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Physics, Astronomy & Cosmology / What do atoms consist of?

« on: 18/01/2009 11:59:47 »

Nicholas - welcome to TNS.

Protons & neutrons form the nucleus of an atom. Protons carry positive charge and neutrons, as the name suggests, are neutral. The vast majority of the mass of the atom is contained within the nucleus.

Protons & neutrons are made of quarks (not perks). There are 6 types of quark but protons & neutrons comprise only 2 - the up quark & the down quark. Neutrons have 2 downs & 1 up, protons have 2 ups & 1 down. These quarks carry different charge. The up quark's charge is +2/3 and the down quark's is -1/3.

You can see that by adding these in the appropriate combinations you get +2/3 * 2 -1/3 = 1; a proton with charge 1 or -1/3 * 2 +2/3 = 0 which gives you a neutral neutron. Quarks are held together by the strong force.

Electrons form a "cloud" around the nucleus. It used to be thought that electrons orbited the nucleus in the same way that planets orbit the sun. However, the advent of quantum physics showed this not to be the case.

Electrons carry negative charge. It is the electromagnetic force that keeps electrons in their place.

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General Science / How many grains of sand are there in the world?

« on: 21/12/2008 10:04:20 »

I've heard the comparison many times that there are more [insert as appropriate] than grains of sand on the Earth.

That got me wondering. How do they know how many grains of sand there are?

Supplemental question - are all grains of sand the same size?