Electrical systems must be "grounded" in order to work. So how can the lights turn on in an airplane at 30,000 feet?
An electrical ground doesn't always have to be "the ground." It can be any electrical conductor that is able to absorb a vast amount of electrical charge. In a circuit, the ground serves as an escape route for dangerous buildups of current; an accidental surge will end up in the electrical ground, rather than frying the device's sensitive components.
Indeed, with its massive size, the Earth itself serves as an ideal ground hence the term but contact with Earth isn't the only way of grounding a circuit.
In airplanes, the electrical ground is the metal body of the plane itself. "The metal frame is used as a conducting 'ground' for all the electrical components inside the aircraft," Bob Erck, a scientist with the U.S. Department of Energy, explained on the DOE website. "So 'ground' on an airplane is usually the frame. A metal frame also protects the occupants from lightning strikes . Parts of the aircraft that are made of plastics or composites are protected with a metal mesh or fibers."
Cars are grounded in a similar way to airplanes: Current can't travel through the insulating rubber tires and into the Earth, so instead, all electronic circuits in cars are connected to their metal frames.
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