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This question already has an answer here:

I am familiar with the physical explanation as to why microwaves heat up food: the rapidly fluctuating electric field exerts a torque on the dipole of the water molecules resulting in vibration and thus and increase in temperature.

As long as all the food in the microwave is exposed to the microwaves, why should it matter how many items are cooking at once? Is there only a finite region on which the microwaves are concentrated?

Lots of food labels contain different cook times based on the number of items you heat at once.

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marked as duplicate by Jon Custer, Kyle Kanos, Cosmas Zachos, stafusa, sammy gerbil Jul 20 '18 at 22:33

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    $\begingroup$ If you put more water in an electric kettle then it takes longer to boil the water as the kettle has a fixed electrical input and heat output. The same is true for a microwave oven with a fixed microwave power output. $\endgroup$ – Farcher Jul 17 '18 at 0:03
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The microwave field distribution in a microwave oven depends on the contents of the oven, so if there are more items in the oven, the average field amplitude is less (the amplitude depends on the input microwave power and the Q-factor of the cavity, and with more items the losses are higher and Q-factor is less).

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