Wikipedia:Reference desk/Archives/Science/2019 February 18

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

February 18

Mass of nucleons

According to my nuclear chemistry textbook, the three quarks in a proton account for ~2% of its mass, with the rest of the mass being due to the gluons. I have two problems with that. First of all, an up quark has a mass of 2.2MeV/c^2 and a down quark has a mass of 4.7MeV/c^2. With two up quarks and a down quark, the mass of a proton due to quarks should be 9.1MeV/c^2 which as a fraction of the known mass of 938MeV/c^2 should be about 1%, not 2%. Secondly, gluons have no mass, so how do they account for the rest of the proton's mass? I know we have the concept of mass/energy equivalence, but given we're dealing with the rest masses of elementary wavicles and expressing the masses in units relating back to the energy anyway, surely that has already been taken into account in the stated masses of the quarks and gluons? 202.155.85.18 (talk) 01:11, 18 February 2019 (UTC)

The relevant details of quantum chromodynamics are extremely complex, but it boils down to there being an attractive force between the quarks mediated by massless particles that's proportional to the distance, just like in case of a spring (a so-called harmonic oscillator). A harmonic oscillator has a zero point energy of ${\frac {1}{2}}\hbar \omega$, this zero point energy is where the dominant contribution to the mass of the proton comes from. Count Iblis (talk) 01:59, 18 February 2019 (UTC)

The mass of the u quark is poorly known. It can even be massless. So, 2% is more an upper limit. If you are confused how a system of massless particles can have a mass just consider a system of two photons having the same energy $\hbar \omega$ and opposite momentums $\mathbf {p}$ and $-\mathbf {p}$. Their 4D momentums are $p_{1}=(\hbar \omega /c,\mathbf {p} )$ and $p_{2}=(\hbar \omega /c,-\mathbf {p} )$. The total momentum is $P=p_{1}+p_{2}=(2\hbar \omega /c,0)$. So, the total mass is defined as $M={\sqrt {P^{2}/c^{2}}}=2\hbar \omega /c^{2}$. The main difference between gluons from photons is that the former are bound to an hadron while the latter are not. Ruslik_Zero 20:47, 18 February 2019 (UTC)

• The point is, gluons are massless, but they have energy, and their energy is the 98% of the rest mass of the proton. It's plain old energy we get from special relativity, from the energy of the gluons themselves. The mechanics of that energy are very complex (see quantum chromodynamics), but at a root level, all mass is either rest mass or relativistic mass (E = mc²). When we get down to the quark-gluon scale, the mass of the proton is mostly gluon energy. --Jayron32 13:46, 19 February 2019 (UTC)

It seems strange that mass should have two different sources (rest + relativity), given that we know massless particles can gain relativistic mass. If the mass here is mostly relativistic, are there theories that can account for the mass of all particles as various kinds of relativistic mass? Wnt (talk) 01:57, 20 February 2019 (UTC)

I'm guessing that string theories would be in that category, since the string tension gives rise to mass. 202.155.85.18 (talk) 02:18, 21 February 2019 (UTC)

Foreskin joining/growing together with the glans penis.

l read that if the foreskin isn’t rinsed/washed for a long time, over time it’ll join/grow together with the glans penis, is this correct?--5.33.0.177 (talk) 01:45, 18 February 2019 (UTC)

Foreskin#Conditions is the sourced information we can give. DroneB (talk) 09:48, 18 February 2019 (UTC)

Heating Oil Consumption

Would raising my home thermostat by 5ºF (say from 70ºF to 75ºF) be equivalent, in terms of heating oil consumption, to the outdoor temperature falling by 5º (say from 25ºF to 20ºF)? hydnjo (talk) 18:47, 18 February 2019 (UTC)

No, because there are only a few small "energy bridges" in the house insolation to the outside temperature while the generated inside temperature does not have to overcome any insolation and to the contrary even has effective heat storage and spreaders (Radiator (heating)). --Kharon (talk) 19:50, 18 February 2019 (UTC)

P.S.: That is why all pioneers build a block hut for the winter (superior insolation) instead of keep using their tents. --Kharon (talk) 20:37, 18 February 2019 (UTC)

• Yes (as a reasonable approximation). Partly because 5ºF is only a small difference.

House energy consumption is complicated, to predict it in detail, and much can be made more accurate by measuring the actual house, rather than trying to predict from principles.

Let's begin by assuming that your house is an insulated box. Heat is added to balance the heat lost and the house stays at a constant temperature overall. The heat needed is then equal to that lost by conduction through the insulation of the walls and roof, minus any heat generated by humans inside, lighting, computers, etc. This is fairly easy to calculate, as R-values and U-values are well-known for standard building materials and construction. The heat lost by conduction for these is given by Newton's law of cooling, i.e. the rate of heat transfer is linearly proportional to the temperature difference, thus the implied assumption in your question holds true.

It's not quite that simple. Many losses are through convection instead, which may (in real-world situations) give a more than linear relationship. There's also radiative transfer which can get complicated. This relies on the difference between radiated heat in and radiated heat out, both of which have complex dependencies on each absolute temperature, rather than temperature difference. This depends a great deal on the house construction. A well-insulated 'greenhouse' may be warmer on a day that's cold, but sunny, as it can use its greenhouse effect for passive solar heating.

Then there are human factors aspects. Humans (if let near the control thermostats) will tend to heat more from a colder temperature, over-compensating to reach a higher evntual temperature. Andy Dingley (talk) 20:17, 18 February 2019 (UTC)

Thanks Andy, that's what I thought but wanted to make sure that there wasn't some wildly non-linear effect going on. You see, I was just trying to justify raising my thermostat a few degrees (for my comfort) by thinking of it as being roughly equivalent to nature dropping the outdoor temperature by that same few degrees :) hydnjo (talk) 21:59, 18 February 2019 (UTC)

I've just remembered I actually have some old thermostats (high tech 1980s) which take the indoor/outdoor offset into account and control their indoor set point accordingly to limit that differential, avoiding excess heating demand. Andy Dingley (talk) 22:05, 18 February 2019 (UTC)

Actually, I’ve found that the main factor influencing my comfort level, and thus the thermostat setting, is the indoor Relative Humidity. Which then begs the question as to the startup and running cost to increase the whole house RH about 30 percentage points vs increasing the indoor temperature about 3ºF. Increasing the RH would be trivial with a forced hot air heating system but far from trivial with my forced hot water system. It's a lot easier to just nudge the thermostat up a “bit” hydnjo (talk) 23:25, 18 February 2019 (UTC)

• It's too dry in Winter? I've heard of this, but never experienced it!

Heating here in the UK is mostly hot water. We have gadgets available (but little use for them) which are semi-permeable ceramic water reservoirs, with a wire hook to hang them from our heating radiators. Or you can dry some laundry on them. Andy Dingley (talk) 17:54, 20 February 2019 (UTC)

Low relative humidity during the winter is definitely a factor in the American Midwest. Maybe not so much in the UK, given the stereotype of perpetual dampness. ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:37, 20 February 2019 (UTC)

We have those too here in Croatia, much cheaper (and probably healthier) than a humidifier, and also have little use of them. When the temperature goes below zero here, humidity is usually >95% anyway. That seems to be common around Europe, probably because we're all so near to some sea coast or other. 78.0.242.164 (talk) 13:36, 21 February 2019 (UTC)

The RH in my house right now is 12%. The heating hot water is forced through "concealed" baseboard radiators which is common in the Northeast US. So no practical possibility of hanging wet stuff to gain moisture in the air. hydnjo (talk) 18:29, 21 February 2019 (UTC)

If the outdoor temperature is 15 C or less, a 25 C indoor temperature guarantees humidity of 50% or less

Some of these comments seem to defy physics. Outdoor relative humidity at low temperatures will indeed be near 100%, but provided heating is actually being used to the degree many of us were assuming, then the indoor humidity should be very low. To be sure, releases of steam from heating would change this; also, cold areas of a house could condense water that evaporated in warmer areas. Wnt (talk) 22:37, 22 February 2019 (UTC)

• Much of physics - from Newton onwards - seems to defy physics. That's why measurement is important, not human observation.

Your graph point is roughly that "Heat air by at least 10°C and you can't have more than 50% RH". That's not a bad theoretical prediction, but it's breached in practice in two ways. Firstly, here in the damp, coastal UK, we don't heat by 10°C (or much over it) except exceptionally. I'm going from an ambient of 8°C outdoors to 18°C indoors. Yes, it gets colder than that too - but I don't heat overnight. If I have South Africans in the house, I set the gas heating to full and shovel coal into the fireplaces for a day beforehand.

Secondly, that's not how "damp UK houses" get damp. Our problem is with damp permeating through masonry, either by penetrating damp from outside, or by condensation of atmospheric moisture. This then turns the walls (which are dense masonry for most UK houses) into moisture reservoirs, usually with black mould problems too. At any interior temperature, such a reservoir can thus increase atmospheric RH to almost any level. Andy Dingley (talk) 23:25, 22 February 2019 (UTC)

There's also the not insignificant contribution of human breathing. 78.0.201.171 (talk) 17:26, 23 February 2019 (UTC)

I'm assuming that's reasonably constant between climates. Although it probably varies (as people / sq. ft. of floorspace) between US & UK houses. Andy Dingley (talk) 17:38, 23 February 2019 (UTC)

18 C -- no wonder! That scarcely counts as heating at all ... I mean, Jimmy Carter never tried to talk people into setting thermostats that low. Actually, I was stunned to see he had them set in federal buildings to 65 F, which isn't very much higher [1] - nowadays, 68 is considered a form of asceticism. ;) But I doubt even empty federal buildings turned their heat off at night... Wnt (talk) 22:55, 23 February 2019 (UTC)

• Jimmy Carter wasn't a Geordie. Andy Dingley (talk) 23:02, 23 February 2019 (UTC)