Wikipedia:Reference desk/Archives/Science/2021 August 3

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August 3

Plants - berry and juices questions.

Strawberries, raspberries, and blackberries roughly have the same lifespan, they decay quickly to room temperature. Blueberries spoil/decay 5x slower, so they can last 5x longer. Why is that? I heard the answer has something to do with respiration. All of them must be kept at 33 F - 35 F or 1-2 C. Problem is if it goes to 0 C, ice forms, but the issue isn't when ice forms, but when ice melts, then expansion damages the fruit.

Then, what about juices? Why are some juices can last much longer, and in room temperature, than other juices? By looking at expiration dates, cherry juice seems to last a year from shelf life. And apple, grape, and pear juices tends to be mixed together, but not with shorter-lifespan juices like strawberry. So, nobody sells a "blueberry-strawberry" juice mix, certainly not at room temperature, not because of the blueberry, but because of the strawberry. I've never seen a cranberry-strawberry, but I do see cranberry-raspberry, which is puzzling to me, because raspberry juice has the same shelf life as strawberry juice? Does cranberry juice actually have an effect that increases the lifespan of raspberry juice, for example? That it would not otherwise do with strawberry juice? Raspberry and blackberry juice can be mixed together because of their same-lifespan / shelf life.

So what is it in strawberry juice that makes it the shortest shelf life, that has to be refrigerated the most? Thanks. 67.165.185.178 (talk) 03:42, 3 August 2021 (UTC).

• Juices are covered by a rather unique Juice HACCP program. Typically they are pasteurized, so I'll bet that the shelf life of strawberry juice is not because it is any more likely to be affected by microbial growth, but by physical changes. I'm guessing it turns an ugly color very quickly. Abductive (reasoning) 08:07, 3 August 2021 (UTC)

Another thing that can go wrong is that a solid deposit comes out of the juice and onto the container, which looks ugly. Graeme Bartlett (talk) 22:58, 3 August 2021 (UTC)

I rather suspect that strawberry juice is less acidic than blueberry, raspberry, etc, which could account for a shorter shelf-life. DuncanHill (talk) 20:02, 4 August 2021 (UTC)

As a related question, what causes certain juices, required to be refrigerated at grocery stores - orange juice, strawberry juice, whereas other juices can be sold at room temperature. 67.165.185.178 (talk) 23:56, 4 August 2021 (UTC).

Since no one has answered, I'll volunteer that I'm guessing this depends a lot on how the juice was processed and packaged, as well as other factors like local regulations, norms, preferences etc. You can definitely buy a lot orange juice that is sold at room temperature in NZ. Most of this is reconstituted from concentrate but I think some are not. These are normally sold either in plastic bottles or Tetra Pak and have a best before date of months from manufacturing date. Strawberry not so much although I'm not certain this is because it's not possible but could be instead an issue of cost etc. Although this source mentions [1] the high-acid factor as the reason UHT is not normally required so DuncanHill might be right on strawberry juice. BTW our article mentions that Cold-pressed juices tend to have a very short shelf life. Also I'm fairly sure apple+strawberry or pretty much any apple whatever mix exists unless it tastes so bad no one wants it (or it's too expensive for the potential market or simply no one has bothered) e.g. [2] [3]. Note the last one may say "Strawberry and Kiwifruit" but if you look carefully it says "with apple base" which was a big part of my point. Surprisingly our article doesn't seem to deal with this but apple juice is a very common "base" for fruit juices. It's cheap with a fairly neutral sweet flavour. So depending on local regulations, it enables manufacturers to claim their produce is 100% juice (or whatever) and has no added sugar while producing a sweet cheap to make product with a small amount of strawberry or whatever else they promote and many dietitians etc are doubtful is much better than most soft drinks. See e.g. [4]. Nil Einne (talk) 17:15, 9 August 2021 (UTC)

Electron flow

Do electrons flow differently in a wire than in a circuit? Here's what I gathered.

In a wire: both electrons and protons travel from high voltage to low voltage.

In a circuit: electrons will flow from a point of more negative voltage to that of a more positive voltage. Electricity travels from high negative charge (lots of free electrons) to low negative charge or even positive charge (missing electrons). High voltage means more difference in charge, low voltage means less difference in charge.

I presume these aren't contradicting? 67.165.185.178 (talk) 05:45, 3 August 2021 (UTC).

Electricity goes from the positive side to the negative side. This is just a definition, made up before electrons were discovered. In wires (and most circuits are made of wires) the electrons move in the opposite direction. The protons don't move. Electrons have a negative charge and a flow of negative charge in one direction gives the same current as a flow of positive charge in the opposite direction. But an electric current isn't always a flow of free electrons. When we send electric current through sea water, we get positively charged sodium ions moving from positive to negative and negatively charged chloride ions moving from negative to positive, both coming from the dissolved salt. PiusImpavidus (talk) 08:15, 3 August 2021 (UTC)

So to give direct answers:

• "In a wire: both electrons and protons travel from high voltage to low voltage." Incorrect. The electrons travel from negative voltage to positive voltage, the protons don't travel.
• "In a circuit: electrons will flow from a point of more negative voltage to that of a more positive voltage." Correct, except that the electric current isn't always carried by electrons – but in a wire, or a circuit made of wires, it is.
• "Electricity travels from high negative charge (lots of free electrons) to low negative charge or even positive charge (missing electrons)." Incorrect, it's the opposite. Electricity goes from positive to negative, although the electrons go from negative to positive. And the current goes from positive potential to negative potential (voltage between two points in a circuit is the difference in potential between those points), not from positive to negative charge, although with non-zero capacitance those are correlated.
• "High voltage means more difference in charge, low voltage means less difference in charge." Almost. In real circuits, capacitance is never zero and the voltage is proportional to charge divided by capacitance. In ideal circuits (and many real-life electric components are close enough to ideal), capacitance is taken as zero (except where, by design, it's not), so there is no charge, but still voltage. PiusImpavidus (talk) 09:04, 3 August 2021 (UTC)

I'm curious where you listed electricity as the opposite direction of electrons. So can we say electricity travels from high voltage to low voltage? (But not electrons). I should ping @Thinking of England:. 67.165.185.178 (talk) 19:01, 3 August 2021 (UTC).

The direction of current is opposite to the flow of electrons. I think PiusImpavidus explained it correctly. Graeme Bartlett (talk) 22:55, 3 August 2021 (UTC)

Isn't that what current is, the flow of electrons? Okay, so that's 3 questions: the flow of electrons, the flow of electricity, and the flow of current. -_- 67.165.185.178 (talk) 23:18, 3 August 2021 (UTC).

No. The most common form of electric current you may be aware of is the flow of electrons through a conductor, and by arbitrary convention we have defined the direction of that electric current to be opposite that of the direction of the flow of electrons. But there are also examples of flow of positive charge carriers (as has been mentioned by other editors here), and those cases also represent an electric current, and by arbitrary convention we have defined the direction of that electric current to be the same as the direction of the flow of those positive charge carriers. -- ToE 23:57, 3 August 2021 (UTC)

Electric current is the flow of electric charge. When positively charged particles move from A to B, there's a current from A to B. When negatively charged particles flow from A to B, there's a current from B to A. It's just that we had already written down all the laws of electromagnetism (the Maxwell equations) before finding out that the main carrier of electric current in daily life happens to be a negatively charged particle. But that doesn't matter, other than being slightly confusing to people new to the subject. Most electrical phenomena other than in vacuum tubes and semiconductors can be fully understood without getting involved in the concept of electrons. PiusImpavidus (talk) 09:05, 4 August 2021 (UTC)

You rang? (I don't know what you expect from me, but perhaps I can rephrase some of the statements already made.)

The net direction of flow of a particular type of charge carrier in a particular system at a particular moment is a well defined physical property. If 6.24×10¹⁸ electrons per second are flowing through a conductor from point A to point B, this direction of electron flow is true, independent of any convention. But by convention, we measure the direction of "electrical current" as being in the same direction as the net motion of positive charge carriers and the opposite direction as the net motion of negative charge carriers. And when we say that 1 amp of "electricity flows" from point B to point A (in the example above), we mean that the direction of current, as we define it by convention, is from point B to point A -- even though in that example the direction of net charge carrier motion is opposite. That does not mean there are some separate physical particles of a substance called "electricity" which are flowing opposite those electrons. We simply define the "flow of electricity" by that convention.

Quoting from Electric current§Conventions:

The direction of conventional current is arbitrarily defined as the direction in which positive charges flow. Negatively charged carriers, such as the electrons (the charge carriers in metal wires and many other electronic circuit components), therefore flow in the opposite direction of conventional current flow in an electrical circuit.

Does that help?

As far as flow from high voltage to low voltage, I'd rather not use those terms, as High voltage has a specific meaning. (-10,000 VDC above ground is high voltage, whereas +5 VDC is not.) So speak instead of electricity traveling from more positive voltage to more negative voltage (or to less positive voltage if you wish), and electrons traveling from more negative voltage to more positive voltage. -- ToE 23:49, 3 August 2021 (UTC)

Okay, your last sentence hit the nail, electricity travels from more positive voltage to more negative voltage, electrons travel from more negative to more positive voltage. Which 1 does current go with? 67.165.185.178 (talk) 01:19, 4 August 2021 (UTC).

Electricity is a bunch of phenomena in electromagnetism. When we say that electricity flows somewhere, we mean that there is an electric current or a displacement current (a current not consisting of moving charged particles, but of a changing electric field). So the flow of electricity is loosely speaking the same thing as the electric current and it goes from positive to negative. Usually. PiusImpavidus (talk) 09:05, 4 August 2021 (UTC)

One note, is that electricity, even in solid conductors, can be the flow of positive charge. See Electron hole and P-type semiconductor. --Jayron32 14:44, 3 August 2021 (UTC)

There are also solid ion conductors, eg for lithium or sodium, but they are special materials and not metals. The main electric flow in a metal like wire is carried by electrons. Electric flow will also be accompanied by a magnetic field. Graeme Bartlett (talk) 22:55, 3 August 2021 (UTC)

"Electricity goes from positive to negative" - well, yes and no. Sometimes electrical current can be thought of as conventional current (positive to negative) or electron current (negative to positive). There are electricity and electronics text books that are available in both versions.

If you are doing simple electrical circuit analysis you can sometimes pick either conventional current or electron flow. But if you want to understand how vacuum tubes, cathode ray tubes, diodes, transistors, etc function, you have to get real with the physics. There is no abstract magical "electricity goes" that is separate from how electron flow happens.

Conventional current works in limited circumstances. Like how the "planets circling a star" model of electrons in atoms works in limited circumstances. It's not physically real, but sometimes you can fudge the math to make it give correct answers.

Conventional current is a historical error. Benjamin Franklin didn't know that electrons existed, and he decided to think of electricity as a flow of positive charges. 85.76.69.130 (talk) 15:44, 4 August 2021 (UTC)

When this something travels from high negative charge (lots of free electrons) to low negative charge or even positive charge (missing electrons), how that be by definition and not observation? Electricity, electrons, and current? How can 1 be defined to the opposite direction of the other? 67.165.185.178 (talk) 19:02, 5 August 2021 (UTC).

There appears to be a thing called electric charge. It appears to be quite real. For example, when we rub some fur against a piece of amber (the Greek word for which is ἤλεκτρον – elektron), the fur and amber get some property that causes them to attract each other, but two pieces of amber treated this way have a tendency to repel each other, and so do separate hairs of the fur. This property is called electric charge. Some further study reveals that electrically charged objects fall in two categories, such that two objects from the same category repel each other, whilst two objects from different categories attract each other and, when combined, neutralise the charge. The charges can be labelled as positive and negative, and we arbitrarily chose to call the charge in the fur positive and that on the amber negative.

As observed, opposite charges can neutralise each other and the objects don't even have to be in direct contact, as electric charge can apparently flow through uncharged, conducting objects, like metal rods. So we have such a thing as a flow of electric charge, also known as electric current or colloquially a flow of electricity (but I don't like that phrase), which runs from positive to negative. That makes sense. If there's a positive amount of something in one reservoir and a negative amount in a different reservoir and we connect them to neutralise, the flow of this something must be from positive to negative. And it appears that we can even make such an electric current without first generating measurable separate charges; we can make a setup to separate charges on demand to create a current as soon as the circuit is closed. All of that is classical electromagnetism. Note that I haven't discussed particles yet.

Now let's add some modern physics. And I really wish people wouldn't mention electrons in books on electromagnetism until they reach the chapter on the photoelectric effect. (OK, you're free to mention electrons when talking about electrochemistry.) So now we introduce the charge carrier. Charge isn't some abstract property of matter that can flow around in so-called conductors; it's a property of some invisible particles that can flow around in so-called conductors. And it appears that the most common charge carrier in daily life electrical circuits is a negatively charged particle, which is appropriately called an electron. Remember that amber got a negative charge? But if a charge carrier, moving to provide an electrical current, has negative charge, it must move in the opposite direction to the flow of positive charge, and therefore to the electrical current.

BTW, the current most important to humans in daily life is the current running through our nerve cells. This current uses positively charged sodium and potassium ions in a watery solution as charge carriers. PiusImpavidus (talk) 14:56, 6 August 2021 (UTC)

That's an excellent description. Just to expand a bit on one thing you noted on "classical electromagnetism". It's a shame, really, that we only deal in two domains of understanding of electrodynamics: "classical" and "quantum". Really, there's three domains: there's the classical-classical domain of things like Ohm's law and Coulomb's law and the like, where the laws governing electrodynamics are all governed by nice, easy algebraic equations with a small number of variables. That pretty much covers everything that was discovered prior to James Clerk Maxwell and Oliver Heaviside, and usually covers the sort of electric theory that is enough for practical understanding of most electrical devices, enough for an electrician to wire a house or for a person to do simple repairs on their own electrical devices, etc. Once we get to the Maxwell's equations, we suddenly get VERY complicated VERY fast, but we still live in a "pre-quantum" world. That's because Maxwell and his equations show that electricity, magnetism, and light are ALL manifestations of the same phenomenon, known as electromagnetism, and you can't really properly describe one in isolation. Ohm's law needs basic algebra to solve, something like Gauss's law (one of the four Maxwell equations) needs multidimensional calculus. Then there's the third domain of understanding, that of quantum electrodynamics, or QED, and now we're getting into Richard Feynman's path integrals and the like. --Jayron32 16:29, 6 August 2021 (UTC)

Okay, I think my question is, what's the difference between conventional current and electron-current? 67.165.185.178 (talk) 19:41, 6 August 2021 (UTC).

I'm not sure I follow the question. Electric current is the net flow of electric charge in the same way that water current is the net flow of water molecules. Just as you don't actually need to track the motion of individual water molecules to be able to understand how water flows through a pipe, you don't actually need to track how individual charge carriers are flowing in a wire to understand electric current works. At the level of things like Ohm's law, you don't even need to consider such things. You might want to check out some of the electricity videos at the YouTube channel "Science Asylum". Nick Lucid, the presenter, does a very good job and the graphics are top notch. Here is his electricity and magnetism playlist. --Jayron32 23:47, 8 August 2021 (UTC)

Then why is conventional current in the opposite direction of electron-current? 67.165.185.178 (talk) 01:12, 9 August 2021 (UTC).

Current moves in the positive direction. Electrons are negative. The sign conventions are necessary to make sure math works out. It's arbitrary WHICH sign conventions are used, and unfortunately, 250 years ago, Benjamin Franklin picked wrong. So now we're stuck with the confusing situation that charge carriers have the opposite sign as the electric current. If you get that time machine built, and talk some sense into him, you can make life much simpler for physics students for the next 250 years. If not, it's just another thing you need to remember. --Jayron32 11:55, 9 August 2021 (UTC)

I don't think my question was ever about the sign of electrons, but the direction. When electrons flows from (lots of free electrons) to low (missing electrons), but electricity flows from missing electrons, to lots of free electrons, I see that as an observation, not a definition. In my mind, electricity is still the flow of electrons, but they (electricity and electrons) go in opposite directions of each other. 67.165.185.178 (talk) 12:11, 9 August 2021 (UTC).

CNN report on breakthru covid infections

[5] Does this story say anything meaningful? It got a fair amount of attention. It says less than 1% of vaccinated people have experienced breakthrough Covid infections, but doesn't give a comparison number for non-vaccinated people. Per [6] the current infection rate in California is 16.4 per 100K, which is far below 1%, and actually suspiciously low (can it possibly be right)? I see all kinds of numbers in the news, that are useless as far as I can tell. They are trying to imply things about $\Pr[{\hbox{infected}}|{\hbox{vaccinated}}] \over \Pr[{\hbox{infected}}|{\hbox{unvaccinated}}]$ without giving enough info to actually compute this important ratio.

ObDisclaimer: I'm not looking for medical advice. I'm vaccinated but know someone who is not, who is hassling about it. Thanks. 2601:648:8202:350:0:0:0:2B99 (talk) 06:53, 3 August 2021 (UTC)

• So, this 1% is a sort of prevalence figure? Abductive (reasoning) 08:12, 3 August 2021 (UTC)
  • I can't tell. There is no data to show where that number came from. There is some stuff on statnews.com that might be more informative and that I'll try to look at when I can, but it is late here now. If the 16.1/100k figure is the number of active infections new infections per day right now, and the average one lasts 10 days, then extrapolating over the ~ 500 day pandemic would mean about 8% of the population was infected at one time or another. I guess I can believe that. 2601:648:8202:350:0:0:0:2B99 (talk) 08:32, 3 August 2021 (UTC) (Edited: fix words to fit math).

Somewhat more worrying in terms of breakthrough infections is this recent paper on the CDC website."Outbreak of SARS-CoV-2 Infections...". doi:10.15585/mmwr.mm7031e2. {{cite journal}}: Cite journal requires |journal= (help). It suggests that the Delta variant can cause infection even in those who were fully vaccinated. Mike Turnbull (talk) 11:24, 3 August 2021 (UTC)

When the vaccines first came available, I don't recall anyone claiming their vaccine was 100 percent effective against catching the virus. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:33, 3 August 2021 (UTC)

In terms of a reference for a Wikipedia article, per WP:MEDRS, CNN is not a qualified source to use for adding such information. If the CNN article cites their sources, I would go to those to learn more. --Jayron32 14:42, 3 August 2021 (UTC)

This article from the Seattle Times gives the source of the study as the Kaiser Family Foundation - the KFF press release is here and the data summary is here. Alansplodge (talk) 17:34, 3 August 2021 (UTC)