Everyday microwave oven physics: Does the energy transferred depend on what is inside?

I've got no real physics education but I am interested in it from a popular science and layperson level. If anyone is able to help with this thought experiment.

My question is, on a given microwave power setting and a fixed duration, is the heat transferred to the contents largely constant regardless of what is inside?

E.g. if I want to heat milk a mug so the milk is warm, are the two the roughly the same:

1. Pour cold milk into a mug until mug is full. Microwave set to 900W and 1 minute duration.
2. Pour cold milk into a mug until mug is half full. Microwave set to 900W and 1 minute duration. Top up mug with cold milk until full.

Will both scenarios result in broadly the same temperature?

Likewise, if a full mug of milk takes 1 minute to warm at 900W setting. If I then want two full mugs of milk, what will be more efficient:
1. Putting both mugs in the microwave at the same time, run at 900W and for 2 minutes duration or
1. Put each mug in one after the other, each mug being heated at 900W and for 1 minute each?

The above thought experiment is for nothing other than to conclude a friendly family debate :-D

The best way to check this is to actually experiment. I have not done the experiment so I don't actually know the answer.

My hypothesis would be that for your first experiment they would not be roughly the same. I would guess that 2 will be substantially colder than 1.

This reminds me of the "tea or milk first" debate. Taking into account evaporative cooling, I would hypothesize that in both scenarios above, option 2 will lead to colder milk.

physicsnoobie79 said:

TL;DR Summary: At a specific microwave power setting and duration, is the heat energy transferred to the contents relatively constant?

My question is, on a given microwave power setting and a fixed duration, is the heat transferred to the contents largely constant regardless of what is inside?

This is a very difficult question, I think. I trawled around for some help and I found this link which I briefly looked at and which would be worth a look. But most of the hits I got from my search seemed more interested in the various modes that the waves have within the cavity than in the actual heating process for the food put in it.

In any system which involves a radio frequency amplifier / source and a load, it's usual to need to know the Impedance of the load in order to know how much power will be transferred to it and how much will be reflected and wasted. A microwave oven uses a magnetron which is a fairly robust device (actually, a pretty magic invention aamof) which can 'handle' a range of load impedances (jugs of water / lumps of meat etc., all of different sizes). But a nominal 900W setting will not necessarily mean that the 900W is getting into the water.
The efficiency factor can vary a lot. This link claims to have values but I honestly suspect the figures may not be too reliable and comparing one appliance with another would need lots of different cooking loads and types. They suggest values 70% up to the high 90's%.
I agree with @Dale who suggests that you could very easily do some experiments with various masses of water in glass jugs. Avoid unspecified crockery which may absorb much of the microwave energy and mess up your results.

Best to operate well below 100C, for good practical reasons and measure the temperature as soon as you can, after removing the jug. (don't leave it inside during cooking!)

sophiecentaur said:

and measure the temperature as soon as you can

After thorough stirring! (Otherwise the temperature of the water will not be uniform and the reading meaningless.)

DrClaude said:

After thorough stirring! (Otherwise the temperature of the water will not be uniform and the reading meaningless.)

Good call! On that topic, I often wonder why they don’t offer a magnetic stirrer (chemistry lab style) in microwave ovens.

I’m sure it would be possible to suppress induced currents in the puck with suitable slots(?).

Just think; perfect sauces with no stop-stir-stop-stir.

sophiecentaur said:

Good call! On that topic, I often wonder why they don’t offer a magnetic stirrer (chemistry lab style) in microwave ovens.

I’m sure it would be possible to suppress induced currents in the puck with suitable slots(?).

Just think; perfect sauces with no stop-stir-stop-stir.

Even easier would be to have it in electric stove tops. I have been wondering about this absence for a long time...

DrClaude said:

Even easier would be to have it in electric stove tops. I have been wondering about this absence for a long time...

Good idea for some applications; I had a Dowe Corning glass casserole for years . Unfortunately, the pans needed for induction hobs are iron content and I would never willingly leave induction cooking in favour of auto stirring.

The 'old' microwave ovens with turntables could work with a paddle suspended from the roof of the cavity. 'Height adjustment' problems in a deep bowl could require a fancy fixture, though

sophiecentaur said:

'Height adjustment' problems in a deep bowl could require a fancy fixture, though

An elbow joint type mechanism ?

If we consider the magnetron as a generator and the coffee cup as its load, then for max power transfer the resistance of the two will be equal. If we now reduce the load resistance to half by adding a second cup of coffee, the total power transferred will drop only slightly. This may be seen if the generator resistance is considered to form a potentiometer with the load resistance. For one cup, Rg=Rl and voltage is one half, but for two cups the voltage is 0.5 Rl/(0.5 Rl + Rg) = 0.5 / (0.5 + 1) = 1/3. The power delivered to one cup is then V^2/R = 1/(4 Rl) and to two cups is 2/(9 Rl). so the comparison is 0.25 in the first case and 0.22 in the second.
From this I would conclude that two cups will take approximately twice as long as one. In other words, the total energy being supplied is not excessively sensitive to the actual load.

tech99 said:

for max power transfer the resistance of the two will be equal

I couldn't disagree with that in principle but there is a significant SWR (evidence in the locally melting ice cream experiments) so affairs it's anyone's guess. The power delivered to a mismatched load on the end of transmission line with a high VSWR will be very dependent on the line length / frequency. I imagine that the dimensions of the cavity would have been chosen for the best performance overall, after a number of tests. (They must be sick of baked potatoes and ready meals.)

The way forward is to do a number of experiments and careful adherence to the above ideas - stirring and quick measurements. It would be good if the OP could get back to us with a brief report of results. ;-)

physicsnoobie79 said:

E.g. if I want to heat milk a mug so the milk is warm, are the two the roughly the same:

1. Pour cold milk into a mug until mug is full. Microwave set to 900W and 1 minute duration.
2. Pour cold milk into a mug until mug is half full. Microwave set to 900W and 1 minute duration. Top up mug with cold milk until full.

Will both scenarios result in broadly the same temperature?

No it won't be the same ( read below ) .

physicsnoobie79 said:

Likewise, if a full mug of milk takes 1 minute to warm at 900W setting. If I then want two full mugs of milk, what will be more efficient:
1. Putting both mugs in the microwave at the same time, run at 900W and for 2 minutes duration or
1. Put each mug in one after the other, each mug being heated at 900W and for 1 minute each?

The above thought experiment is for nothing other than to conclude a friendly family debate :-D

Hello there . That's an interesting question . In my opinion :
What's important to realise in first place is that there's no heat transferred to food ( you really don't have any heat flowing ). I get what you mean , but it's important to realise why it's wrong to talk about heat transfer , because this insight will help understand the answer to your question better .

What the microwave oven does is to fill it's inner space with standing EM waves ( the oven chamber acts like an EM cavity ). When you put food inside this cavity you basically force food's molecules to vibrate due to the oscillating EM field inside the cavity . The vibration of food's molecules results in heat production .

An important detail here is that not all molecules can actually vibrate . At least not as much as others . The molecules should pocess a property called dipole moment . In very simple terms there should be separation of a positive and a negative charge ( could also be induced to some degree by the field itself if not inherent to the structure of the molecule )* .

So yes , the content of the microwave oven definitely affect whether the object will be heated and if energy is going to be drained from the cavity . Therefore...

If a full mug of milk takes 1 minute to warm at 900W , then you should place both mugs in the oven for just 1 minute . The energy is there , in the cavity . Placing a second mug of milk won't affect the heating of the other mug . You are just draining more energy from the EM field which is already stored in the cavity .

*Conductive materials can also heat in the oven . Extremely fast to be honest , so you have to be extremely careful and never , ever put something conductive in a microwave oven like for example aluminum foil .

It is not necessary to have a resonance in the box for a magnetron to supply energy to food. When you switch on, the resonant energy builds up to a maximum over many cycles. When you switch off, the box then supplies energy to the load and to the magnetron over many cycles. When the magnetron supplies energy to food, a travelling wave goes from the magnetron to the food. Resonance can be useful to provide transformer action but is not fundamental to microwave heating.

sophiecentaur said:

...I often wonder why they don’t offer a magnetic stirrer (chemistry lab style) in microwave ovens.

Things in microwaves are generally - and imminently - headed for someone's mouth, not a beaker.

In order to do its job, an impellor feature would have to be engineered into the stove. i.e. not just a gadget you buy at Bed, Bath and Beyond.

And that would mean the stove company would have to take responsibility for every person (pro-cook, absnt-minded senior or naive child) who forgot to take the impeller out of their food and accidentally broke a tooth - or worse. Much worse.

I can see why such a feature would not get off the drawing table.

DaveC426913 said:

responsibility

That was my first thought as well. The times we live in...

DaveC426913 said:

Bed, Bath and Beyond.

This would be "Beyond" right?

So microwaves has a rotating turntable to even out the exposure. Not exactly a stirrer, but how do you stir piza anyway?

Vanadium 50 said:

So microwaves has a rotating turntable to even out the exposure. Not exactly a stirrer, but how do you stir piza anyway?

The very idea of a pizza in a microwave gave me a problem. Isn't pizza all about a crust, for crust's sale?

Avaro667 said:

never , ever put something conductive in a microwave oven like for example aluminum foil

Not on its own, Nor touching the sidewalls.

Thin aluminium pie plates of metal with food in them are OK, as evident by the sale of such products as being microwave ready. I cook my fish sticks on a pie plate and it does not come out 'red' hot.

sophiecentaur said:

The very idea of a pizza in a microwave gave me a problem. Isn't pizza all about a crust, for crust's sale?

A friend showed me this: to reheat leftover pizza, just put the slice in a large skillet on the stove. Medium low, and I like to cover it for the first few minutes. The crust comes out nice and crispy, not that flubbery microwaved bread yuck-o.

http://www.sfu.ca/phys/346/121/resources/physics_of_microwave_ovens.pdf

Looks like my potatoes take longer to cook than my carrots. ( All things being equal, no doubt difficult to keep in line for any microwave experiment at home )

256bits said:

Not on its own, Nor touching the sidewalls.

Thin aluminium pie plates of metal with food in them are OK, as evident by the sale of such products as being microwave ready. I cook my fish sticks on a pie plate and it does not come out 'red' hot

Once i made the mistake to put inside the microwave oven food wrapped up in aluminum foil. It didn't touch the walls . I literally saw plasma inside the oven and the food obviously caught fire . About the aluminium pie plates now...i'm not sure why they don't catch fire . I guess it has to do with the thickness and maybe they have also made changes to the material ?

Avaro667 said:

food wrapped up in aluminum foil.

You can't beat one of those big globe (real) filament bulbs. Firework night but only for a few seconds as the bulb gets dangerously hot.

Not physics, but last week I did what I should have done years ago: I took the back off of our microwave, located the piezo, and twisted it until it tore loose from the controller board. No more beep beep beep!

Avaro667 said:

Once i made the mistake to put inside the microwave oven food wrapped up in aluminum foil. It didn't touch the walls . I literally saw plasma inside the oven and the food obviously caught fire . About the aluminium pie plates now...i'm not sure why they don't catch fire . I guess it has to do with the thickness and maybe they have also made changes to the material ?

Thats kinda funny. Maybe not.
Must of scared the crap out of you. It would me!
Wrapped in foil - no way for the microwaves to get into the food. They reflect off of instead.
The pie plate has an open top, so waves can attack the food.
There is some induced current in the pie plate, but nothing I have noticed as being overly hot, actually more like as warm as the food.
They also have "browners", if that is what they are called, which has a metal surface that heats up which, if in contact with the food, is supposed to give it the baked in the stove look.

sophiecentaur said:

You can't beat one of those big globe (real) filament bulbs.

I think I have some old-style flashbulbs around here. I wonder what will happen when....

Vanadium 50 said:

I think I have some old-style flashbulbs around here. I wonder what will happen when....

zirconium fire bad juju

Vanadium 50 said:

I think I have some old-style flashbulbs around here. I wonder what will happen when....

https://encyclopediadramatica.online/Oh_noes

sophiecentaur said:

You can't beat one of those big globe (real) filament bulbs. Firework night but only for a few seconds as the bulb gets dangerously hot.

You just reminded me of this :

256bits said:

Thats kinda funny. Maybe not.
Must of scared the crap out of you. It would me!
Wrapped in foil - no way for the microwaves to get into the food. They reflect off of instead.
The pie plate has an open top, so waves can attack the food.

The really funny thing is that happend during my internship which was also RF/microwaves related . So the embarrassment was double . I could be an excellent example of how to get fired in no time , lmao . But yeah , i was lucky because i realised it almost immediately . I can't imagine what could have happened if i had just left , continuing whatever i was doing at that time . It took no more than 5-10 secs to catch fire . The smells were awful btw . The burger was okay though , after removing the burnt parts of course . And luckily i didn't die .

256bits said:

There is some induced current in the pie plate, but nothing I have noticed as being overly hot, actually more like as warm as the food.

That's some useful piece of info , thanks . Next time i will definitely try someting like a pie plate . Doesn't it block some of the EM power though ?

256bits said:

They also have "browners", if that is what they are called, which has a metal surface that heats up which, if in contact with the food, is supposed to give it the baked in the stove look.

That's pretty cool actually . Is there any difference to the taste of the food or is it just an aesthetic emulation of the stove look ?

Avaro667 said:

Once i made the mistake to put inside the microwave oven food wrapped up in aluminum foil. It didn't touch the walls . I literally saw plasma inside the oven and the food obviously caught fire .

Yeah. I had my back to my microwave, saw the whole room light up with the flash.

Avaro667 said:

About the aluminium pie plates now...i'm not sure why they don't catch fire . I guess it has to do with the thickness and maybe they have also made changes to the material ?

I think it as a lot to the do with the shape of the material. Pinch points (such as corners and creases) are prone to concentration of current, which is often where the flashes occur. A large surface with no corners may be less prone to having radiation focus on a small area.

I think.

Avaro667 said:

That's pretty cool actually . Is there any difference to the taste of the food or is it just an aesthetic emulation of the stove look ?

Yes, it's still crunchy, a little. You're converting the radiation to heat, and drying out the surface of the food at particular points, which effectively toasts it. A little.

DaveC426913 said:

Yeah. I had my back to my microwave, saw the whole room light up with the flash.

A small trinity test , lmao .

DaveC426913 said:

I think it as a lot to the do with the shape of the material. Pinch points (such as corners and creases) are prone to concentration of current, which is often where the flashes occur. A large surface with no corners may be less prone to having radiation focus on a small area.

Oh...i believe you must be absolutely right . It didn't cross my mind but it's true , aluminum foil is full of crests , creases and corners . Large curvature means larger local electric field strength , which inevitably leads to the breakdown of air , and hence the birth of an arc/plasma .

An experiment would be interesting , and we could verify it , but on the other hand i don't want to start a fire again . I will trust theory on this one .

DaveC426913 said:

Yes, it's still crunchy, a little. You're converting the radiation to heat, and drying out the surface of the food at particular points, which effectively toasts it. A little.

It's a very smart trick . As long as it doesn't burn the food . But that can happen even in a regular oven if don't know what you are doing .

Sometimes a jar of peanut butter, kept in the refrigerator, will be too hard to spread. The 'cure' of course is to heat it - in the microwave.

This often works well -- unless the is a scrap of that heat-bonded foil seal left on the edge of the jar. The jar rim is about 2mm (0.08in.) thick.

If it is just a speck of foil, it generates a minor plasma explosion and all is well.

Around 12mm (0.5in) long, there is a continuous plasma discharge as the edge of the plastic jar decomposes (Polyethylene Terephthalate), leaving behind the Carbon, which of course is conductive. Conductive enough to sustain continued burning of the plastic!

The audible indications are an initial 'Pop' followed by a loud 'Hum' as the power transformer in the oven is overloaded by the short circuit in the plasma cloud.

I have never let the process go to completion as I don't want to replace the oven, it also takes some effort to remove even 1 seconds worth of the burned plastic stench from the oven interior.

Now I usually remember to check for left-behind foil before inserting in microwave!

Cheers,
Tom

p.s. IIRC, the heating effect for non-conductive materials is proportional to their di-electric constant. Water, falling around 78-80, is usually what heats the most.

^^^ Yep. That's one of the reasons I stopped buying TJs peanut butter and started filling up an empty jar at the bulk bin of the local co-op. It was frustratingly difficult sometimes to remove that plasma inducing metallic residue from the rim. I used to scrape it half-effectively with a butter knife and then end up with tiny bits of foil in the peanut butter.

The other reason is that the bulk peanut butter from the co-op tastes a *lot* better than the stuff TJs sells.

JT Smith said:

the bulk peanut butter from the co-op tastes a *lot* better than the stuff TJs sells.

Thats thanks to the ... anonymous supplements .... dust, grime, bugs, hand juice ...

Why would Trader Joes peanut butter have all that stuff in it? :-)

Seriously, the bulk stuff is clearly a higher quality product. It doesn't even have salt in it which I was certain I had to have in my peanut butter in order to like it. I was quite surprised when I learned after buying and eating it that didn't have any. I guess I don't need salt in my peanut butter after all -- at least if it's good peanut butter.

I could go around the corner to shelves in the store and buy the exact same product from the same producer already in a jar for only slightly more money. I know I'm not saving the world by reusing a jar. I just like making a big sticky mess in the store.