Wikipedia:Reference desk/Archives/Science/2018 December 27

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December 27

Why are bricks and concrete blocks so different in weight and size?

I have heard that the most common Imperial brick size is about the best size for cubic feet built per hour by a professional bricklayer. Then why are concrete blocks so big? Are pyramid blocks huge because they didn't have mortar? Sagittarian Milky Way (talk) 03:54, 27 December 2018 (UTC)

You may find Concrete masonry unit informative. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:19, 27 December 2018 (UTC)

Mortar was certainly known long befor since is is just a variation of building with Loam. The choice for big Blocks may have simply been the observation that they last for centuries. The Egyptian's build allot more pyramid's with different technical approaches prior to the big ones crowded by tourists today. See our Article Egyptian pyramid construction techniques for more. --Kharon (talk) 19:49, 27 December 2018 (UTC)

Those big blocks in the pyramids are indeed stone, not brick. When you use smaller blocks, you get more surface area for the same volume and all that surface area had to be created using simple tools. Using bigger blocks simply means less work to do in the quarry. Using cubes or even truncated octahedrons would have reduced surface area even more, but the latter are a bit hard to cut from solid rock and the beams they used give a more stable structure. PiusImpavidus (talk) 10:50, 28 December 2018 (UTC)

That makes sense, pyramid blocks were carved to the accuracy that you couldn't put a knife between them while you only have to make brick molds once. Are concrete blocks actually more efficient than bricks since you can add so many bricks of volume at once and there's less pieces to straighten and mortar? Of course concrete blocks with prism-shaped holes to give brick-thickness "plates" to grip weren't available in most of history. Why didn't ancients build larger brick molds with holes for convenient gripping then? Did large bricks take too long to dry or fire? Are they weaker (similar to how 2 materials are sometimes put in a composite matrix for better strength)? Sagittarian Milky Way (talk) 14:38, 28 December 2018 (UTC)

What's the difference between ventricular bigeminy and electrical alternans?

The two situations seem very similar, but our articles on one don't mention the other so it's easier to grasp the contrast. Yanping Nora Soong (talk) 23:23, 27 December 2018 (UTC)

To my eye the articles and illustrated EKGs seem pretty straightforward, with the exception that the EKG used to illustrate bigeminy does appear to be displaying the other phenomenon on one or two leads duh, I should have zoomed the figure, there are different leads nearly in a line horizontally, so there's no actual change in polarity. Bigeminy means two beats for reasons involving physiological transmission of signals in the heart and electrical alternans means a change in the direction of current (not necessarily one beat to the next; our example seems to have about six beats the same before changing in one or another axis) as measured from one or more pairs of leads on the chest. They don't seem related at a conceptual level. Wnt (talk) 00:43, 29 December 2018 (UTC)

In bigeminy, as the article notes, the most common cause is an ectopic beat following each normal beat. This results in pairs of beats on the ECG. While the two beats in the pair might look different in terms of shape (and in fact, most likely they will, given the ectopic beat is often a premature ventricular contraction, which looks different from a normal contraction triggered from the sinoatrial node), each pair should look more or less similar to other pairs. On the other hand, the most common cause of electrical alternans is cardiac tamponade, when the heart is swinging in fluid in the pericardial sac. This leads to amplitude differences between beats because the heart is turning towards and away from the ECG leads measuring the electrical signal in the heart, so that the vector component travelling towards/away from a lead changes between beats. However, if that person isn't also having ectopic beats on top of their tamponade, each beat will still be a normal beat generated from the sinoatrial node, so you would expect to see them all to have identical morphology with the only difference being the amplitude (how tall they are).

If you look at the illustrations on the two articles, this difference is pretty well demonstrated. Looking at just the bottom lead on each ECG (the rhythm strip) so that you can compare multiple beats from the same lead over time, you can see that in bigeminy, each pair looks pretty much the same. However, the two beats in each pair look very different: the first beat has a normal QRS complex followed by the T wave, whereas the second beat is much wider and is followed by this huge dip with no T-wave. Contrast that with the rhythm strip for electrical alternans, where you can see that each beat has a normal QRS-complex followed by a T-wave (i.e., sinus rhythm), with the only difference being that the tallness of each QRS complex varies (because the heart is swinging back and forth). Hope this helps! Brambleclawx 19:45, 29 December 2018 (UTC)