3.10 FLOWING WATER
A characteristic of all liquids is that they flow. When rain falls, the water flows downhill over the ground, and through the earth, until it reaches a river. The river also flows downhill until it reaches a lake or the sea. There it spreads out and the water creates a level surface.
There is a well-known saying that "water always finds its own level". This means that water always flows until it reaches the lowest level it can find. And it keeps flowing and spreading out until no part of the surface is higher than any other part.
If we pour water into different containers, joined together by pipes, the water fills all the containers to the same level. The diagram (right) shows a piece of apparatus found in some school laboratories. Notice that the water level is the same in all the containers; it is not affected by their shape or size.
The siphon. A siphon is a tube used for taking water (or any liquid) from a higher level to a lower level in a particular way. Look at the diagram below. When the tube is filled with water, water flows from the high container to the low one. An easy way to fill the tube is to suck at the lower end. When the water enters your mouth, pinch the end of the tube between your fingers to stop air entering. Now put the end of the tube back into the lower container and release it. The siphon will start working. Water will flow from the high container to the low one. In a siphon, the water actually flows uphill to start with! But this only happens when the tube is full of water. And it only happens when the outlet end of the tube is lower than the water level in the high container.
A siphon is useful for making water flow steadily from a container that has no tap. Sometimes truck drivers use a siphon when they have run out of petrol. They may be able to siphon petrol from the tank of another vehicle.
· 1. We say that water always finds its own level. Explain, in your own words, what this means.
· 2. When does water flow uphill (without a pump)?
· 3. Why is it very dangerous to play with petrol?
· 4. Describe how you would decant clean water from a large container to a small one, using a siphon.
3 - 10