# Can a satellite generate electricity by using a planet's magnetic field? [duplicate]

I am entering AP Physics E&M and don't have much knowledge about electricity but always had an idea based on my limited knowledge.

If a satellite had a metal coil inside of it, wouldn't it have an induced electric current since it orbiting Earth's magnetic field in the same way that sliding a magnet next to a coil would?

I would imagine this would be useful for spacecraft in the outer reaches of the solar system that could generate electricity like this since solar power would be far less efficient at such distances.

• Commented Jun 30 at 20:08
• Note that the electrodynamic tether only works in Earth's ionosphere. It relies on ambient ions to carry the return current. Commented Jul 1 at 3:54
• This question is similar to: Can the Earth's magnetic field be used to generate electricity?. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. Commented Jul 1 at 10:56
• see this write-up on NASA’s space tether experiment. Commented Jul 1 at 13:08

The Earth's magnetic field is very weak compared to that produced by a magnet you might '[slide] next to a coil' and the time over which the field will vary is long (> ~90 minutes for one orbit), so even in an optimally oriented coil the induced current would be very small. In the 'outer reaches of the solar system' any induced current would be orders of magnitude smaller still.

But in terms of using induced current as a power source there is a bigger problem - any energy extracted from the coil would be at the direct cost of the satellite's kinetic energy. Extraction of electrical energy would act like a (very inefective) brake on the satellite's motion.

As a postscript based on some of the comments below. In a uniform magnetic field a current loop (regardless of the magnitude of the current) produces exactly zero nett force, although it can produce a torque. So in deep space it could not produce any measurable acceleration or deceleration, as the sun's magnetic field is both very weak and almost uniform over any reasonable distance. For example: an enormous superconducting current loop with the same diameter as the earth (13,000 km), flying through space about half way between the orbits of Earth and Mars and carrying a current of one million amps, would experience a maximum force from the sun's magnetic field of approximately 20 Newtons. ...and that would drop to a milli-Newton about the time it passed Neptune's orbit.

• An SF author could use that for braking or even accelerating (by running a current through a superconducting gigantic wire). Commented Jul 1 at 9:34
• @Peter-ReinstateMonica It is used in the form of Magnetorquers, but the effects are so small, that it is purely used to control angular momentum.
– mlk
Commented Jul 1 at 12:09
• @Peter-ReinstateMonica in that vision, did you mean a huge wire wrapped around the earth or ???? Commented Jul 1 at 12:34
• @Fattie Not sure. But if it brakes it can also, reverted, accelerate, as a matter of principle. (Btw, I was initially thinking of mag-sails but those interact with the stellar wind, not (directly) with an external magnetic field.) Commented Jul 1 at 13:28
• It sounds very sci-fi, excellent! Commented Jul 1 at 13:32