Recent comments in /f/askscience

zutnoq t1_j67q8kj wrote

Reply to comment by VainVeinyVane in [Electricity and magnetism] For a half-wavelength dipole antenna, why is the voltage distribution a quarter wavelength out of phase with the current? by VainVeinyVane

A capacitor is essentially two plates of metal separated by a layer of insulator. The antenna is acting like one of those plates of metal, the air is the insulator and (the) ground is acting as the other plate.

Edit: in this specific case the other arm of the antenna is the other plate, if that wasn't obvious.

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Naive_Age_566 t1_j67kjnu wrote

Reply to Why do planet's rings match their rotations? by Ultimaurice17

you have to start with planet formation.

you have this cloud of dust. some of the dust clumps together and form - well - clumps of matter. those clumps further collect dust and grow.

all the parts of the cloud have more or less random kinetic energy - aka: swirling around in random motion. but as those parts collide, some of that kinetic energy canceles out. but it is highly unlikely, that it canceles out *exactly*. in the end, the cloud as a whole has some little "intrinsic" angular momentum - and that momentum has to be conserved. after all the parts have shed their excess kinetic energy, all that remains is this intrinsic angular momentum. this means, that if you wait for long enough, all parts of the cloud will move in the same direction - aka, rotate around the common center of mass.

in the end, your initial cloud will form a kind of disc, that uniformely rotates. in the center, the stuff will clump together further and will form the planet. stuff, that is far enough away, will stay in orbit and eventually form small moons. but the key is: they still have the initial angular momentum from the cloud - aka, the have the same rotation direction.

it's not exactly clear, how the rings around planets form. the rings of saturn are probably "fed" by water ice, that is squeezed out from some nearby moons. it could also be leftover material from the original cloud (unlikely - that orbit should not be stable enough - aka: the stuff should have fallen down on the planet or moved farther away and dispersed). of it could be small moons, which came too close to the planet and have been shredded by tital forces (see roche limit).

anyway: regardless of the exact physical process: angular momentum has to be conserved. always. if there is no external source of kinetic energy, in the end, everything in this system will move in the same direction. planet, moons, rings.

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