Science desk
< October 19	<< Sep \| October \| Nov >>	October 21 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

October 20

Electric current and power

With electric current we always talk about power in Watts and not energy in Joules, why? — Preceding unsigned comment added by Malypaet (talk • contribs) 04:56, 20 October 2022 (UTC)[reply]

Because situations where it is useful to speak about the power (the rate of energy transfer) are more common than ones where it is useful to speak about the amount of energy transferred. In particularly, we commonly talk about situations where a certain level of power is, or needs to be, maintained for an extended time period. --174.95.81.219 (talk) 07:59, 20 October 2022 (UTC)[reply]

Why does one use amperes when measuring electric current and not charge in coulombs? Basically for the same reason. --Lambiam 08:16, 20 October 2022 (UTC)[reply]

An ampere is a flux, a bit like a frequency in electromagnetic radiation. How could amps be used to calculate energy ? Malypaet (talk) 22:18, 20 October 2022 (UTC)[reply]

The energy a current (plus voltage) delivers depends on how long the current flows. That is why it is usual to use Joules per second (Watts) instead of Joules. Note: I notice in your other questions you sometimes simplify things by assuming t=1. While this can be useful, you shouldn't forget that time is still in the equation. 195.50.139.86 (talk) 08:58, 20 October 2022 (UTC)[reply]

There's also the practical issue that the Joule and the coulomb are pretty small. It's common to use Ah (amp hours) and kWh (kilowatt-hours) as more everyday units. A 12 V 100 Ah battery should be able to continuously provide 5 amps for 20 hours while maintaining a voltage of at least 10.5 volts, what would a 360 kC battery supply? [assuming the reader doesn't think it's a temperature!] Likewise a 2kW electric fire will take 2 units of electricity to run for an hour and cost 68p (UK), any idea what that is in Joules and how much does a Joule cost? As well as being everyday units that people are familiar with, these are also long standing statutory units that are required by law. Changing them means re-educating the whole population for no very good reason other than to appease some SI fundamentalists. Martin of Sheffield (talk) 10:48, 20 October 2022 (UTC)[reply]

It's more that seconds are so small, and most people prefer hours over kiloseconds, leading to an inconvenient conversion factor that's not a power of 10. Ampèrehours and kilowatthours are a way to absorb that inconvenient factor 3600 into the unit. Else we could just use megajoules and kilocoulombs. If only the French had succeeded in introducing decimal time... PiusImpavidus (talk) 09:00, 21 October 2022 (UTC)[reply]

I often quote t=1s, because people often forget that the frequencies of electromagnetic radiation are in Hz and therefore defined over a time of 1s. It's like magnetism and electricity: inseparable. So I try here to do the //, because for the electric current which carries a flow of energy, the use of power seems natural (for ampere, t=1s), (whereas this is not always the case for the electromagnetism) . I did not find an example where one does not speak of power, but of energy in this field. A contradictory example? Malypaet (talk) 22:11, 20 October 2022 (UTC)[reply]

An electric circuit involves a flow of energy in the form of electric current. No flow, no circuit. This means usually when talking about circuits what we care about is the rate at which this energy flows—power—because we are wanting this energy to flow through the circuit over time in order to perform some kind of work. How much energy does your TV use? In formal terms there is no single answer. It depends on how much energy it draws and for how long. If you have it sitting unplugged, it's currently using zero energy (except maybe a tiny bit from an internal battery, if it has a clock). --47.147.118.55 (talk) 04:00, 21 October 2022 (UTC)[reply]

EMF in closed circuit

Can we call potential difference in an closed circuit as EMF if there is no charge in the circuit thus producing no current? Exclusive Editor _{Notify Me!} 13:11, 20 October 2022 (UTC)[reply]

If it is a closed circuit, there is still a voltage source/EMF driving the electricity. A circuit implies the existence of an EMF. --Jayron 32 13:25, 20 October 2022 (UTC)[reply]

There are ALWAYS charge carriers present, because those are just the electrons of the material. If you apply a potential difference, those charge carriers are going to move. So the premise of your question (potential difference but no charge), doesn't really make sense. PianoDan (talk) 14:27, 20 October 2022 (UTC)[reply]

Writing that there is no charge in the circuit is like writing that your circuit is made of matter with no electrons and protons ! What is this ? A neutron star ? Malypaet (talk) 21:35, 20 October 2022 (UTC)[reply]

If there's no charge, there's certainly no movable charge, which means it's an insulator. If your circuit is made of insulators, it's not a circuit in the normal sense. Still, electric fields can exist there. PiusImpavidus (talk) 09:10, 21 October 2022 (UTC)[reply]

Electronegativity

I'm trying to help my daughter, reviewing her AP Bio coursework with her, but I'm having some trouble myself with the notion of electronegativity. The higher up and the more to the right on the periodic table of the elements, the more electronegative the element, but is that before or after it grabs an electron. My daughter's teacher told her that more electronegative means more negative...but once a chloride atom, for instance, becomes a chloride ion, it's more negative, but is it more electronegative? Doesn't electronegativity describe the atom's affinity for grabbing electrons? And once it's got a full valence shell and is negatively charged, won't Cl^- repel electrons? Thanks!
I know that the article defines it as a property of an atom while it's in a bond, but is that the only way of understanding it? Can it not be said that atoms are more electron-hungry because they are more electronegative, but that once they are "satiated" they would technically drop in electronegativity because they are no longer electron-hungry? DRosenbach ^{(Talk | Contribs)} 21:49, 20 October 2022 (UTC)[reply]

The EN of an element is its attracting power before it grabs an electron, although it is worked out based on the element in compounds. Sanderson (writing in Chemical Bonds and Bonds Energy, 1976, 2nd ed., p. 9) says that the EN of each element in e.g. KCl is well represented by the geometric mean of the EN's of each element before they became bonded. For e.g. KCl this is the square root of 0.82 x 3.16 = 2.59 = 1.61. He adds that:

"In the chlorine atom the effective nuclear charge is reduced and forced to act over a longer distance, which means that the electronegativity decreases. This is exactly what we should expect, for the attraction an atom exerts on extra electrons must certainly diminish to the extent that the atom succeeds in acquiring them. But in the potassium atom the now larger effective nuclear charge is acting over a shorter distance, which means that the electronegativity increases. The reasonable assumption is made that these adjustments cease at the point where the two electronegativities become equal through the process of uneven sharing of the bonding electrons."

The teacher's advice that more electronegative means more negative is unhelpful. Higher EN just means increased attraction for electrons (which have a negative charge).

— Sandbh (talk) 01:02, 21 October 2022 (UTC)[reply]

Electronegativity: how much an atom "hogs" electrons when bonded to other atoms. Electronegativity is defined for the elements themselves, but is a quality only relevant when those elements form chemical bonds. When the atom "has a full valence shell", that means it's bonded and is part of a molecule. It is no longer an individual atom and you have to talk about the molecule it's part of. Electronegativity is not something that we hook up a detector to an atom or molecule and measure; it's calculated. Asking "what is the electronegativity of this atom after it forms a chemical bond" is a category error. The point of these electronegativity numbers is to tell us how much the element "hogs" electrons within a molecule. It's not a measure that we define for every individual atom and then follow each atom around and track for it as stuff happens. Remember, things like electronegativity are thing we humans make up in order to describe the world around us. In reality there is no tiny little counter within each atom that keeps track of its electronegativity. "The measures are made for man", not the other way around; with any kind of measurement, what matters is why we make it and for what purpose. --47.147.118.55 (talk) 03:50, 21 October 2022 (UTC)[reply]

Electronegativity is a way we express how polar a covalent bond is. When atoms are sharing electrons, they often don't share them equally. In water, for example, the electrons in the molecule are more "attracted" to the oxygen atom and less so to the hydrogen atom. That creates a charge imbalance, where the oxygen side of the water molecule is more negative and the hydrogen side is more positive. Electronegativity is a number that we give to each atom to say how "attractive" it is to electrons in a covalent bond. It is important to note this is a VERY limited application; it ONLY applies to atoms involved in COVALENT bonds (i.e. those in molecules) and does not really apply to other situations (i.e. lone atoms or ions or anything else like that). To measure how attractive an atom is to electrons in ionic situations, we use a different measure, electron affinity or ionization energy, which measures how likely an atom is to form a negative or positive ion respectively. --Jayron32 12:42, 21 October 2022 (UTC)[reply]
- Covalent vs ionic is a continuum. Otherwise EN values for metals would not make any sense: when could you use them? :) Double sharp (talk) 00:02, 22 October 2022 (UTC)[reply]
  - That is very true; one application of EN is to measure where on the covalent-ionic continuum a bond will lie; but in trying to determine things like "how likely is this atom to form an ion" IE or EA are better measures of that; whereas EN will tell us something like "when these two atoms are bonded, to what degree will one atom or the other attract the shared electrons". --Jayron32 20:46, 26 October 2022 (UTC)[reply]
    - @Jayron32: You know how pH and pOH are like oppose ends of the pH scale, and can be easily converted? Would this be the case for electronegativity, and electron affinity/ionization energy? In the covalent-ionic continuum. 67.165.185.178 (talk) 22:52, 28 October 2022 (UTC).[reply]

Re the OP – actually Ne is more electronegative than F in the sense of attracting a bonded pair of electrons, despite being significantly less electron-hungry, to put it mildly. :) Double sharp (talk) 18:28, 26 October 2022 (UTC)[reply]

Only in some sense; electronegativity is meaningless in atoms that cannot form traditional chemical bonds, and as yet no such covalently bonded neon compounds have been formed, the kinds of things we think of as neon compounds are rather exotic and don't fit into the traditional concepts that EN is well suited to dealing with; for example the "Allen electronegativity" of neon would predict it to make a nearly purely ionic bond with most metals; in reality it forms only very week ligands with a few transition metals in some very specific conditions. Electronegativity has several different definitions depending on the technique used to measure it; so some of those methods produce putative electronegativities for things like neon and helium, even if it's a meaningless number. --Jayron 32 20:44, 26 October 2022 (UTC)[reply]

@Jayron32: Getting bonds to Ne is no problem. It's getting it done in a neutral compound that is a problem. There are plenty of charged species with bonds to Ne; they've been known since 1977 at least (and they're even in the neon compounds article too). Double sharp (talk) 22:24, 26 October 2022 (UTC)[reply]

I wouldn't say it's no problem. It requires some exotic conditions to maintain stability and/or some pretty convoluted chemistry. "No problem" would be how I would describe forming bonds with carbon. But yes, you continue to be correct. You aren't going to become less correct with more explanation. No need to continue with doing so then. --Jayron 32 22:41, 26 October 2022 (UTC)[reply]