E" may be a common abbreviation among engineers, but not among the public.

Chem-E I get, but it gets even weirder when they start abbreviating civil engineer as Dum-E.

[-Velocity-]

"EE" is common for an Electrical Engineer, yes. But as for the others? "ME," in my experience, more commonly refers to either a Medical Examiner (also known as a Coroner, a doctor who investigates deaths for the police) or a particularly horrible version of Microsoft Windows. I've never seen anyone abbreviate Chemical Engineer to anything before now; "Chem-E" may be a common abbreviation among engineers, but not among the public.

Oh, ok thanks. I don't "get out much" so I couldn't answer his question :/

[*Aqua*]

Thanks again! I just learned something new.

In my country 'engineer' is commonly associated with constructing something sturdy, like buildings. There is no 'Chem-E', we usually call him chemist. And there is no 'ME', we call him pathologist or simply doctor (also called 'gods in white [lab coat]').

[peadar1987]

In British English, chemical engineer is usually someone who works with "big chemistry", so huge processing plants, oil refineries, etc.

Research chemists are those who work developing new drugs and compounds in laboratories.

"Chemist" is generally a synonym for "pharmacist"

We call electrical engineers "sparkies", and mechanical engineers "clankies". I've heard chemical engineers referred to as "sludgies" once or twice, but I don't think it's too common.

[SelectHalfling0]

Nasa actually did an experiment with this. The wire was nearly 1 mile long. The current was so strong, a bolt of electricity jumped the line to the space shuttle and broke the connection. Then nasa watched the multi-million dollar satellite drift away (they wanted to reuse it).