Some physics related to everyday life

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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, fix, or tell me)

Note: There are a lot of images you can find that are much nicer than this.

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

Perhaps most broadly, you can see the spectrum as

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

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 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

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).
on names and radio:
Microwave came in as a "well, turns waves this small are also usable for radio-like uses" (they were initially harder to generate) (side note: the food cooker we call microwave is way more specific, namely 2.45 GHz, because that frequency heats water specifically)
radio in general had similar naming issues before, growing many extra names over time, which is why we ended up with HF, VHF, SHF, UHF, EHF (High, Very High, Super High, Ultra High, and Extremely High Frequency), and to the other side MF, LF, VLF, ULF, SLF, and ELF (Medium, Low, Very Low, Ultra Low, Super Low, Extremely Low
each associated with a few typical uses, for practical reasons
sometimes as simple as "size of the antenna" (roughly why Mhz to GHz are practical for domestic systems)
directionality of typical antennas is also related
and for reasons like maximum emission due to previously established rules - though this is more fluid
for a general idea of radio frequency use (the spectrum grew to very fragmented over time)
e.g. the very low frequencies are very low information, but still useful for things like navigation, time stations, and since it penetrates water better, submarine navigation
speech and audio is usually above ~30kHz, because below that the very limited bandwidth is impractical
AM music radio typically within 500MHz..1500MHz (60cm .. 20cm)
TV and FM radio typically within 50MHz .. 1GHz (6m .. 30cm)
mobile phones are mostly in a few places between the ~1GHz to 40GHz range
and lots of gaps are filled by mobile, aeronautical, amateur
3GHz to 300GHz is a mix of many things. Things that are more unique to it include astronomy and satellite use
Radar is more a technique than a frequency
there are (very distinct) Radar-like uses between 3Hz and 100GHz[1]
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 (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
Quite a large range, and there is more than one sub-categorisation, but we frequently go by 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 it's only perhaps half of the sun's thermal delivery, and only part of IR is particularly warm

  • 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 range so very wide (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, from sun causes relatively 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
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 special glass to even pass UVC. So yes, the regular purple UV does nothing in terms of germs.
  • 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

Up until ionizing, it's hard to mess with people.

The best you can do is deliver enough energy that you heat them up enough to cook them.

If you wanted to do that, you would choose higher frequencies, ebcause they are an easier carrier for more energy. That energy is photon energy * amount of photons, but it turns out we have a lot of regulations about the latter. Standing arm's length away from a powerful narrow beam microwave transmitter might melt the chocolate in your pockets, but that's about the limit.

There is no accumulative damage except while you are actively cooking them, and you'll notice 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, for charged particles, specifically though ionizing.
But don't worry - they're rare enough individual events, so they bother computers more than us.

Everyday device and EM

Full-body scanners

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, fix, 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

Ionizing radiation

Nuclear radiation

See also