# Why doesn't the Earth's magnetic field affect electronics?

Why doesn't the Earth's magnetic field affect electronics in the same way that a magnetic field affects/exerts a force on a wire with a charge at a perpendicular angle? Isn't that why electric motors work?

Well, it does. The Earth's magnetic field is about half a gauss, or $0.5\times10^{-4}\rm\,T$. So if you have a meter of wire carrying one ampere of current from east to west, it'll feel a magnetic force of $0.5\times10^{-4}\rm\,N$ in some mixture of upwards and the north-south direction that depends on the tilt of Earth's field at your location. (I'm in the US Southeast and the field is about 70° from the horizontal, so if I did the experiment the force would be mostly to the north.) That force corresponds to the weight $mg$ of an object with mass $m=5\rm\,mg$, which is pretty small compared to any real one-meter wire that wouldn't melt under an ampere of current. Furthermore, most mains current is rapidly alternating, and most current carriers are cables which have both the supply and return currents right next to each other, which are both effects that make it harder to measure the force.

Earth's magnetic field can be a big annoyance if you're designing or using an instrument where free charges move and their exact trajectories matter, like a photomultiplier or a cathode-ray tube.

• Generations of CRT televisions and video monitors were aligned (if one was careful) facing north-south before making fine adjustments. – Whit3rd Jun 8 '17 at 7:00

Earth's magnetic field is just too weak. half a gauss is nothing compared to your standard refridgerator magnet.

• Thanks! It won't let me accept the answer. Too new on Physics stack exchange... – oeste Jun 8 '17 at 1:35

Actually, the Earth's magnetic field does affect electronics, probably, in a positive way, as it protects it from the effects of cosmic rays.

• But wouldn't a simple magnet provide protection from rays within its influence? Like if we were on the moon, could we just generate a field to do the same? – oeste Jun 8 '17 at 2:22
• @oeste That is a different question that could be posted as such. – gerrit Jun 8 '17 at 9:42
• @oeste: A simple magnet can only act within a volume with a dimension of, say, 1 m. The Earth's magnetic field acts within a volume with a dimension of, say, $10^6$ m (comparable to the Earth's radius). Therefore, to deflect charged particles of the same momentum you need a "simple magnet" with a magnetic field that is $10^6$ stronger than the Earth's magnetic field, i.e., tens and hundreds of Tesla. This is impracticable for protection of electronics (as far as I know, the field of the strongest laboratory DC magnet is 45 Tesla). – akhmeteli Jun 8 '17 at 13:07