篇篇学之7：Hydroelectricity 2

The energy that may be extracted from water depends not only on the volume but on the difference in height between the water impoundment(or source) and the water outflow.This height difference is called the head.The amount of potential energy in water is directly proportional to the head.For this reason,it is advantageous to build dams as high as possible to produce the maximum electrical energy.The machanical power of falling water is an age-old tool.Of all the renewable energy sources that generate electricity,hydropower is the most often used.It is one of the oldest sources of energy .

WORDS:
extract 获得 water impoundment 库水 proportional 成比例的 generate 产生