THE THEREMIN

Invented by the Russian physicist Leon Theremin, the Theremin is the only instrument which is played without touching. The different frequencies and amplitudes of sound are produced by moving your hands near two antennae. This changes the capacitance (and thereby the resonant frequency) of the instrument's LC circuits because your hands act as the grounded plates of capacitors: if your hand moves closer to the the antenna, you are increasing the capacitance; move it further away and the capacitance reduces. Your body is effectively acting as a variable capacitor.
(remember that capacitance is inversely proportional to distance according to C=ε0εrA/d)
The weird and ethereal sound of the Theremin has been used in popular and classical music, and horror and sci-fi films.

NPBHS CREST IN SPACE

The NPBHS tiger-jacket crest presented to NASA by Charley Miles in 2006 is currently in space, aboard the shuttle.
TVNZ Video:

WHAT DO PHYSICS TEACHERS DO IN THEIR HOLIDAYS?

Filmed at the Dunedin Physikos conference 3-6 July 2007.
With thanks to Dr. Des Duthie.

COW'S EYE DISSECTION

Today we dissected cows' eyes.
Although our scalpels and scissors were a bit blunt, we were able to cut up the eyes and see the main features, including the

• lens,
  
• cornea,
  
• aqueous and vitreous humours,
• retina,
• tapetum (the shiny layer that looks like paua shell and reflects light),
• optic nerve.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

Thin Lens Java applet

Have a look at this java applet that simulates spherical lenses and mirrors. Thin Lens Java applet

CHAMPAGNE SPARKLY EFFECT

Total internal reflection can also be seen in a stream of water. By shining laser light through Gladwrap into a stream of water we simulate the fibre optic cable, using water instead of glass. The laser light reflects off the water/air interface and stays inside the stream of water.

REFRACTIVE INDEX & TOTAL INTERNAL REFLECTION

By sending a narrow beam of light through a perspex block, you can easily see the refraction (bending) of the light as it re-enters the air. This is called refraction. As the light speeds up as it emerges from the block, its wavelength increases and the angle with the normal also increases.








Now if we increase the angle of incidence, we begin to see a lot more reflection, as the light begins to strike the tiny irregularities on the perspex/air interface at angles greater than the critical angle.












Finally, if we increase the angle of incidence so that all the lght strikes the perspex/air interface at an angle greater than the critical angle. we see the phenomenon called
Total Internal Reflection

Students collect data for a large number of incident and refracted angles and use MSExcel spreadsheets to calculate the sines of these angles.

Angle in perspex=P

Angle in air=A

Because SineA/SineP is the refractive index of perspex relative to air, a graph of SineA vs SineP will have a gradient equal to the refractive index of the perspex.

Using a data-projector the resulting graph can be shown to other students.

Good experimental practice results in a good fit to the regression line!

BALLISTIC PENDULUM

The ballistic pendulum consists of a suspended block which is struck by a fast-moving projectile. It is an example of "combined mechanics" meaning that it involves physics concepts from a range of topics.
The topics are:
conservation of momentum (when the slug hits the block)
conservation of energy (when the block rises up after being struck by the slug).
Data collected by 12PHYMC classes:
Mass of slug = 0.50g
Mass of block = 252.0g
Height change of block after being struck = 6.0mm.


Calculations
As the block moves upwards, kinetic energy is converted to potential energy
½mv²=mg∆h
=> v=√(2g∆h)
=> v=√(2*10*0.006)=0.3464m/s
So we now know the block had momentum immediately after being struck of 0.2525*0.3464=0.08747kgm/s.
As the block had no momentum before being struck, this must also be the momentum of the slug immediately before it hit the block.
So now we can work out the size of the slug's velocity:
ρ=mv
=>v=ρ/m=0.08747/0.00050=174.9m/s (170m/s to 2 sf)

Ballistic pendulum
Video sent by johnmc2

"Physics for Future Presidents"

An interesting lecture which touches on many of the topics we are looking at this term.

NASA SPACE SCHOOL

Akshay and Leela were interviewed by the Daily News yesterday and their story appeared on page two of this morning's edition. Congratulations to these NPBHS and NPGHS students on being selected to represent us at the NASA Space School later this year.

Photo: Daily News