The flow of electricity through wires and cables from the main supply can be compared with the flow of water through pipes from a water tank.
When the tap is turned off, the water does not move, but when a tap is turned on the height of the water in the tank exerrs pressure on the water in the pipes and forces it through the outlet. This water pressure is comparable with voltage in electricity. The rate of the water flow, similar to the flow of current, is controlled by two things: the pressure and the size of the outlet. For example, a very narrow spray allows less wager to flow than a wide pipe. Similarly in electricity, a very thin wire restricts or resists the flow of current. Electricians measure this resistance in ohms and the flow of the current in amperes (amps).
These are the similarities between the flow of electricityand water.
电流和水流
从供电处通过电线或电缆发出的电流可与从水塔通过水管流出的水流作一比较。
水龙头关上的时候，水就不流出来，但是水龙头一打开，水塔的高度就给水管里的水加上压力，将水从出水口挤出。这个水压可与电压进行比较。水流的速度，同电流相似，由两方面来控制：压力和出口的大小。比如说，细水管比粗水管流过的水就少。同样，很细的电线会限制或阻断电流。电工以欧姆来测量这个阻力，以安装来测量电流。
这就是电流与水流之间的相同之处。
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