Kettles are among the simplest of household appliances. Lift the lid and peer inside and you'll see, at the very bottom of the water container, a coil of thick metal called the heating element. When you plug the kettle into an electrical outlet, a large electric current flows into the heating element. The element's resistance (the tendency any material has to stop electricity flowing through it) turns the electrical energy into heat. In other words, the element gets hot. Since it's in direct contact with cold water, the heat passes to the water by conduction and rapidly warms it up too.
How long does a kettle take to boil?
You can boil water in all kinds of ways—even in a simple pan on an open fire or stove—though an enclosed kettle is usually much faster: it stops heat escaping, allows the pressure to rise faster (remember that water boils when it saturated vapor pressure equals atmospheric pressure), and helps the water to boil more quickly. But do you ever get frustrated at how long it takes your kettle to boil? Don't! The amazing thing is that your kettle boils as quickly as it does—and here's why.
If you keep pumping heat energy into the bottom of a kettle (faster than heat is escaping through the top and the sides), sooner or later the water inside it will boil. A basic law of physics called the conservation of energy tells us that if you need to boil a liter of water, starting from the same temperature, you'll always have to add the same amount of energy to do it.
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