What is a centrifuge?

Have you ever watched clothes in a spin drier? The clothes all lift from the bottom. They go to the sides and stick to the drum as it spins round. When the drum stops spinning, the dry clothes fall down to the bottom again, where you would expect them to be. Why does this happen?

The answer is that the spin drier is a simple centrifuge. A centrifuge is a machine that pro­duces something very similar to gravity.

Spin a ball on a piece of string round your head. As you spin it faster, you will feel a force pulling you off-balance. This is because a freely-moving body will always travel in a straight line, and if you force an object to move in a curved path it exerts an equal force against you in a continual effort to fly off. This force keeps the ball on the piece of string as far away as possible. This is the force at work in the spin drier, and it is often called 'centrifugal force'.

When the drier is at rest, gravity holds the clothes at the bottom of the tub. When the drier starts moving, the clothes will move out-wards like the ball on the string when you start spinning it. And just as when you spin the ball you exert a force on the string that continually prevents the ball flying off, so the walls of the spin drier prevent the clothes flying out. The motion of the clothes outwards is stopped by the inwards force exerted by the walls. The clothes revolve round the centre of the spin drier in much the same way as the moon is kept revolving round the Earth. A body which is forced to move in a circle at a constant speed is always being accelerated towards the centre. (See: acceleration, gravity). It is the Earth's gravity that keeps the moon in its orbit and so the force that the spin drier exerts in keeping the clothes in orbit' is very like that of gravity. In fact, it is usually measured in terms of g, the acceleration due to gravity (approx. l0 m per sec.2) Even a small spin drier will develop as much as l0g, and centrifuges that are used in medical research laboratories can develop more than one million g.

Specially designed centrifuges are used to separate liquids, like oil and water or milk and cream. If you stand a mixture of oil and water in a glass, you can see the oil separate from the water. After a while, all the water drops to the bottom of the glass, with the oil floating on top. This is because oil is lighter than (not so dense as) water (see: density). The pull of gravity makes the denser liquid sink to the bottom. But this is a very slow way of separating liquids, because the pull of gravity is not very strong.

Centrifuges separate liquids quickly because, just like the Earth's gravity, they throw dense liquids to the sides of the centrifuge more strongly than less dense liquids. But the force is much greater than the Earth's. Separation is much quicker. Today, huge centrifuges are used widely in industry for separating water and oil, removing dust or bacteria from liquids and to separate the cream from milk.