Transcript Subversive Physics - HRSBSTAFF Home Page

Covering today’s outcomes with
today’s Physics
Today’s Outcomes in Physics 12
 326-3: apply quantitatively the laws of conservation of
momentum to two-dimensional collisions and explosions.
 328-5: analyse, qualitatively and quantitatively, the forces acting
on a moving charge and on an electric current in a uniform
magnetic field.
Today’s Physics
We are a long way from the idea that protons, neutrons and electrons are
the fundamental particles in nature.
Physicists now know that there are a multitude of sub-atomic particles.
They can observe these particles in a bubble chamber.
A bubble chamber is a detector filled with a liquid close to its
boiling point (superheated), where the ionizing particles'
trajectories materialize in the form of tracks made of
bubbles.
Today’s Physics: Bubble Chambers
Just like the salt produced trails showing
its path, particles will initiate boiling by
ionizing the atoms in the liquid as they
pass through the liquid. These trails,
coupled with some Grade 12 Physics
enable us to observe sub-atomic particles.
Today’s Physics: Bubble Chambers
So let’s see what we can figure out. First, the rules of the
‘game’.
1) The photos we will use are from CERN’s BC site:
http://teachers.web.cern.ch/teachers/archiv/HST2005/bubble_chambers/BCwebsite/index.htm
2) The chamber is filled with liquid hydrogen. Negative kaon particles (K-) were
shot into it. These particles can hit the protons or electrons of the
hydrogen or, of course, simply pass right through them.
3) The trails are the result of a charged particle causing the
hydrogen to boil.
4) Charge and momentum are conserved. The charge is either +1 or -1
5) The constant magnetic field will exert of force on the moving
charged particle whose magnitude is found with F = qvB and
whose direction is found with the right-hand rules.
So let’s play…
Since the interactions
must take place
“downstream”, the
particles must be
moving toward the
top of the page.
a) Up
b) Down
Here we see the
kaon ‘beam’
Which way are
they traveling?
What is the direction
of the magnetic field?
a) Into the page.
b) Out of the page.
a) To the right.
The negative
kaons are pushed
to the left.
Back of hand
thumb
b) To the left
The spirals are
caused by an
interaction between
a particle and
something in the
liquid. What are the
particles hitting?
a) Protons
b) Electrons
c) Neutrons
The particles must be losing
energy since the tracks spiral.
They must be charged.
The tracks spiral
to the left.
The trail on the left
The trail on the right
a)
A negative particle
A positive particle
b)
A positive particle
A negative particle
c)
A negative particle
A negative particle
d)
A positive particle
A positive particle
A collision happens
at A. One particle
travels to the right
and another to the
left. Which answer
summarizes what
happened at A?
The path on the
left is curved.
The charge of the
kaon must be
conserved.
In the interaction at A, a kaon
produced a positive and a
negative charge. How can a
negative particle produce a
positive and a negative charge?
The kaon must have interacted
with a positive charge.
Why didn’t we see the positive
charge before the collision?
a) It was moving too fast
b) It was moving into the The chamber is filled with
liquid hydrogen. There are
page
lots of stationary positive
c) It was stationary
protons.
Which of the tracks from A
shows the particle with the
smaller momentum?
F = qvB
qB 
mv 2
Fc 
r
mv
r
qBr  mv
pαr
So the smaller
the radius (the
more curved the
path) the smaller
the momentum.
How many particles were
created in the collision at A.
a)
b)
c)
d)
2
3
At least 2
At least 3
Notice B
There may have
been a neutral
particle produced
at A.
What happened at B?
The neutral particle
a) Collides with a neutral
particle
b) Collides with a positive
particle
c) Decays into a positive
and a negative particle
d) Decays into two
negative particles
We see that two charged
particles are produced. One is
positive and one is negative.
The total charge
after the interaction
is neutral, so the
total charge
beforehand must
be neutral
The
hydrogen
contains
only protons
and
electrons
In using the bubble chamber diagram, students have used the
concepts of the conservation of momentum in 2D and their right
hand rules. They have also related the force on a charge moving in a
magnetic field to its momentum.
For more examples and questions check out:
http://epweb2.ph.bham.ac.uk/user/watkins/seeweb/Bubble.htm
Today’s Outcomes in Physics 11
 325-2: analyse graphically and mathematically the relationship
among displacement, velocity and time
 325-7: identify the frame of reference for a given motion
Today’s Outcomes in Science 10
 325-1, 212-7, 325-2 : using linear experimentation with appropriate technologies, analyse
graphically and quantitatively the relationship among distance, time, and speed and the
relationship among position, displacement, time, and velocity
Today’s Physics
GPS is everywhere. It is estimated that every day 1 billion people
use this technology: farmers, skiers, police, treasure-hunters and
surely Physics teachers!
What is it based on?
Relativity of course!
So let’s model how GPS works.
Your GPS receiver has picked up signals from three different satellites.
One satellite sent the signal when it was above Vancouver, another went
it was above Churchill Falls, and the third when it was above
Charlottetown. The signals tell us that you are…
2154 km from Vancouver
1879 km away from Churchill Falls, Labrador.
2464 km away from Charlottetown, PEI.
With your map, we can find out where you are located.
But first, we’ll need a scale.
A scale resizes the diagram. In this diagram
a line is drawn to represent 1000km. How
long, in centimeters, is this line?
This means that
What is the value of 1 cm on the map?
_______
3.25 cm = ______
1000 km
3.25
3.25
1 cm = 308 km
Every cm on the map means
308 km on the Earth’s surface.
 You are 2154 km from Vancouver
So if we’re 2154km away that’s…
 1cm 
2154km
  6.99cm
 308km 
here
here
here
You could be…
We see that there are three locations on this map 2154 km
away from Vancouver. (There are lots of other locations that
distance as well but we know we’re somewhere on the
map.)
We need to narrow this down. We also know you are
1879 km away from Churchill Falls, Labrador.
 1cm 
1879km
  6.10cm
 308km 
here
here
Now you could be…
Well we’re not in the Northwest Territories!
We’re either in Sandy Lake, Ontario or Churchill, Manitoba.
We also know that you’re
 2464 km away from Charlottetown, PEI.
 1cm 
2464km
  8.0cm
 308km 
You are here!
You were in Churchill, Manitoba. We used distances from three
satellites to discover your position. Now the question becomes,
How do the satellites measure
those distances?
We know that GPS satellites orbit the Earth at a
height of 20 200 km.
The satellite sends a
TIME signal to the
receiver indicating
the time the signal
was sent.
The receiver measures
the time when it
receives the signal.
How do the satellites measure
those distances?
The difference in time is used, with the speed of the signal to calculate the distance
between the receiver and the satellite.
Now, using Pythagoras’ Theorem we
can find the distance between the two
2
cities.
d  21000km  (20200km) 2
d  5741km
So GPS is all about WHEN not really about WHERE!
The timing of these devices needs to be very precise. Therefore
they must take into account
That time elapses faster when you’re
high above the ground
That time elapses slower when
you’re moving really fast
We now know enough to work this through
completely.
Three satellites have given your receiver this
information:
Satellite A is 20 200km above Fort McMurray, Alberta. t = 0.06821s
Satellite B is 20 200km above Iqaluit, NWT. t = 0.06764s
Satellite C is 20 200km above Thunder Bay, Ont. t = 0.06758s
Find
1) The distance between the
satellite and the receiver.
3) Where the receiver is.
2) The distance between the
city and the receiver.
A) 0.06821s 300000km / s   20463km
d
20463km2  20200km2
B)
0.06764s 300000km / s   20292km
2
2
d  20292km  20200km
d  3270km
d  1930km
 1cm 
3270km
  10.61cm
 308km 
 1cm 
1930km
  6.27cm
 308km 
C)
0.06758s 300000km / s   20274km
2
2
d  20274km  20200km
d  1731km
 1cm 
1731km
  5.62cm
 308km 
Today’s Outcomes in Physics 12
 327-11: summarize the evidence for the wave and particle models
of light
 115-7 explain how scientific knowledge evolves as new evidence
comes to light and as laws and theories are tested and
subsequently restricted, revised or replaced.
The video explores the fundamental proof that light is a
wave: Young’s double slit experiment.
This shows that light
cancels itself out as
only a wave can.
The bright spots or
fringes show the
constructive
interference whereas
the dark spots show
destructive
interference
The video then explores the results when “particles” like
electrons or atoms or molecules are fired at the double
slit.
This same pattern
shows that the
electrons are
cancelling themselves
out!!
There is a high
probability that the
electrons will hit in the
“bright fringes” and a
low probability that
they will hit in the
“dark fringes”.
The double slit experiment shows that light is a wave but it
also shows that particles act like waves. The idea of what
matter is must be changed!
Particles…
For hundreds of years were considered localized
quantities of matter. They are in one spot but not
another.
The double slit experiment shows that light is a wave but it
also shows that particles act like waves. The idea of what
matter is must be changed!
Particles…
For hundreds of years were considered localized
quantities of matter. They are in one spot but not
another.
The double slit experiment shows that light is a wave but it
also shows that particles act like waves. The idea of what
matter is must be changed!
Particles…
they must be considered as waves as
well.
And this reality gets crazier…
When physicists fire one electron at a time, the
interference pattern is still formed
When physicists try to measure which slit the single
electrons pass through, the interference pattern is
destroyed. The electrons act as if they are localized
once more.
When physicists turn the intensity of the light down
(so that one photon at a time hits the double slit) the
light hits the screen as a series of discrete bundlesphotons. The interference pattern builds up over time
like it did with the electrons.
And this reality gets crazier…
So clearly, light is a wave (double slit pattern) unless
the intensity is really low then it acts like a localized
particle.
And equally clear, is the fact that electrons are
localized particles until we turn our backs on them
then they act like a wave.