Some physics related to everyday life

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Practical questions

Things that I sort of know but can't recall offhand.

Or happened to have more interesting details.

Around the microwave oven

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

The most minimal microwave oven would consist of

  • a 2.45GHz source (a magnetron)
  • a chamber that is insulated for that frequency, and
  • a way to guide waves from the source to the chamber (a waveguide)

Microwaves deliver energy most efficiently into polarized molecules, creating friction and thereby heat.

There are various molecules we can get to, but the most abundant in the list that is present in (most) food is water (though also fat and sugar(verify)).

It turns out that at 2.45GHz (wavelength of approx 12.2cm, 4.8")) we heat primarily water.

So any food with high enough moisture content can be heated in a microwave.

Unaffected materials include most types of glass, paper, ceramic, and many plastics, though there are exceptions.

Uneven heating

Simpler designs will have some amount of standing waves, implying there are also areas receiving much less energy than others.

This is one reason you may see unevenly warmed food.

The rotating platter reduces this by moving your food through these spots, but isn't perfect.

Microwaves commonly have a stirrer, sort of a metal fan that helps jumbles the waves around, making for less pronounced standing waves. This helps, but isn't perfect either.

Instructions might include stirring halfway through, or letting it sit a while after cooking, depending a little on what kind of food it is.

Denser food sees only 2-3cm (~1 inch) of penetration.

If heating something large, the inner parts may need to be cooked via conduction from already-warm parts. If you want to do that evenly in a microwave, you'd probably turn the power down and wait a lot longer -- but at this point, you might as well use a regular oven, or pan.

It also turns out that ice does not absorb energy as easily as water, (it melts, but mostly because the energy gets delivered to water on its surface) so a frozen product will effectively warm slowly at first, then heat faster, and when something was partially frozen thing, that can make some parts be very hot and cooked while others are still frozen.

Combined with the last, it means cooking a large piece of frozen something isn't great.

Bad ideas

Some plastics

Whether a plastic is microwave safe is not about it getting too hot because of microwaves, it's about what it might contain and release when hot (mostly because of hot food).

So if it has a microwave label, it's an indicator of what it does not contain: additions known to not less-than-great for you, or even just taste bad. (The main names here seem to be BPA, some phthalates, and some others)

Metal (partly)

A few foods

A few ceramics

Empty microwave

Other potential problems

Sudden boiling liquids

It is possible for some liquids to superheat, which for these purposes means their temperature is above their boiling point but do not look like they are boiling.

This doesn't happen easily. Pans and heaters avoid this because they heat locally, and it takes quite-uniform heating to avoid creating local nucleation sites for the bubbles (and movement) we call boiling to start.

It also requires fairly pure water - this isn't easy to do with most tap water.

The movement involved in picking it may be enough to shift things and start the boiling, as may adding something at a different temperature (spoon, another ingredient).

Some more notes


Microwave ovens are well isolated by the internal metal walls, by the door's grated metal sheet (because of the microwave wavelength), as well as the chassis.

The little microwave EM that gets out is several orders of magnitude weaker than what's happening inside; it's milliwatts rather than the kilowatt-ish rating the microwave probably has, and rather smaller than what is known to be able to hurt us, or even be perceptible as heating.

Microwaves won't get out the air exhaust, largely because of the wavelength involved combined with the designed physical characteristics.

Pacemakers could in theory be affected, with the small wires acting as antennae. It can't hurt to be overly careful, and you might want to avoid a job at a microwave radar site, but microwave ovens are quite unlikely to cause problems.

Microwave frequencies mean this is non-ionizing EM radiation, meaning that they won't destroy cells, just heat them (comparable to cell phones).

You wouldn't want to stick body parts inside of an over, because it will heat you just as well as it heats food, which working organs won't like. But the little energy that escapes is unlikely to be even felt.

Ions, Ionizing radiation


Where do ions appear?

So what's the difference between ions and electric charge?

Why can ions be good or bad?

Negative ion generator

So what is an ionizer?

Ionizing radiation

Nuclear radiation

How much ionizing radiation is normal, how much is bad?


See also

EM spectrum

Quick(ish) reference

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Perhaps most broadly, you can see the spectrum as

  • radio
  • light (IR, visible, UV)
  • harmful radiation (UV, X-ray, Gamma)

Note: There are a lot of infographics you can find that are much nicer than the below.

(This is here for an experiment which I have not nearly finished yet)

A broad table (based on one from wikipedia) for some overview:

Name Wavelength Frequency (Hz) Photon energy (eV) grouping ionizing or not? Notes
radio 1 meter – 100,000 km 300 MHz – 3 Hz 1.24 μeV – 12.4 feV thermal [1] no under ~30kHz (1km) there are only a few niche uses that can barely be called radio
Microwave 1 mm – 1 meter 300 GHz – 300 MHz 1.24 meV – 1.24 μeV
Infrared 700 nm – 1 mm 430 THz – 300 GHz 1.7 eV – 1.24 meV thermal/optical (the thermal/optical split is sort of arbitrary)
Visible 400 nm–700 nm 790 THz – 430 THz 3.3 eV – 1.7 eV optical
Ultraviolet 10 nm – 400 nm 30 PHz – 790 THz 124 eV – 3.3 eV bond breaking yup bond breaking starts in the higher range of UV
X-ray 0.01 nm – 10 nm 30 EHz – 30 PHz 124 keV – 124 eV  damages life
Gamma ray less than 0.01 nm more than 30 EHz more than 124 keV damages life

[1] - 'thermal' is the classical name, named not for feeling warm, but the mechanism: thermal radiation is EM that originates in thermal motion of matter. This is much wider than heat/infrared. But yes, you can warm things with much of this range if you're really trying.

In a little more detail:

  • Below 3kHz or so
static field
easily influenced by atmospheric changes, and by noise from devices, and the antennas would have to be impractically large doesn't have a lot of uses
  • Radio wave is a broad part of the spectrum
most broadly 'Radio waves' is 3Hz .. 300 MHz (100000km .. 1m), but as mentioned above, real uses start above a few kHz
and if you consider microwave part of it, up to 300GHz (1mm).
a lot of the Radio-Frequency band names come from incremental amounts of use during the era that radio and TV were in active development
but there are a lot more uses in this range than radio and TV
microwave came in as a "well, turns waves this small are also usable for simialr uses" (they were initially harder to generate)
speech and audio basically needs to be above dozens of kHz (but are usually higher), video in MHz or more range, because that's the order of data you have.
TV, AM and FM radio typically within 50MHz..1500MHz (60cm .. 20cm)
mobile phones are largely at a few GHz (order of 10 cm)
frequency choice is also influenced by practicality of antenna size
Radar (and radiolocation) is more a technique than a frequency
there are Radar-like uses between 3Hz and 100GHz (see e.g. SUMMARY OF RADAR OPERATIONS in [1], though their practical use is fairly specific to range)
though a lot of it is on the order of hundreds of MHz (longer range) to dozens of GHz (shorter range)(verify)

  • the terahertz gap
roughly 0.3cm .. 30um, 0.1 THz to maybe 10 THz
so roughly "the bit between microwave and lower infrared"
called this because generation and detection here is inefficient for practical reasons, and not in mass production
sub-mm wave, is an often narrower part of this, used in astronomy

  • Infrared, 1mm .. 0.75um, 0.3THz..400 THz. There is more than one sub-categorisation of IR, but we often go roughly by:
FIR (far infrared) (1mm..14um, ~0.3THz..20THz)
IR-C (14um..3um, ~20..100 THz)
IR-B (3um..1.4um, ~100..200THz),
IR-A (1.4um..0.75um, ~200..400THz)
Infrared is called thermal, but
that's partly because it's part of the (much larger) range of thermal radiation.
If you meant actual warmness
it's only perhaps half of e.g. the sun's thermal delivery
only part of IR is particularly warm, or practical to warm people - preferably far, but mid also works

  • Optical
~700nm, 450THz - red light
~600nm, 500THz - orange light
~580nm, 520THz - yellow light
~530nm, 560THz - green light
~480nm, 650THz - blue light
~450nm, 700THz - purple light

  • Bond-breaking / ionizing
'UV' is a very wide range (0.3um..10nm, 790 THz .. 30 PHz) that it is usually subdivided into UVA, UVB, UVC based on the varied effects.
UVA - 315–400nm, ~800THz, (tanning beds. The little UVA from the sun that makes it to the surface causes at most minor skin damage)
UVB - 280–315nm, ~1000THz, (sunburns, skin cancer)
UVC - 100–280nm, ~1200THz (used for sterilization, ozone generation. Will burn flesh if strong enough, and damage your eyes. The ozone it generates from oxygen isn't ideal either)
there are other classification, like far/middle/near UV
and there is extreme ultraviolet, 10-120nm
Because of the scale, the numbers are not very precise. UV could be said to start maybe around ~750THz, while e.g. 700 THz is still just visible purple
EM starts to be ionizing somewhere above UVA, so UVB and UVC are considered ionizing. Luckily...
Our atmosphere blocks almost all UVC, a lot of UVB, and relatively little UVA.
Blacklights often straddle visible and a bit of UVA, so are mostly harmless
you can get UVC germicidal sterilizing lamps, but you don't want to [2]. You can usually tell by them having clear glass, because you need fairly special glass to even pass UVC. So yes, the regular purple UV does nothing in terms of germs.
window glass take out most UVB, though only some of UVA. This is part of why you won't easily get sunburned indoor - and your body produces less Vitamin D.
  • X-ray is roughly (10nm..10pm) 30 PHz to 30 EHz
Hard X-ray (above ~5eV, 3EHz) have better penetration but do more damage
Soft X-ray are those with lower energy (30PHz to 3EHz)
  • gamma waves is above 30000000THz+ (30EHz), shorter than 10pm

On EM and damage

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Ionizing (UV and higher, e.g., Gamma (nuclear), X-ray, cosmic rays) damages people.

This is also usually easy to stay away from, and hard to generate accidentally.

Below that (radio/thermal, optical) can, at worse, warm people up. Doing that enough to cook them is hard to do accidentally and without noticing (pain receptors being what they are).

Heating someone that much also takes a very strong transmitter that you generally won't be near enough to (dozens to hundreds of Watts - the only one I can think of is a microwave cooker, which is why those are isolated) Most other things don't neary have the enerfy.

Say, your phone because tops out at at most a Watt. You may get half of that in your face or hand, but that's an amount of energy lower than holding a warm hand next to your face.

If you want to heat people, you would generally choose the highest frequency you can easily generate, because it more easily carries energy. Below ionizing radiation, that basically must means infrared range.

There is no accumulative cell damage except while you are actively cooking them to a char, and you'll notice it way before that.

It's only once it ionizing where each individual photon arrival causes damage, which means more-than-tiny amounts of any of it is more easily an issue.

The table above lists photon energy because it's around ~10eV (in the UV range) that EM quanta are strong enough to be ionizing, that is, break molecular bonds.

Our bodies can deal with a small amount of that - and continuously do. But we want to keep that small.

Note that there are other reasons for things to have more energy. For example

cosmic rays, which are actually particles, have lots of energy because the ones that we detect are typically going near the speed of light, making their effective energy high enough to be damaging and, when these are also charged particles are then ionizing.
But don't worry - they're rare enough individual events, so they bother computers more than us.

Where are everyday devices on the EM spectrum?

Variations in IR

Variations in UV

Variations in X-ray

Variations in full-body scanners