[CliffordK (OP)]

Take a small cup of water, and a large cup of water (both at the same starting temperature), and put them in the microwave at the same time.

Which will boil first?  By how much?  Why?

[CZARCAR]

gotta be the bigger cause this smells like a trick question?

[CliffordK (OP)]

Not really a trick question.

As we know...  if you put in ½ cup vs a full cup, the smaller cup (½) will boil faster than the full cup.

If you put in a full cup, vs two half cups, one would have the same amount of liquid, just different containers, so one would expect them to all take the same amount of time to boil.

Likewise, if one put in ¼ cup, and 3/4 cup, one still has 1 cup of water, and thus everything boils about the same.

There will be slight differences, which I would attribute to the uneven distribution of microwaves inside the microwave, as well as the ability of the container to act as a heat sink.

[CZARCAR]

Not really a trick question.

As we know...  if you put in ½ cup vs a full cup, the smaller cup (½) will boil faster than the full cup.

If you put in a full cup, vs two half cups, one would have the same amount of liquid, just different containers, so one would expect them to all take the same amount of time to boil.

Likewise, if one put in ¼ cup, and 3/4 cup, one still has 1 cup of water, and thus everything boils about the same.

There will be slight differences, which I would attribute to the uneven distribution of microwaves inside the microwave, as well as the ability of the container to act as a heat sink.

assuming the presence of a rotating base?+phase transition requires much more energy

[daveshorts]

Yes there will be an effect where you can get a single full cup at the centre of the microwave, where the microwaves are normally more intense, where as the 2 cups will be circling around it

[William McCormick]

Yes there will be an effect where you can get a single full cup at the centre of the microwave, where the microwaves are normally more intense, where as the 2 cups will be circling around it

There is also another effect present, if you create microwave voltage through water, the greater the distance, the greater the difference in polarity, can be created, in the water. To the maximum distance, that still increases the voltage potential, across the waters physical distance or volume. Distance can cause a certain difference in the way, the microwaves are utilized. Voltage may be utilized more then any amperage in that scenario. Making the most use of the wattage potential of the microwave output.

I found that by dropping an extension chord, at the connection, at the male and female plugs, in sea water, that you will get tiny little bubbles, forming on the terminals inside the extension chord plugs. Very mild bubbling action. It does not blow the circuit breaker, it does not even seem to be drawing any amperage. Yet when you touch, parts of the boat that are all connected to the (-) terminal of the battery, what we call ground, in a boat, like the rudder, propellers and bow rails. And then also touch the earth ground, the ground from the power company, and copper pipes in your house, you get awesome power, between the two points.

You can also use the vibrating sander, the whole time, and not even know that the chord that you thought you had secured, had fallen into the water. You will get a severe cramping in your chest, because you cannot feel the AC current going from the boat ground back to the earth grounded casing of the sander, until you shut off the vibrating sander. Just something you should know working near salt water.

I already knew this, and it still happened to me. Ha-ha

I have fooled with AC current through salt water, and I could not get an AC electric power company meter to read the draw. Yet it started to boil the water. Just something you may wish to try.

Kill everything in your panel shut off all breakers and just leave one breaker that you are experimenting with, on, and see what happens. I had my own electic meter for the experiments at the time. It may just be too small a draw to read, but then again the meter, read some very small electronic devices that do not make anywhere near that amount of heat.

Before you just turn back on all the breakers, make sure you wait at least 4 minutes, to let any refrigeration equipment, equalize pressure between the high and low side of the system. Some older systems and older thermostats do not automatically do this. Most refrigerators will also have a problem if you disconnect power to them while they are running, and then try to turn them, right back on again. That is often why equipment fails in wind storms, that short out power cables and force equipment to try to restart, under a load.

                      Sincerely,

                            William McCormick

[bizerl]

Does surface area play a part? ie. If I have a cup of water, will it take longer to boil than the same amount of water poured into a shallow dish?

[damocles]

I got 2 identical polyester beakers with capacity 400 mL to the brim. I put 100 mL of water in cup A and 200 mL of water in cup B. I placed them in my (rotating base) household microwave, centred on a diameter and equidistant from the centre, and started irradiation at full power. Cup A started to bubble at 2 min 10 sec, cup B not until 2 min 45 sec.

There may be all sorts of electromagnetic and geometrical considerations involved, but the answer to Will's original question appears to be "the smaller cup, by about 20% if it contains half the volume of the other".

[bizerl]

I got 2 identical polyester beakers with capacity 400 mL to the brim. I put 100 mL of water in cup A and 200 mL of water in cup B. I placed them in my (rotating base) household microwave, centred on a diameter and equidistant from the centre, and started irradiation at full power. Cup A started to bubble at 2 min 10 sec, cup B not until 2 min 45 sec.

I'd be interested to see if this difference is exaggerated if you heated them separately - ie. only have 100ml of water in the microwave and then compare that to only having 200ml of water.

[damocles]

bizerl,

I thought it important to stick with Will's specifications for my current experiment. I know from past experience with heating cups of tea, soup, porridge, etc. in the microwave that smaller quantities cook quicker, but not quite half the quantity, half the time. In my experience cooking time goes down by about 35-40% for a half quantity.

A lot of the recipes in my microwave cookbook are expressed as "{x} minutes plus {y} minutes per {500} gram", where x is smaller than but of similar size to y.

[William McCormick]

Here is one, that you can try easily. Take a quarter pound stick of butter out of the refrigerator, along with a 1/16th pound or a quarter stick of butter. Nuke each in a 1000 watt microwave for 14 seconds. Note the results. Very fascinating.

It is the surface area in my opinion. The smaller piece has more surface area to volume. So the entire piece of butter still in the wrapper is polarizing equally, because it has a lot of surface area to volume. The larger stick still in the wrapper is not polarizing equally and therefore gets heated better.

I did some experiments today and the small piece did not even melt. While the larger stick had a whole melted core in the center of the butter.

                      Sincerely,

                            William McCormick

[JP]

I got 2 identical polyester beakers with capacity 400 mL to the brim. I put 100 mL of water in cup A and 200 mL of water in cup B. I placed them in my (rotating base) household microwave, centred on a diameter and equidistant from the centre, and started irradiation at full power. Cup A started to bubble at 2 min 10 sec, cup B not until 2 min 45 sec.

There may be all sorts of electromagnetic and geometrical considerations involved, but the answer to Will's original question appears to be "the smaller cup, by about 20% if it contains half the volume of the other".

I wonder how big the geometry effect is.  I've heard that most foods will only absorb microwaves in the first 2-3 cm of their surfaces.  If you put the entire volume of water near the surface (using tall narrow containers such as test tubes, for example), then you'd expect them to boil at more or less the same time (the power absorbed would increase proportionally to volume).