Q1.
Make your own centrifuge with a cardboard disc and thick thread. You will be able to see how the heavier particles move outwards. It is a fun and hands-on way to understand the science behind separation. Which mixture would you like to separate using this mini centrifuge?
Answer
Muddy water is the best mixture to try, because you can see the result with your own eyes and it needs nothing that is not already in the classroom.
How to do it
- Push a thick thread through two holes near the centre of a stiff cardboard disc and tie the ends to make a loop, as in the paperfuge of Fig. 5.20.
- Tape a small, tightly capped plastic tube of muddy water flat on the disc, close to its rim. Tape a second, identical tube on the opposite side so that the disc stays balanced.
- Hold the loop at both ends, wind the disc up by swinging it, then pull the strings apart and let them come together, over and over. The disc spins one way, then the other, at high speed.
- Spin for two or three minutes, then stop and hold the tube upright.
What you will see: a layer of mud packed at the far end of the tube — the end that was pointing outwards — with much clearer water above it. The same tube left standing on the table would take far longer to become that clear.
Why it happens: A particle going round in a circle is constantly being pulled towards the centre by the tube walls. From the point of view of the mixture inside the spinning tube, this feels like a strong outward push — the centrifugal force. Its size grows with the mass of the particle and with the square of the spinning speed, so at a few thousand revolutions per minute it is hundreds of times bigger than the particle’s weight. The heavier mud particles are therefore driven to the outer end far faster than gravity alone could ever manage, while the lighter water stays nearer the axis.
Other mixtures worth trying: chalk powder in water, curd separating into thick curd and whey, or the pulp in unfiltered fruit juice. Safety first: the tubes must be sealed and taped firmly, the disc must be balanced, and everyone should stand clear while it is spinning.