The heating element is near the bottom of the kettle because of the way it heats up the water. The water is heated through the flow of electricity meeting resistance in the form of the element. The heat is created through convection currents, which warm the water. Heating any fluid causes it to expand; in other words, it gets less dense but takes up the same amount of space, which has to mean that it gets lighter. In the same way, when the water in the kettle gets hotter it loses density, which in turn causes it to rise to the surface. (You can do this as an experiment, by putting different coloured dyes into cold or boiling water, putting them into the same vessel and seeing which one rises). On reaching the surface it starts to cool, which causes it to sink down, where it is then again heated by the convection currents from the element. As more of the water is heated the total temperature rises, so that eventually there is not enough water cooling down fast enough to sink, and the water then comes to the boil.
You can see how this works if you try and imagine a kettle with the heating element in a different place. Suppose you put the heating element near the top of the kettle. In the first place, it would only work at all if the kettle was filled right up to the top, as of course the element can only heat water that is already covering it (it doesn’t work like a naked flame, which can heat a pot of water placed above it – a heating element is designed to work in liquid, and it must be covered by the liquid or it will just get hotter until it melts or burns out). Filling the whole kettle up every time is a waste of electricity to start with, as you should only put in as much water as you need each time, and more often than not you don’t need a full kettle.
Apart from that, even if this imaginary high-placed element was covered by water, it wouldn’t work well. This is because, as mentioned earlier, when the water gets hot it becomes less dense and rises. Imagine if this newly heated water is already just an inch or half an inch below the surface; it will go up to the surface and sink again all the way to the bottom of the kettle – but will not find any convection current waiting there to heat it. Heating water by this method would take a very long time if it worked at all – in fact if the element wasn’t very powerful, the water might cool down again faster than the element could push the heated particles upwards.
If you look at different types of kettle you will see that many of them use a flat element, like a plate, which simply heats up the whole of the bottom of the kettle and all the water that comes in contact with it. Others have elements in the form of tubes through which the current passes; but in either case, you will find them at or near the bottom for the reasons explained.
If you click here you can learn more about the way a kettle element works. You can also learn more about different types of element and how to look after them; for instance, how to remove limescale and other deposits that can build up on kettle elements over time.