[chris (OP)]

Naked Scientists podcast listener Ryan has appealed to us for help on this question combining the old tradition of tea making with the much more recent tradition of boiling your water in a microwave!

I like herbal tea, usually when I am sick. I do not currently possess a kettle. You can see where this is going.

When I microwave a mug of water, it takes usually 1:25 to get piping hot. However, when I microwave two mugs of water, and they orbit each other around the rotating plate, it takes about 30-45 seconds longer than double that time to heat them both.

So my question: why does it take longer than double the time to microwave double the water?

I assume it has something to do with one mug blocking the other from microwaves, but that doesn't seem quite right to me. I apologize for my apostasy in neglecting the kettle.

Can you help?

[alancalverd]

You probably put the single mug in the centre of the rotating plate. If you microwave an uncooked poppadom for about 30 seconds you will see that the centre and possibly the edge are cooked but there is an almost raw ring betwen the two, where the microwave amplitude is at a minimum. When you put two mugs of water in the cooker, they span this low-power region so take longer to heat than if you put them singly at the centre.

A commerical microwave with a stirrer rather than a rotating plate, will cook more evenly.

[chris (OP)]

I suppose we should also explain for people who might not know why adding more things to the microwave oven increases the cooking time at all.

The simple answer is that anything you put into the microwave oven that can absorb microwaves will absorb energy, meaning that there is "less to go around" or be shared between all of the things in the oven. This means that the rate of heating - i.e. the rate of energy delivery - per item is lower, so the things cook more slowly.

Superimposed on that is the point that Alan's making, which is that the microwaves are not homogeneously distributed throughout the oven's interior. The wavelength of the waves produced in a household microwave oven are about 12cm. This means that there is an energy maximum and minimum (peaks and troughs of the waves) every 6 centimetres. So if you don't optimally position your food / beverage then it can end up with the majority of the cookable (wet) material in an energy minimum, further retarding the rate of heating.

Meanwhile, for interest, here's a video we made about how a microwave works:

[evan_au]

As the water temperature approaches boiling point, the rate of evaporation increases significantly.
Evaporation carries away the most energetic water molecules from the surface, cooling the remaining water.

With twice as much water in the microwave, you expect the time to boil to double. But the longer time spent near boiling point means that more energy is carried away by evaporation, which means that it takes more than double the time to boil.

I imagine you could test this via an experiment. Something like:

• Mark the points where you would place two cups.
• Place a single cup at one of these points, and measure how long it takes to boil.
• With a cold cup of cold water, place it back at the same spot, but place a saucer over the cup, so that the evaporating water will condense and fall back into the cup. This reduces heat loss via condensation.* Measure how long it takes to boil (I expect it will be less).
• Now repeat the experiment, but with two cups on the marked spots, and see if the time is closer to double with the saucer on top.

*Note: Do not try to seal water in a closed container and put it in the microwave. It could explode in your hands, or explode in the microwave. Your body can't be replaced like a microwave.

[PmbPhy]

Naked Scientists podcast listener Ryan has appealed to us for help on this question combining the old tradition of tea making with the much more recent tradition of boiling your water in a microwave!

I like herbal tea, usually when I am sick. I do not currently possess a kettle. You can see where this is going.

When I microwave a mug of water, it takes usually 1:25 to get piping hot. However, when I microwave two mugs of water, and they orbit each other around the rotating plate, it takes about 30-45 seconds longer than double that time to heat them both.

So my question: why does it take longer than double the time to microwave double the water?

I assume it has something to do with one mug blocking the other from microwaves, but that doesn't seem quite right to me. I apologize for my apostasy in neglecting the kettle.

Can you help?

Each object in a microwave oven attenuates the EM field inside and thus the heating time. If this wasn't so then energy wouldn't be conserved.

[hamdani yusuf]

Maybe you can try keeping the first mug in the center and the second mug right next to it, and see if they boil together?