HOLIDAY ROCK COLLECTION - ROCK GUIDE

Here's an online rock guide you can start using to classify your rocks.

LIGHT + LIGHT = SHADOW

One of the most puzzling physics demonstrations can be one of the most illuminating!
How can a bright flame have a shadow?
It's an example of how electrons can increase their energy state by absorbing photons of light energy and then, when they "fall" back down to their lower energy state, emit a photon of light.
This is a good way to understand quantum theory. The electron can only exist in certain, defined energy states and so the photons absorbed or emitted as they move between any two states will always have the same energy.
Photon energy = Planck's constant (6.63x10^-34 Js) x frequency of light E=hf

And because the frequency of light determines the wavelength (λ=c/f where 'c' is the speed of light), and wavelength determines "colour", these photons will all have the same colour, for any given energy transition.

Which is why, when salt (sodium chloride) is burnt when illuminated by a sodium lamp, the sodium atoms will emit yellow light in the flame, but also absorb the yellow light produced by the sodium atoms in the lamp.

The result is that the flame produces a shadow on a screen.

NEW ZEALAND FROM THE SPACE SHUTTLE

Enjoy this photograph, which the space shuttle commander took for NPBHS student Charley Miles. Space shuttle on the left (tail fin pointing towards Earth) New Zealand on the right!

This makes a superb desktop image.

Click on the photo for the full version.

GEISSLER TUBE

This is a discharge tube. It consists of a partially-evacuated chamber containing an inert gas at low pressure. If a high Voltage (we used an induction coil) is applied across the terminals, the Voltage will strip electrons from the anode and create a stream of electrons. The electrons interact with the gas atoms, producing a visible beam of light.







Now if a magnet is brought close to the tube, the beam of electrons will deflect (moving charge in a magnetic field) and because this deflection is at right-angles to their motion, they move in a circular path.

The closer the magnet, the stronger the magnetic field and the greater the deflection. The force on a moving charge in a magnetic field is F=Bqv .

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

MOMENTUM PROBLEM

LEN LYE

Not really about physics (any more than anything isn't!) but this picture is historically interesting, especially to residents of New Plymouth. It shows Len Lye (in black) attempting to erect an early (12m) version of his famous "Windwand" in New York in 1960. The technology back then simply wasn't up to Lye's vision. Now you can see a 45m Windwand on the New Plymouth foreshore. Lye's sculptures show an instinctive understanding of physics: if you get the chance, go and see his works. They are particularly interesting to students studying harmonics and wave-forms. The gentleman assisting him in the photo, wearing a white suit, is the English writer Robert Graves (author of "Goodbye to All That" and "I, Claudius"). In 1942 Robert Graves' daughter Catherine married the New Zealand nuclear physicist Dr. Clifford Dalton, who was later to invent the fast-breeder reactor. After Dalton's death in 1961, Catherine alleged that he had been murdered for his part in breaking the American monopoloy on nuclear technology, and even that the death of another New Zealand-born physicist, Dr. Gilbert Bogle, in 1963 was part of a cover-up to prevent Bogle from investigating Dalton's death. However it is generally accepted that Dalton died of cancer, and that Bogle's mysterious death beside a Sydney river was probably caused by hydrogen sulphide poisoning.

GOOGLE PHYSICS VIDEOS

Hi everyone
I have added a new "element" to my blog - videos from Google with the keyword "physics". I don't yet know how useful/suitable they will be, but the first lot are pretty good.
I hope you enjoy these videos, and that they make you want to know more about the physics behind them.
There is one here which is silent and therefore requires some explanation - it involves pouring liquid air (or nitrogen) onto a material which then becomes supercooled, and shows the phenomenon called "superconductivity" - essentially it has no resistance.
The silvery disc is a magnet - when it is placed above the superconductor it will fall towards it, inducing a current in the superconductor which will oppose the change inducing it and produce a magnetic field that opposes the field of the magnet, making it "levitate" . Year 13 students who have studied electromagnetism (Physics 3.6) will recognise this as Lenz's Law in action.
You can achieve the same effect by replacing the superconductor with a permanent magnet, but in this case you have to spin the "levitating" magnet so that gyroscopic precession takes care of any derangement of the floating magnet's spin.
Towards the end of the clip, the magnet is pulled upwards away from the superconductor. In response to this, the currents induced in the superconductor set up a magnetic field that tries to hold the magnet closer - again opposing the change!
Then as the superconductor heats up again on the side of the vessel, the currents in it will reduce because its resistance is increasing, and the magnet gently falls.

IMPULSE (CHANGE IN MOMENTUM)

When a force acts on an object for a period of time, it will change the object's momentum.
This quantity (change in momentum, or the product of force and time) is called IMPULSE.
We can easily derive a formula from Newton's second law. F=ma
a=Δv/Δt so F=m Δv/Δt
gives FΔt=mΔv
mΔv is the change in momentum, Δρ
In your textbooks the formula is given as FΔt=Δρ.

SAFETY FIRST

10SCAMC modelling the latest in laboratory eyewear


















Come on guys let's be serious!

USING COMPUTER DATA-LOGGING

Data-logging

Using the "datastudio" software we can find the acceleration of a free-falling object. The sensor we used was a picket-fence and photogate.
Angus shows the screen display of graphs and tables produced.

CENTRIPETAL FORCE (YR12)

The Yak 'planes looping over New Plymouth demonstrate circular motion.
Circular motion requires a centripetal force, which is a force directed towards the centre of the circular motion. This force is provided by the control surfaces of the aircraft pushing against the air, and by the aerofoil effect (lift) of the wings
At the top of the loop, gravity will provide some of the centripetal force, so the pilot will feel some "weightlessness"; but at the bottom of the loop, the 'plane has to have enough lift to overcome gravity and provide the necessary centripetal force and the pilot will experience a strong upward force from the seat, making him feel "squashed down".

Looping is also interesting from an energy perspective.
As the 'plane climbs into the loop it loses kinetic energy and gains gravitational potential energy. It needs enough kinetic energy at the bottom, to supply the potential energy at the top, and still have enough kinetic energy to keep flying.

DOPPLER EFFECT (YR13)

This video shows the Doppler effect, and explains the ideas of red-shift and blue-shift.
BUT...
Don't forget, that as the entire electromagnetic spectrum produced by a star will be shifted, the star as seen from Earth will NOT change colour!
For red-shift, for example, the UV will be shifted into the visible, so you will still see the violet end of the visible spectrum.
It is the shifting of spectral lines (gaps in the spectrum) that enable us to measure red-shift or blue-shift.


Here's a neat animation of the shifting of spectral lines

PHYSICS (YR10)

The Bugatti Veyron has a top speed in excess of 400km/hr. You need a good broadband connection to watch this movie. You may get better results by double-clicking the screen and watching it direct at dailymotion.

Bugatti Veyron at top speed

It accelerates from 200km/hr to 300km/hr in 9.4s

To convert from km/hr to m/s you need to divide by 3.6.

1) What is the average acceleration in m/s² over the 9.4s?
2) If the mass of the Veyron is 1950kg, what is the average force applied by the car on the road over the 9.4s?

Post your answers as 'comments'.

DIFFRACTION GRATING (YR13)

The diffraction grating (a thin film with hundreds or thousands of tiny parallel lines) produces interference patterns (Young's Experiment) with very sharp, bright fringes compared to a double-slit apparatus.
This is because each fringe produced by a grating is a result of the superposition of light from hundreds of lines.
The first-order fringe is produced when the path difference for light coming from adjacent lines is one wavelength.
The second-order when the path difference is two wavelengths, and so-on.

If the diagram on the left shows the first order fringe, then:

bc=λ
de=2λ
fg=3λ.

If it shows the 2nd-order, then:
bc=2λ
de=4λ
fg=6λ.

The angle of the n-order fringe can still be found using nλ=dsinθ, or the approximation nλ=dx/L

Young's type interference can also be seen with CD's, in which the pits act like the lines of the diffraction grating. The photograph on the left shows incident laser light (strong vertical beam) striking a CD and producing antinodal lines. The zero, first and second-order lines can be seen very clearly.

Homework:
If light of λ=632nm is incident on a CD, and the distance between the two first-order fringes is 98.4cm on a screen 1.20m from the CD, what is the distance between pits?



Link to Hyperphysics

Link to Diffraction Grating Applet

RUSTING (YR10)

The results of our rusting experiment:

The tube on the left has a stopper, and a small amount of Calcium Chloride to exclude and absorb any air (Oxygen)
This nail has no rust.

The middle tube has no stopper and is half-full of plain water. The nail is therefore open to reaction with Oxygen and Water. This nail had a lot of rusting.
The tube on the right is filled with boiled water (to remove dissolved Oxygen) and covered with a layer of oil. This nail had only a very small amount of rust

Conclusion:

Both Oxygen and Water (as well as Iron, of course! - Thanks to Ken "what's the frequency?" Lockhart) are needed for rusting to occur.

Wikipedia link

BEATS (YR13)

• At the top of the diagram, you see the waveforms created by two sources with similar, but slightly different frequencies. Notice how they are sometimes in phase, and sometimes out of phase.

• The blue waveform at the bottom of the diagram, shows the superposition of the two.

• The antinodal forms that result are called 'beats'. The frequency of the beats is equal to the absolute difference between the frequencies of the two sources.

• fb=f1-f2

Check out this link to Hyperphysics website