Month: February 2012

high frequency sound on a tv advert

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A dog food company has launched the first TV advert aimed directly at dogs themselves.  The advert’s soundtrack includes sounds with frequencies in the range 18 000 to 20 000 Hz (18-20 kHz).  Dogs can hear sound with frequencies greater than 20 kHz, we call this ultrasound.

I found a copy of the advert on Youtube.  The upper limit of human hearing is 20 kHz, so some young people should be able to hear the sounds for themselves even though adults can’t.  Can you?

You might have used a signal generator in class to discover your personal upper frequency limit.  The video below has a soundtrack that starts at 1 Hz and sweeps up to 20 000 Hz, before sweeping back down to 1 Hz again.  For how long can you hear the sounds?

 

nuclear fission

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We looked at generating electricity using hydroelectric and nuclear power stations today.  Click on this image of a hydroelectric station to see how they work.

Nuclear fission occurs when a large nucleus splits into 2 or more smaller nuclei.  In nuclear reactors, splitting of the large uranium nucleus is achieved by adding a neutron to make the large nucleus unstable.

 

Animation showing fission of U-235 by Stephan-Xp

The animation above shows a (blue) neutron destabilising a large (red) uranium nucleus to cause fission. When the 2 smaller nuclei (also red) are released, notice that three (blue) neutrons are produced as part of the fission process.  These new neutrons can proceed to interact with other uranium nuclei and cause further fission to occur.  If the reaction continues to take place, we have a chain reaction.

This youtube video uses mousetraps and table tennis balls to demonstrate a chain reaction.  Notice how the reaction keeps going after the first ball is added and only stops when all the mousetraps have sprung.