Some physics related to everyday life

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 — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

The most minimal microwave oven would consist of

• a magnetron at 2.45GHz
• 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, flipping these molecules, adding kinetic energy and thereby heat; this is a case of dielectric heating.

There are quite a few molecules that that gets to, and microwaves will heat all of them to some degree, but of that list, the one that is most abundant in most food is water (though also fat and sugar(verify)), and 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 (that didn't enclose water when made), and many plastics, though there are exceptions.

Uneven heating

You will have some amount of standing waves, simpler designs more so, meaning there are areas receiving much less energy (heat) than others.

This is one reason you may see unevenly warmed food.

The rotating platter helps, but isn't perfect.

The microwave stirrer -- sort of a metal fan that helps jumbles the waves around, making for less pronounced standing waves -- also helps, but isn't perfect either.

Microwave cooking instructions might include stirring liquids halfway through, or letting food sit a while after cooking, both to let the heat conduct out more evenly.

Denser food sees only 1-3cm 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 longer, though at some point you might consider using a regular oven, or pan.

A frozen product will heat very unevenly.

Microwaves may be one of the fastest way to dump energy into frozen food, but it is also one of the most uneven - arguably in part as a result.

The physics of this are a messy mix, but you can see it as a shield near the surface (in part because ice has good penetration but lower-temperature water that melts at the surface does not(verify)).

So while it may seem to melt, that is primarily the surface.

Again, slower is better, but defeats part of the point.

Bad ideas

Some plastics

Whether a plastic is microwave safe is less about it getting too hot because of microwaves, and more about what it might 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)

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

Isolation

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.

EM spectrum

Quick(ish) reference

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

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)

Perhaps most broadly, you can see the spectrum as

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

Or, a little more detailed:

Name	Wavelength	Frequency (Hz)	Photon energy (eV)	grouping	ionizing or not?	blocked by atmosphere?	Notes
Radio	1 meter – 100,000 km	300 MHz – 3 Hz	1.24 μeV – 12.4 feV	thermal [1]	no	lower wavelength radio is blocked, higher end not	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				many applications are much like classical radio
Infrared	700 nm – 1 mm	430 THz – 300 GHz	1.7 eV – 1.24 meV	thermal/optical		mostly blocked	(the thermal/optical split is sort of arbitrary)
Visible	400 nm–700 nm	790 THz – 430 THz	3.3 eV – 1.7 eV	optical		mostly passed
Ultraviolet	10 nm – 400 nm	30 PHz – 790 THz	124 eV – 3.3 eV	bond breaking	yup	mostly blocked	ionizing/bond breaking starts in the higher range of UV; some UV still isn't
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 for being Thermal_radiation - which names "if it's above absolute zero, it emits EM". It's not really bounded, though for things at human-sane temperatures this sits around heat/infrared.

'Thermal' seems contrasted with 'optical' but that's a fuzzy distinction too.

See also the similar black body radiation

And yes, objects that we consider cold do emit a broad spectrum, including radio

it's broad-spectrum, though, and human-friendly temperatures imply have a peak that is infrared

And yes, you can warm things with much of the EM range if you're really trying - in the end any EM is energy so enough of it will heat and will damage. But you may have trouble finding a strong source of some, or even generating some.

In a little more detail:

• Below 3kHz or so

static field

power-line

easily influenced by atmospheric changes, and by noise from devices, and the antennas would have to be impractically large

...so doesn't have a lot of radio-like uses

• Radio wave is a broad part of the spectrum

most broadly 'Radio waves' is 3Hz .. 300 MHz (100000km .. 1m)

and if you consider microwave part of it, up to 300GHz (1mm).

there are few practical radio-like uses below a dozen or so kHz

technically ULF, SLF, ELF, and VLF are even below a few kHz, but can be considered exceptions

in practice most interesting things are above 30kHz

those band names have been a mess for many decades - a lost of them come from incremental amounts of "Oh I guess we'll use that too", e.g. around radio and TV were in active development

including microwave - they were initially harder to generate but at some point became another step in "well, turns out waves this small are also usable"

Examples:

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

~600nm, 500THz - orange

~580nm, 520THz - yellow

~530nm, 560THz - green

~480nm, 650THz - blue

~450nm, 700THz - purple

• 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)

Notes:

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, and 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. (No, blacklight/purple style 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 if you never go outside.

• X-ray is roughly (10nm..10pm) 30 PHz to 30 EHz

Soft X-ray are those with lower energy (30PHz to 3EHz)

Hard X-ray (above ~5eV, 3EHz) have better penetration but do more damage

• gamma waves is above 30000000THz+ (30EHz), shorter than 10pm

On EM and harm

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

For context

We are used to 'radiation' meaning 'nuclear' and 'danger'.

But any EM radiates. From the harmless visible light, to heat, to radios, to the harmful UV, gamma, and other nasty things in space.

We are used to 'photon' meaning 'visible light'.

But in physics the term is valid for all EM, from radio to IR to visible to gamma.

Harm how?

EM radiation can damage in one of two ways, and the distinction turns out to be a specific place, a specific threshold in the EM spectrum, which happens to lie in the UV range.

Below that threshold (what we call radio, thermal, optical), the only way to harm people is for there to be so much of it it heats them up too much. This is easiest with a specific range of IR, but possible with any.

Doing that enough to cook them is hard to do accidentally, and hard without them noticing (pain receptors being what they are).

Most natural sources aren't strong enough to bother us much - the sun's heat being the strongest example down here on earth's surface.

Most man-made sources of these are often not strong enough due to regulations, or if they are, they typically have fences around them.

But perhaps more practically, if the only mechanism is heating, it can't be damaging without first being noticeable (there are stories radio tower engineers having snacks melt in their pocket but not really noticing themselves), then painful, and only then being harmful. So this is not generally not an issue.

The only everyday example I can think of being potentially risky is a microwave - which is why those are shielded, and have interlocks based on its door.

Whereas e.g. your phone tops out at about 2 Watt (and it lowers it as much as the distance to the tower allows, to save battery), and most variants have a duty cycle of order of 1:8 or so[3], meaning the average power (which is what matters here) is an eight of that peak power.

Smush that between your face and hand hard enough that they recover all that power, half in your face and half in your hand, and that's still not an amount that matters. Your hand puts out more than that (you put out ~100W overall when relatively idle, hands are ~1% of your body surface).

Above that threshold, it turns out that it's no longer about collective effect - every individual arrival (and they are tiny and many) individually has enough energy to react with molecules.

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

This means that any amount of it can and will do damage to our cells.

We typically call this ionizing radiation, because that's how it does that damage to our cells (and to other things).

If you really wanted to heat people, you would generally choose the highest frequency you can easily generate, because it more easily carries more energy.

Below ionizing radiation, the easiest happens to be the infrared range.

There is no accumulative cell damage until you use temperature that start cooking people, and people will notice that.

Our bodies can deal with a tiny amount of ionizing damage - and continuously do, in the form of background radiation and the small amount of UV(-B) that isn't blocked by our atmosphere.

and we still like to keep this low.

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

Aurora are charged particles (which?(verify)) from the sun, which need to go moderate speed to even make it into our atmosphere(verify).

But don't worry, it mostly happens 100-200 kilometers up, and only a small amount of it makes it to ground level, rarely enough to affect our infrastructure much (verify)

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.

How much gets to us from space?

Broadly, see this image from NASA, via wikipedia:

Infrared notes

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

IR refers to infrared.

Infrared is part of the EM spectrum, right next to human-visible light

Infrared is in fact a large range in EM, and one end does different things from the other.

So it is useful to split into different parts (and confusingly, we've even done that split in different ways[4] - more on that below)

So many real-life cases of saying 'infrared' actually refers to a fairly specific part of it. Say,

• infreared remote controls, most IR communication, are NIR

• "night vision" in camcorders, infrared-sensitive photo and video cameras, and security cameras is NIR

and then primarily the near-visible part of that (unless specified otherwise - there is some variation here)

infrared lighting for these photo/video cameras will also be near-visible NIR

if there's still a faintly visible red glow, it's probably lower, closer to 850nm(verify)

if specifically trying to avoid that (e.g. around animals), it may be closer to 940nm(verify)

• more professional night-vision also tries to amplify what little is there -- see terms like residual light amplifiers and image intensifiers)

these seem to prefer somewhat higher-frequency NIR(verify)

and may add much higher-frequency IR to add heat signatures - but that's a fairly fancy feature

• passive infrared motion detectors (because they focus on moving body heat) - have broad sensitivity but the peak is in mid-infrared

• active infrared motion detectors are typically based on NIR emission (800..950nm(verify))

• Electric terrace heating (almost-incandescent ceramic elements) and space heaters are also broad

but mainly in the mid-infrared range

most things we call 'infrared heat' are around here(verify)

• panel heating is often more to the far-IR end(verify)

but it is easily overstated how different they are

more practically, most things we use for heating are broad-spectrum, so some degree of all of this. We can shift the peak around a bit to do more of one than the other.

more theoretically, almost all of the EM spectrum can warm you up when there is enough of it, it's just that it's not easy or common to get something to heat you up that is narrowband and powerful (perhaps a microwave transmitter)...

more pratically again: ...because most things that heat you up easily are broader, like fire, or the sun (bigger fire)

-->

• Infrared from space mostly gets blocked by the atmosphere

so most sources of it are made by us, or fire (which is hopefully usually made by us)

More on the IR splits

For example astronomers have some further reasons to distinguish them.

Optical astronomy cares more about NIR(verify), but there are specific interests in the other regions.

One common seems to be the near/mid/far split (near/far relative to the visible spectrum, 380nm..740nm), like:

• Near-Infrared
  • Wavelength approx 740nm to approx 2500nm
  • IR LEDs are usually near-IR, typically somewhere within 700...1000nm

• Mid-Infrared
  • 2500–25000nm

• Far-Infrared
  • 25000–1000000nm(verify), getting close to microwave region

See also:

• http://www.ipac.caltech.edu/outreach/Edu/Regions/irregions.html
• http://en.wikipedia.org/wiki/Infrared

Infrared and cameras

See Photography_notes#Infrared

Communication

Two-directional communication is typically half-duplex because a device can easily be blinded or confused by its own signal.

Consumer IR (TV remotes and such)

• Often uses a intermittent patterns of pulses, not continuous sending
  • Pulsing with low duty cycle also means you can pulse the LEDs with more current without destroying them.
  • sending and a specific speed of pulsing is something that won't occur naturally, so helps confusion from environment IR.
    • Carrier usually 38kHz. More generally it's somewhere in 33..40kHz or 50..60kHz, often 38kHz, 40kHz, or 36kHz
• In the case of remotes there are hundreds of variant protocols (that is, bit patterns that are specific to brands and devices)
  • Universal remotes usually have a lookup table from brand-and-model to one of hundreds specific code sets that the remote supports
  • and occasionally the ability to learn codes from an example

http://en.wikipedia.org/wiki/Consumer_IR

IrDA

• Speed: 2.4 kbit/s to 1 Gbit/s (faster speeds primarily at close range)
• Modulation: baseband, no carrier
• Has a few different layers

http://en.wikipedia.org/wiki/Infrared_Data_Association