School Students

This page is designed to contain interesting ideas from Chemical and Process Engineering that could by tried by school students sometimes in conjunction with their teachers.

This page is currently just a start but it will continue to grow.

Why is it that toilets flush and plug holes drain in different directions in different hemispheres?

This phenomena is caused by the Coriolis Effect and people say that sinks should drain in different directions in different hemispheres but that would happen only in a perfect world. It does not happen in practice. A good explanation of what really happens is given in the web site Bad Coriolis.

The theory is that moving objects (and water) have inertia and try to continue moving in the same direction at the same velocity. When the object is rotating it is more complex but you can see what happens by trying some experiments.

Go to a playground with a round-about. Stand on the outer edge of the round-about, get someone to rotate it and try walking quickly to the centre. You should find that you are thrown in the direction of rotation even faster than the roundabout is moving. That is because you body wants to keep going at the same speed relative to the ground. You might have learned that your speed can be calculated by multiplying the circumference by the number of revolutions per second, so the greater the circumference the greater the speed. The edge of the round-about is travelling a lot faster than the centre.

Now stand on the edge with a water pistol, rotate the round-about and shoot at the centre. Does the water jet seem to go ahead of the round-about? It is trying to keep the same velocity relative to the ground so it seems to go faster than the round-about.

Lastly get a flat bottom round container with a 6 mm hole in the bottom. Hold the container filled with water at the centre of the round-about with your finger plugging the hole from underneath and get someone to rotate the round-about. Put some small pieces of dry grass on top to see how steady the water is. When the water is rotating at the same speed as the round-about remove your finger. Does the water drain out in the same direction as the round-about? Does it go in the same direction as the water pistol jet. Try rotating in the other direction too, though you might get someone else to do this as you'll be feeling dizzy by now.

All these three experiments show that objects want to maintain the same velocity compared with the ground. Before you leave the round-about please wipe the water off it.

The Earth is just like a large round-about but it rotates in space only once per day. If there were no other forces or disturbances, the same effect would occur, but in practice the shape of the sink or basin and very small flows in the water determine the direction of draining.

Some Strange Fluids

Not all fluids are the same. Some are thick and some are thin. Some get thicker when you stir them, some get thinner. The thickness of a fluid is referred to as its viscosity which has units of centipoise (cP). The viscosity tells us how hard we have to push the fluid to get it to move.

Here's some typical viscosities

  • Water 1 cP
  • Air 0.001 cP
  • Golden Syrup 25000 cP
  • Engine oil 200 cP

You can have fun with corn flour or custard powder (some of the cheaper brands don't work so well).

Put some dry corn flour or custard powder in a bowl. Add just enough water so that you can pour it out (into another bowl). You've probably alread found that it gets harder (more viscous) as you mix it. The hard you try the harder it becomes. If you want to create a mess try rolling some into a ball between your fingers and then between your hands. As soon as you stop rolling it, or when you pass it to someone else it will start to flow. Yuk. We refer to this fluid as a shear thickening, or dilatant, fluid.

The opposite behaviour is so common you don't even notice it. Think of thick yoghurt, for example. When you are not stirring it is hard to pour but when you stir it, it seems relatively thin. A lot of foods, or fluids that contain long molecules are like this. we call them shear thickening, or pseudoplastic, fluids.

What about toothpaste or mashed potato or putty? If you don't push them they don't flow. They are solid until you push hard enough and then they become liquid. Yes they are solid and liquid. We call them Bingham Plastic fluids.

Golden syrup is quite boring by comparison. It seems thick and it is thick. When you stir it, it stays just as thick. Its a normal, Newtonian, fluid.

For more information contact Dr Ken Morison.

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