Everyday microwave physics

  • #1
physicsnoobie79
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At a specific microwave power setting and duration, is the heat energy transferred to the contents relatively constant?
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
 

Answers and Replies

  • #2
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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.
 
  • #3
DrClaude
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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.
 
  • #4
sophiecentaur
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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!)
 
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  • #5
DrClaude
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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.)
 
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  • #6
sophiecentaur
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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.
 
  • #7
DrClaude
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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...
 
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  • #8
sophiecentaur
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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
 
  • #9
pinball1970
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'Height adjustment' problems in a deep bowl could require a fancy fixture, though
An elbow joint type mechanism ?

1669728760472.png
 
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  • #10
tech99
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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.
 
  • #11
sophiecentaur
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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. ;-)
 

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