Can the Earth's magnetic field be harvested? What the physics allows

The Earth's surface magnetic field, of order 25–65 microtesla, is often proposed as a free energy source. This note sets out why it is not one and where it is instead valuable. A static field does no work: by Faraday's law an electromotive force requires changing flux, so any power drawn through the geomagnetic field is supplied by whatever moves the conductor. The field's stored energy density is of order a millijoule per cubic metre, and it carries no power flux of its own, in contrast with sunlight at roughly a kilowatt per square metre. Flux concentrators improve coupling but create no energy. The field's real value is as a global reference for navigation, anomaly detection and space-weather monitoring, and the note lists the elements of a checkable ambient-harvesting claim. An earlier version of this note was published on the author's blog: https://hrohera27.blogspot.com/2026/09/can-you-harvest-earths-magnetic-field.html