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Physicists to the fore, please!
I haven't used a microwave oven for over 20 years, but in light of a comment made on the eggs thread about the source of heat when cooking eggs not mattering, I was wondering if it actually does.
I remember when we did have a microwave using it to heat water in a mug, and finding the mug too hot to touch when it was done, but also that the water seemed to cool down very quickly.
It was the same with other foodstuffs if I remember correctly. We used to call it 'microwave heat' to distinguish it from something properly cooked.
Was it just my imagination? Or is heat from a microwave oven different somehow to heat generated a different way?
If something is 60degrees its 60degrees.
How fast it cools is to do with its mass and the ambient air temperature it will cool down and the room will warm up until equilibrium is reached.
The biggest question here is why the chuff were you heating water in a microwave.
Was it just my imagination? Or is heat from a microwave oven different somehow to heat generated a different way?
Heat is heat. But there may be things you observed that could explain it... within a microwave oven you get hotspots (its why the food rotates in conventional microwaves). If you pop a large bar of chocolate in a m/wave with the rotating plate removed you can actually measure the wavelength of the m/waves because it melts the chocolate in spots and the rest stays cool. Obviously if you leave it the heat then travels from the hot to the cold parts and it all reaches some equilibrium point. The same will be true, but potentially less obvious, for anything you bring out the M/Wave. Its one of the reasons the instructions for ping food usually say to stir and or stand - you are letting the heat "even out". Another factor can be the vessel you heat it in. When you heat something on the stove the vessel becomes hot first then the contents, take it off the heat and the vessel is still hot. In the microwave, good M/Wave friendly crockery is "transparent" to m/waves so doesn't get heated directly by them. As a result when you turn the m/wave off the heat travels from the contents into the container, which depending on its "thermal mass" can be quite a significant heat loss.
Heat is heat. But there may be things you observed that could explain it… within a microwave oven you get hotspots (its why the food rotates in conventional microwaves)... It's one of the reasons the instructions for ping food usually say to stir and or stand – you are letting the heat “even out”. Another factor can be the vessel you heat it in. When you heat something on the stove the vessel becomes hot first then the contents, take it off the heat and the vessel is still hot. In the microwave, good M/Wave friendly crockery is “transparent” to m/waves so doesn’t get heated directly by them. As a result when you turn the m/wave off the heat travels from the contents into the container, which depending on its “thermal mass” can be quite a significant heat loss.
This makes sense. Thank you!
M/Wave friendly crockery is “transparent” to m/waves
At the same time, this is sort of why I asked the question. I mean, if heat is heat - or, as @joshvegas put it, sixty deg. is sixty deg. - then shouldn't all crockery be subject to it in the same way? Obviously it isn't, but to a non-physicist, it did make me wonder what the difference was.
then shouldn’t all crockery be subject to it in the same way?
Well no, because microwaves heat things differently than gas (just like infra red patio heaters keep you warm in a different way than a log burning chiminea does).
just like infra red patio heaters keep you warm in a different way than a log burning chiminea does
Um... infra red patio heaters and chimineas both warm you primarily by infra red radiation.
A microwave warms food by making water molecules vibrate. The water molecules will pass on this energy (vibration) to the other molecules in the food.
Was it just my imagination? Or is heat from a microwave oven different somehow to heat generated a different way?
Heat is heat, but the transmission of energy to the food being heated is different. Microwaves dont transmit heat through the air between the food and the magnetron they fire out electromagnetic waves which turn into heat energy when they reach the food.
Ovens on the other hand create connective currents of hot air within the oven itself which warms the food.
So yes, it's different in the sense that a microwave oven doesnt generate any heat, it just emits waves of energy that happen to turn into heat energy when certain materials are placed inside it. I think the mechanism is called dielectric heating?
Um… infra red patio heaters and chimineas both warm you primarily by infra red radiation.
But a chiminea heats you via both radiated and convected heat surely?
But a chiminea heats you via both radiated and convected heat surely?
How can a fire outside heat you my convection, unless you were sat right on top of it.
Convection works indoors as there's nowhere for the hot air to go. Outdoors the chimenea (or any fire) would actually be cooling you by convection as it draws cool air in.
A microwave warms food by making water molecules vibrate. The water molecules will pass on this energy (vibration) to the other molecules in the food.
Strictly speaking it's not water, it's any polar molecule (or functional group).
Also......
Earthenware ceramics with non negligible water contents will heat up, whereas stoneware and porcelein which is fired at higher temps won't.
Glass is full of impurities (most importantly metals and metal ions) which also absorb microwaves.
Plastics in general won't absorb microwaves, but will melt. Especially as microwaves are prone to superheating liquids. You can get water significantly above it's boiling point as there's no nucleus to form bubbles arround, which is why you should never microwave water (or reheat your tea). It's an unfortunately common injury to end up in hospital with when your cup subsequently does boil (a splash trapping a bubble of air, or younstirring it with a spoon) as soon as you take it out and move it to your face!
If you pop a large bar of chocolate in a m/wave with the rotating plate removed you can actually measure the wavelength of the m/waves because it melts the chocolate in spots and the rest stays cool.
Science for the kids:
then shouldn’t all crockery be subject to it in the same way? Obviously it isn’t, but to a non-physicist, it did make me wonder what the difference was.
presumably you are familiar with the fact you shouldn't put metal in a microwave? so clearly a metal tankard and a china mug don't behave the same way. The materials they are made from interact with the microwave energy (in the same way that different coloured objects interact with different wavelengths of light, different materials interact with different parts of the electromagnetic spectrum). But then different materials then also have a propensity to heat up/store energy (specific heat capacity), to radiate it, and to conduct it - so even although some materials may be at the same temperature they may feel at different temperatures.
However, if I took a mug of water and put in the microwave, and at the same time I take an identical mug of water and put a small kettle element in it and heat them both the to same temperature at the same rate - you'll have no way to tell those two mugs apart. The source of the heat is irrelevant (but the fact the water heats before the mug, might mean you can observe effects differently from heating the mug on top of the STW obligatory woodburner).
You can get water significantly above it’s boiling point as there’s no nucleus to form bubbles arround, which is why you should never microwave water (or reheat your tea). It’s an unfortunately common injury to end up in hospital with when your cup subsequently does boil (a splash trapping a bubble of air, or younstirring it with a spoon) as soon as you take it out and move it to your face!
You see, I'd say that's why you should be careful and stir a hot drink from the microwave before consuming it, rather than never microwave water. If you've heard it that much it will be too hot to drink anyway...
HEAT (energy) is not the same as temperature.
I once microwaved a jar of nutella, a bad idea made even worse by the remnants of metal foil left around the rim.
How can a fire outside heat you my convection, unless you were sat right on top of it.
Because heat from the fire warms the air around it and not just directly above it.
Unless they’re in space/vacuum, radiators, despite their name work by convection, not radiation.
Thank you Daffy - that had always been my understanding too. Admittedly physics was never my strongest subject at school but sometimes I accidentally listened.
As far as cooking goes, microwaves are mostly lousy because they just heat the water in the food, so it tends to turn soggy. In an oven or frying pan, you can brown the food at high temperatures, which gives it the delicious flavour. That's through radiation (in an oven or grill) or conduction (in a frying pan). You can't do that in a microwave. If you put some bread in a grill, you make toast. If you put it into a microwave, you make soggy bread.
Because heat from the fire warms the air around it and not just directly above it.
Unless they’re in space/vacuum, radiators, despite their name work by convection, not radiation.
Yes because there is a ceiling! In the absence of anything to stop the hot air rising (as is normally the case outside) the chimnea will be ineffective at creating convection. It will lose heat by convection but that heat will not make it to a person to warm them up (some very small amount may due to turbulence in the air but its just as likely that the rising air and turbulence draw in cold air).
I went through a phase of warming plates in my microwave (when I was having food that didn't require the gas oven being on). Can confirm 2 minutes is enough to cause an earthenware one to crack in half :p
Because heat from the fire warms the air around it and not just directly above it.
Unless they’re in space/vacuum, radiators, despite their name work by convection, not radiation.
As poly said, that only works indoors as the warmed air rises, is trapped by the ceiling, then replaced by warmer air untill eventually the habitable bits of the room are at the correct temperature.
Outdoors that column of hot air just shoots directly upwards and cold air is drawn in from the surroundings to replace it.
You're probably right that with the lid off (or an open fire) that a lot of the heat is lost by convection, but that doesn't do anything to heat the people stood arround it.
But temperature is a measure of the average kinetic energy of the substance.
A microwave below the freezing point of water won't heat/melt ice. As the polar bonds (hydrogen bonds) holding the crystalline water in place vibrate at a different frequency.
as above, most mugs will have water in the ceramic, even if they're glazed, they'll absorb moisture from the air and water from washing through the ring it sits on on the bottom that isn't glazed.
The small amount of moisture that's in the part of the mug around the rim and in the handle will be heated by the microwaves and, without the heat sink of the contents to take that heat away, will get much hotter, much quicker than the contents and the parts touching the contents.
The atoms in everything are jiggling about all the time. The more they jiggle, the hotter it is. That's it - there is no other kind of heat. You can get the atoms to jiggle in various ways - either by putting them next to lots of other jiggling atoms (i.e. the ones in the hotplate on your hob or the ones in the hot air in your oven), or by using microwaves to make them jiggle, or even by simply hitting them to jiggle them about. Things heat up when you hit them, a bit. You can cook chicken by slapping it:
A microwave below the freezing point of water
A what now?
A microwave below the freezing point of water won’t heat/melt ice. As the polar bonds (hydrogen bonds) holding the crystalline water in place vibrate at a different frequency.
I'll try this and see what happens. Thing with ice is that the energy of the molecules varies. Sublimation occurs when molecules get enough energy to break away from the ice crystal structure. If molecules can sublime, then surely a microwave can heat them as they do and they will tend to transfer heat back into the ice, so you will get a very thin surface layer of water. That will keep absorbing energy until the ice melts. That's my theory. I'll see how it works.
Well, I put a cupful of icecubes into a Pyrex dish and microwaved them. Now I have a dishful of water.
That slapping chicken video was great, and amusing. thanks for sharing
The biggest question here is why the chuff were you heating water in a microwave.
Isn't it the *most* efficient* way to heat water - since you're heating exactly the amount you need and no more. Rather than boiling a kettle bring many cups of water to boiling point pouring just one wasting all the energy used to heat the rest as it cools back to room temperature*.
*IIRC microwaves waste produce very little waste heat. though I don't know what happens to the 'energy' that isn't transferred to the water. Do they bounce around until they hit something? heat parts of the oven structure?
*of course the cooling waste water does heat your home/room but weather that's a benefit or issue depends on the season
Aye it's the water that's the issue. But put the microwave oven in the freezer, let it get down to temperature, then try it (making sure everything used is just as cold).
I really hope I'm right. I use the image to teach IR spectroscopy to AH pupils. Actually wonder if ice has different IR absorbance than water.
Edit: apparently not OH stretch still appears where you'd expect in both ice and water, so the lattice must move.
https://iopscience.iop.org/article/10.1088/0067-0049/184/2/361
Isn’t it the *most* efficient* way to heat water – since you’re heating exactly the amount you need and no more.
Probably not. You have to power the electronics, the turntable, and the magnetron, which won't be 100% efficient. A very quick google suggests it converts about two thirds of its input electricity into microwave energy.
A kettle on the other hand is about 98% efficient, because it's nothing but a wire with resistance, and all resistance does is convert electrical energy into heat. The 2% or so is from heat escaping from the side of the kettle before it's boiled.
Regarding boiling many cups - every kettle I've had since about 2000 has had a flat base so you can boil only one cup if you want. They used to advertise them as 'one-cup' kettles but they don't even bother any more.
Because heat from the fire warms the air around it and not just directly above it.
Unless they’re in space/vacuum, radiators, despite their name work by convection, not radiation.
I think you're getting convection and conduction mixed up daffy.
A kettle on the other hand is about 98% efficient, because it’s nothing but a wire with resistance, and all resistance does is convert electrical energy into heat. The 2% or so is from heat escaping from the side of the kettle before it’s boiled.
No way is heating one mug of water in a kettle 98% efficient. You are hearting the kettle and it's element which takes energy and is of no benefit.
"Regarding boiling many cups – every kettle I’ve had since about 2000 has had a flat base so you can boil only one cup if you want. They used to advertise them as ‘one-cup’ kettles but they don’t even bother any more."
Do you measure the volume of water precisely? Or do you actually boil 1.5 cups worth...
Aye it’s the water that’s the issue. But put the microwave oven in the freezer, let it get down to temperature, then try it (making sure everything used is just as cold).
I really hope I’m right. I use the image to teach IR spectroscopy to AH pupils. Actually wonder if ice has different IR absorbance than water.
Edit: apparently not OH stretch still appears where you’d expect in both ice and water, so the lattice must move.
With microwaves you aren't directly vibrating/stretching the OH bond; the whole molecule rotates within the microwave field so its local diploe aligns the field - thus increasing the kinetic energy of the molecules. Within ice you have very limited ability to rotate so much harder to make them move, but the bonds can stretch.
If you put some bread in a grill, you make toast. If you put it into a microwave, you make soggy bread.
That however is a good way to bring some slightly stale bread back to a usable state.
To complicate matters, heat is not heat. Infrared is split into three main wavelengths - IR-A, IR-B and IR-C. IR-A is near visible and is what the sun predominantly gives off (as do incandescent lightbulbs) and it is this heat that 'warms your bones'. It is why sunbathing is more pleasant (for most people) than just sitting in a warm room (together with the psychological effect of being in the sun).
It is also why reptiles bask in sunlight to get nice and warmed up for the day. IR-A penetrates muscle and flesh much better than IR-B and IR-C does.
IR-B sits somewhere in between the two and it is what bar heaters and other 'low light' heating solutions emit. It is why when you get close to one it feels like it is singeing the top layer of flesh but is quite effective at heating air mass as well.
IR-C comes from non-visible IR and comes things like radiators. It is not very penetrative but is great at heating air mass hence why your CH makes the room nice and warm but you do not feel 'warmed tot he bone' like you do in sunlight. IR-C is also what is stored and radiated back from land mass etc.
just saw this randomly and thought interesting
A typical microwave oven uses more electricity keeping its digital clock on standby than it does heating food
and because everything on here must be referenced nowadays, or it's dismissed as lies....
Source: QI Elves
A typical microwave oven uses more electricity keeping its digital clock on standby than it does heating food
Over time, I assume.
You can turn the standby clock off on mine.
heat is not heat
Heat is heat. What you're talking about is EM radiation which is not all the same, no, of course.
Do you measure the volume of water precisely? Or do you actually boil 1.5 cups worth…
Not precisely, but I fill a cup with water and pour it in the kettle.
No way is heating one mug of water in a kettle 98% efficient. You are hearting the kettle and it’s element which takes energy and is of no benefit.
It's an easy calculation. Put a litre of water in at a known temperature, time how long it takes to boil then multiply that by the specific heat of water and get the number of Joules that went into the water. Then multiply 2.2kW by the number of seconds it took and compare the results. Try it - I haven't, but it's something my Dad used to teach so I took his word for it.
The specific heat of water is unusually huge, and the specific heat of the kettle walls and element is pretty low by comparison. But that's why it's not 100%.
EDIT there is some variability in the Google results for this, presumably depending on kettle but also perhaps the amount of water involved.
[As an aside: this thread has turned out WAY better than I expected! I don't always understand physics (and the other sciences), but I LOVE the fact that they exist, and can lead to such discussions. Now, as you were.]
20 years ago I designed a low cost switchable impedance matching network for a domestic radio frequency oven that could deal with typical domestic loads. It would switch components in and out depending how much energy was being absorbed. You could cook a chicken in half the time with no preheating necessary as the radio frequency waves penetrated instantly much deeper than microwaves. There was a conventional element too to brown the surface.
Similar RF ovens for fixed loads are used to cook biscuits in factories.
Miele have put one out, it's a bit pricey yet but they are the future. Far more energy efficient and faster than conventional cooking or microwaves.
Softening butter is a tricky process. It goes from solid to liquid within one second.
It’s an easy calculation. Put a litre of water in at a known temperature, time how long it takes to boil then multiply that by the specific heat of water and get the number of Joules that went into the water.
you also have to multiply it by (100 - whatever the starting temp was).
The specific heat of water is unusually huge
4.2 kJ/kgK if anyone wants to do the Maths.
E=mc∆T
Energy required to heat = mass of water in kg x specific heat capacity of water (4.18kjkg-1°C-1) x change in temp of the water
I didn't know about the difference between water and ice. I've found by experience that the best way to thaw frozen food in a microwave is in short bursts; microwave for a minute, leave it for three, zap it again, leave it again, etc. Once it's all thawed, it can be cooked on one go. But I didn't know why - and now I understand. Just straight microwaving means the bits that thaw first continue to pick up more energy, exactly what you don't want, but leaving it to spread the heat and thaw the next bit allows that to absorb the waves better.
Kind of a timely thread as I'm thinking a bit down the line about a microwave boiler.
Has to be the mainstream solution for gas boiler replacement as an easy like-for-like.
but leaving it to spread the heat and thaw the next bit allows that to absorb the waves better.
Well its heat transfer. What you're doing is actually heating up the liquid water on the surface of the ice then that is in turn melting the ice beneath through heat transfer of the hot liquid water to the solid ice. So by leaving it a bit you are preventing superheating the liquid water and allowing the heat to transfer into the ice.
I suspect microwave is far more efficient in heating water. A typical kettle is about 3kw and a typical microwave is around 850w. I would hypothesise that they would heat up the same volume of water in around the same time, so alot more energy wasted in the kettle.
Specific heat ckpacity doesn't really help you here - all that does is tell you how much energy you need to get into the liquid to heat it up, but doesn't tell you anything about the method of getting that energy into the water and how efficient that method is. Microwaves directly impart energy into the water. Kettle heats up elements which in turn uses heat transfer and convection to get the energy into the body of the liquid. That heat also transfers to all the components in the kettle thereby sucking up some of the energy you put in and diverting it from the job of heating up the water and wasting it. Microwaves minimally heat up surrounding environment and components so more energy emitted goes into the water to change its energy state. I would conduct an experiment myself but don't have a kettle (Quooker for the win!).
I’ve found by experience that the best way to thaw frozen food in a microwave is in short bursts
It's not just that - the microwaves only penetrate the first cm or so of food even if it's not frozen. The rest is done by convection just like a normal oven.
I suspect microwave is far more efficient in heating water. A typical kettle is about 3kw and a typical microwave is around 850w. I would hypothesise that they would heat up the same volume of water in around the same time, so alot more energy wasted in the kettle.
No need to hypothesize, it's been done a lot and whilst my report of 98% seems optimistic, every experiment reported that electric jkettles were faster.
but doesn’t tell you anything about the method of getting that energy into the water and how efficient that method is
The point I am making is that because it's a simple resistive load, ALL the energy is lost as heat, and because the element is in the water ALL that heat goes into the water, at least before some of it goes into the surroundings.
The microwave oven itself gets warm, because the process of converting electricity to microwaves isn't 100% efficient of course.
I suspect microwave is far more efficient in heating water. A typical kettle is about 3kw and a typical microwave is around 850w
Resistive electric heaters (the kettle) are 100% efficient at turning electricity into heat. The only losses are any heat that goes somewhere other than the water, so not a lot.
Microwave I don’t know, but I assume 80-90% efficient?
Either way the microwave will be putting nearly 850w into your water, the kettle nearly 3kw into your water. The kettle will win. Neither device will create free energy and neither device will lose much.
For this reason I’m curious about why a microwave domestic hot water boiler is any better than a resistive electric boiler already widely available. Or am I missing something?
From a physics point of view, heat is a thermodynamic concept (the concept of energy moving from one thing to another). This is not exactly in accordance with the everyday language meaning. But as the energy can be transferred via a few mechanisms, I think it is reasonable to say that, whether talking strict physics or more generally, heat can manifest itself in a few different ways.