## Physics Teatime 2: On The Making Of Tea

We talked before about the shape of the teapot; now, another question has come up online: Why does your tea water heat differently in the microwave than in an electric kettle?

Several years ago, Nadia Arumgam mentioned this problem in an article for Slate on making tea. One thing that contributes to the taste of tea is how hot the water is when it is brewing. When you heat water with an electric kettle or on a stove, the heat source is at the bottom. The hot water at the bottom rises, forming convection currents, drawing the cold water downwards to be heated in turn. Over time, the entire pot of water reaches the same temperature. When you see the water boiling at the surface, you know that the entire container is boiling.

In a microwave oven, though, the molecules are heated by electromagnetic agitation at points throughout the container of water, with warm and cool spots. Some of these points may begin to boil while other areas of the water are still cool. When you take the water out and make tea, it may not be as hot as you intend.

Let’s see what this looks like on a larger scale. We have a Dewar flask, an insulated glass container, filled with water. We lower a small heating element into the water so that it heats the top layer of water. We can lower a thermometer probe down to the bottom of the water.

As you can see in the picture, the bottom layer of water is still cold! 23°C, right about at room temperature, and not very appealing for a cup of tea.

The top is boiling; when we move the probe to the top, the temperature is much higher, 97°C. (Not quite 100°C, though, and here we’re only a few centimeters from the heating element.)

This can be a surprising result. We are used to thinking of water as conducting heat well, and it certainly does conduct heat better than air or glass; but not nearly as well as metals or many other materials. In most circumstances, though, most of the heat transfer in water isn’t from conduction, but from convection. In a big container, if your heat source is near the top, there may not be enough convection to make up the difference.

So put on the kettle, curl up with a nice cup of tea, and enjoy the snow!