Wikipedia:Reference desk/Archives/Science/2024 February 15

= February 15 =

Is salted whipped butter more friable and/or sticks less to bread?
If so then why? Solid pieces of unsalted whipped butter seem to fall off less often.Sagittarian Milky Way (talk) 01:06, 15 February 2024 (UTC)


 * Than what? Salted whipped butter? Unsalted unwhipped butter? Salted unwhipped butter? {The poster formerly knwn as 87.81.230.195} 176.24.45.226 (talk) 12:06, 15 February 2024 (UTC)
 * Salted whipped butter vs unsalted whipped butter Sagittarian Milky Way (talk) 14:22, 15 February 2024 (UTC)
 * Take it with a grain of salt: my short answer without references. When pressed and/or placed onto bread, some of the butter liquefies and is wicked into the bread giving it additional contact and surface tension (see Capillary action) causing the butter to stick. At the point the bread becomes saturated, the butter tends to slide off because a fluid film forms between the saturated bread and the intact butter solids. Added salt suppresses the melting points of solids such as ice, so it likely significantly speeds up the end result.
 * Modocc (talk) 14:50, 15 February 2024 (UTC)
 * Taken unsalted, your answer may stick better. --Lambiam 17:17, 15 February 2024 (UTC)
 * in my experience, whipped butter sticks to EVERYTHING more. i guess the air bubbles or something maybe make it more sticky? mushi ( ? ) 16:47, 29 February 2024 (UTC)

Magnetic compass
Not homework but I'd like to know how to answer this at the level of an introductory E&M physics class or that sort of thing. Basically a magnetic compass is a magnetized needle with a pivot in the middle, sitting in the Earth's magnetic field. The needle has mass M and length L and I guess we can ignore most subtleties.

My question is, how do you calculate the torque around the pivot, at least dimensionally? My first thought was that it would be quadratic in L (by integrating along the needle) but maybe that's wrong, and I just don't understand magnets well enough.

Oh yes, I guess the needle material itself needs to have some physical magnetization parameters specified. How would I find those, for whatever permanent magnet material is generally used in not-fancy compasses? Do fancy ones use fancier materials like rare earth magnets?

Motivation for asking: if I get a small cheap compass, say 1 inch in diameter, it will tend to get stuck easily, because the torque from the magnet isn't enough to overcome the friction in the pivot. Compasses with better (lower friction) pivots cost more. If I get one of similar quality that's 2 inches diameter, it will have 2x the friction in the pivot (because the needle is twice as heavy) but I wondered if it would have 4x the torque, similar to a moment of intertia calculation. That is, I'm wondering whether big cheap compasses work better than small cheap compasses.

Someday I'll try to work through a textbook on this magnetism stuff. Thanks. 2601:644:8501:AAF0:0:0:0:2F14 (talk) 03:55, 15 February 2024 (UTC)


 * The torque depends on the magnetic moment, which for a permanent magnet is the remanence times the volume (I think; you should read the articles to check). catslash (talk) 11:13, 15 February 2024 (UTC)


 * That's
 * $$\boldsymbol{\tau} = \frac{\text{vol}}{\mu_0} \mathbf{B}_\text{rem} \times\mathbf{B}_\text{earth}$$
 * catslash (talk) 12:08, 15 February 2024 (UTC)
 * I asked about this in 2019 and didn't get a good answer. Sagittarian Milky Way (talk) 14:29, 15 February 2024 (UTC)
 * Since the earth's magnetic field and the permeability of free space are outside our control at present, it seems we just need to maximize the volume of the compass magnet and the remanence of it's material. catslash (talk) 14:43, 15 February 2024 (UTC)
 * Increasing the needle's mass M by increasing its width or thickness increases its magnetic torque by the same factor, which does not help if the stickiness is due to pivot Stiction (static friction) whose resistance to motion is proprtional to the weight of the needle. Increasing needle mass instead by increasing its length moves the effective centers of application of torque further from the pivot, thereby adding a further increase in the moment of the torque to give faster settling. Another way to overcome stiction is to vibrate the compass. Philvoids (talk) 14:14, 17 February 2024 (UTC)