A few questions

I will update this more and more as I get questions...

Hi, I was just wondering:

Since mass causes gravity, elements which are spread throughout the universe will eventually be drawn towards each together. Now, as they all come together at one point - their mass is increasing each time and they gain more atoms and as a result of the increased mass, more gravity.

I don't understand - how do they get increased mass? If we start off with 4 hydrogen atoms which come together at a central point, how do they gain mass and become more atoms?

Or is it actually referring to all the elements that come together - all the other elements are attracted to this central point and eventually the mass of that central point gets larger and larger as more elements attract to it? - I think that is what it actually means.

(edited 7 years ago)

Reply 1

Joinedup

Is this a hypothetical question?

you can get local clumping together of mass e.g. to form a star... but at the large scale the matter in the universe is getting further apart.

Reply 2

Mik3yMcFly001

Yes - I think you're right. If that's something you found written down somewhere, I think it's just (not very clearly) trying to explain that as atoms come together gradually under the gravitational attraction between them, the matter focused at any one point in space 'snowballs'. The resultant increased density of material in any one region means that the gravitational attraction of that matter collectively becomes greater than when the matter was sparsely diffused through space.

As Joinedup says though, on the largest cosmic scales, the expansion of the universe is in fact accelerating: in the very-distant future the Milky Way will drift aimlessly and alone (along with the rest of the local galactic group). Hubble's Law summarises this accelerated Universal expansion, stating that recessional velocity is actually directly proportional to displacement through the cosmos. That is: v = Hs.

(edited 7 years ago)

Reply 3

Chittesh14

Original post by Joinedup

Original post by Mik3yMcFly001

Thanks guys, it was from this video:

https://www.youtube.com/watch?v=TzWkEFgtP9w

within the first minute of explanation.

Reply 4

Joinedup

Original post by Chittesh14

Thanks guys, it was from this video:

https://www.youtube.com/watch?v=TzWkEFgtP9w

within the first minute of explanation.

I think he's just talking about a small section of the universe tbh. It'd be the wrong idea if you came away thinking atoms of H from opposite sides of the universe end up inside the same star as each other.

The effect of the process is that if you've got a greater than average density of H in a region of universe it'll contract under self gravity until it becomes a nebula and eventually a star.

Reply 5

Chittesh14

Original post by Joinedup

No, don't worry, I wasn't thinking of it like that - not different atoms from different stars haha.
Just in general, stars coming together etc.
So the more denser (the more hydrogen you've got in the universe), the more hydrogen will contract itself under gravity and come to a central point and then clump together into a nebula and then turn into a star eventually right?

Reply 6

Chittesh14

Original post by Joinedup

Can you explain this please?

P-n junctions are formed by joining n-type and p-type semiconductor materials, as shown below. Since the n-type region has a high electron concentration and the p-type a high hole concentration, electrons diffuse from the n-type side to the p-type side. Similarly, holes flow by diffusion from the p-type side to the n-type side. If the electrons and holes were not charged, this diffusion process would continue until the concentration of electrons and holes on the two sides were the same (Does that mean if holes were not positive and if electrons weren't negative - so if they had a neutral charge?), as happens if two gasses come into contact with each other. However, in a p-n junction, when the electrons and holes move to the other side of the junction, they leave behind exposed charges on dopant atom sites, which are fixed in the crystal lattice and are unable to move. On the n-type side, positive ion cores are exposed. On the p-type side, negative ion cores are exposed. An electric field Ê forms between the positive ion cores in the n-type material and negative ion cores in the p-type material. This region is called the "depletion region" since the electric field quickly sweeps free carriers out, hence the region is depleted of free carriers. A "built in" potential V_bi due to Ê is formed at the junction. The animation below shows the formation of the Ê at the junction between n and p-type material.

I don't understand any of the second part highlighted in bold, can you help me please?
Does it simply mean that when the electrons move from the n-type side, the positive charges are exposed - so when an electron leaves, its like +1 on an ion - so a positive ion core is exposed lol. Similarly, in the p-side as a hole leaves - its like an electron can come in, -1 charge exposed?

(edited 7 years ago)

Reply 7

Joinedup

Original post by Chittesh14

Seems unnecessarily confusing IMO to talk about what would happen if the charge carriers weren't charged... or maybe it's just a bit of an awkward use of language :unsure:

strikes me it's a bit like saying "if my dad had boobs he'd be my mum"

This is, iirc pretty much a first year lecture in PN junctions... but with sexier graphics.

the important outcome really is that there is a depletion region between the P and N materials which forms a barrier the charge carriers can't cross unless you help them with an external pd - the bit the video illustrates with skateboarders going up ramps.

obviously it's a didactic model (a simplification for educational purposes)