Suppose I boil water in an electric kettle in order to add hot water in a cup containing a tea bag. Should I cover the metal ring with water or is it ok if the level of water is halfway through the metal ring (the one which heats)? Suppose you only make a small amount of tea (say one cup).
You should completely cover the metal ring, otherwise you risk burning the element out. You should only boil as much water as you need above this minimum though, to save electricity. Check inside your kettle, they often have minimum and maximum levels marked somehow.