Question: When you boil water, does it matter where you add the heat?

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Answer: Imagine using an immersion heater, like this one: 

to boil a large container of water. Does it make a difference where in the container the heater is? At the bottom? At the top?

The electric heating element heats the water immediately around it by conduction - that is to say, it transfers its heat energy to the water molecules directly touching it. As water heats up, though, its density decreases. Lower-density, warmer water will tend to float on top of higher-density cooler water.

If the heating element is at the bottom of the container, the hot water will rise through the container, forcing cold water back down closer to the heater. This process of circulation or convection, gradually transfers heat throughout the container, heating it evenly. If the heater is at the top of the water, though. the water around it will heat up much faster, but the water at the bottom of the container will still be cold!

 

 

Related demonstration:

https://lecdem.physics.umd.edu/i/i2/i2-24.html

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