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upstart writes:

Best Way to Make Tea - Kettle or Microwave?:

Because you're typically heating a liquid from beneath, like setting a kettle over a stovetop, the liquid at the bottom of the container warms up, becomes less dense, and floats to the top. At the same time, the cooler liquid at the top starts to sink closer to the heat source. Eventually, you get a uniform temperature throughout.

But microwaves are an entirely different beast. A magnetron inside the metal box generates microwaves, the kind of electromagnetic radiation that gives the appliance its name. Microwaves fall along the electromagnetic spectrum, just like the ultraviolet light you protect your skin from in the summer, or the X-ray scans your doctor might take.

From there, the magnetron converts electricity from your home into some pretty intense radio waves. A channel called a "wave guide" blasts the radiation into the microwave box to heat your meal or, in this case, water. A turntable rotates the food in question to evenly heat it.

The microwaves bounce all over the reflective metal walls until they ping your food or drink. Then, they blast right through the food, just like radio waves can travel through the walls in your home. This excites the molecules inside the food, causing them to vibrate more quickly, creating heat.

Because the electromagnetic waves are coming from all angles of the microwave unit, rather than just from the bottom, convection doesn't occur at all. Instead, the liquid at the top is much hotter than the liquid at the bottom, which isn't optimal for the seasoned tea connoisseur.

Journal Reference:
Peiyang Zhao, Weiwei Gan, Chuanqi Feng, et al. Multiphysics analysis for unusual heat convection in microwave heating liquid [open], AIP Advances (DOI: 5.0013295)

Original Submission