CERN's hands-on
activities
"Drôle de Physique"
"Drôle de Physique" is a collection of hands-on activities started by James Gillies and now
available for schools on request. The activities have proved extremely popular with visitors
to CERN open days and similar events (e.g. Passeport Vacances). They are currently carried
out in Microcosm where a platform has recently been devoted to them.
The activities are tailored for young students/visitors. The Drôle de Physique activities
are animated by a guide. A session lasts about 30 minutes and ends with delicious ice
cream made with liquid nitrogen!
If you are interested in becoming a guide for Drôle de Physique, do not hesitate to send
us an e-mail. You will be asked to follow a training session where you will be shown the
hands-on and will discover the spirit of the Drôle de Physique, or how to have Fun with
Physics!
You are also welcome to contact Dominique Bertola responsible for the Drôle de Physique
programme.
Please note that Drôle de Physique activities imply the use of the Liquid Nitrogen.
Before handling any dangerous material, guides must read the CERN official document
(IS47) and the International Chemical Safety Card for the Liquid Nitrogen.
We offer visitors 6 different hands-on activities. The table below shows you the
equipment needed for each experiment along with the title and the number. It will help you
to identify the box containing the material for each activity.
Number Title/Subject
Equipment
Liquid Nitrogen
Demo
1
States of matter
2
3
4
5
6
Rutherford
scattering
One large (20 liter) Dewar of liquid nitrogen. One small
Dewar to use for the experiments. One circuit with platinum
resistance. Rubber hose. Ping-Pong balls. Modelling balloons.
Hammer. Protective gloves, glasses. Wooden spoon/tongs.
Optionally: Flowers. Metal bowl. Plastic cups and spoons.
Tupperware liquid container. Ice cream mixture.
One 'particle bagatelle' board, one cover, marbles
Vortex Generator Vortex generator (big box in plexiglass), candles,lighters.
1
.
Human
Gyroscope
Rotating stool. Bicycle wheels with handles. Gyroscope
The TV set
1 television monitor, 1 VCR, 1 video cassette,
1 oscilloscope, 1 permanent magnet.
Lead Glass
One lead glass block from OPAL, one colourful picture stuck
to the desk.
Liquid Nitrogen Demo - What to do
If there are children in the audience, start by asking for two or three volunteers. Tell
everyone that we're going to talk about states of matter and ask what states of matter
there are. Answer: solids, liquids, gases. Say that we're going to turn the volunteers into
solids liquids and gases, starting with solids.
Link arms tightly with the volunteers and ask them to try and move about. It's difficult.
Each person is an atom of a solid. Now say were beginning to heat up and melt, loosen the
grip on the arms until you're holding hands (or wrists). Now moving about is easier, you're
a liquid. Now let go altogether, moving about is very easy, you're a gas. Ask people what
state of matter air is. Gas, they'll say (we hope!).
Then let them look into the liquid nitrogen in the small Dewar and tell than that that is air
too (air is about 80 % nitrogen), but that it is nearly 200 degrees below zero (-196C), and
at that temperature it is a liquid. Let them look, but not touch!
Next, get a balloon in the shape of a dog (easily made, instructions on the packet). Ask
them what's inside the balloon. A gas. Why does the balloon stay blown up? Explain that its
because the gas atoms are dashing about, just like the volunteers were earlier, and exerting
a pressure on the walls of the balloon. Ask what will happen if you put the balloon dog into
the liquid nitrogen, then show them. The dog deflates because the atoms stop moving
around so quickly inside, some of the gas (air) even turns to liquid inside the balloon. When
you bring the balloon back out though, the dog pops back up. Tell them that they can't do
this with their own dogs and give the balloon to some suitable member of the audience. Tell
them that nitrogen gas has a volume 700 times greater than nitrogen liquid.
Next, get some rubber tube and pass it round so that people can see it's just ordinary
tubing. Put one end in the liquid nitrogen and find a volunteer to take the hammer. When
the tube comes out, get the volunteer to smash it with the hammer. You can again let
people feel the cold rubber to see how hard it is. Explain that the rubber becomes brittle
when the atoms are very cold and cling onto each other very tightly.
Refer back to the volunteers at the beginning (Option: if flowers available, can put them in
liquid nitrogen and smash them on the table top). Next, put a Ping-Pong ball with a spiral
drawn on it and a tiny pin-prick in it into the Dewar. You will need to hold the ball down for
a while with the tongs so that air inside escapes and the ball fills up with liquid nitrogen.
Take the ball out and put it on the table top, inside the 'particle bagatelle' is a good idea.
After a while it will begin to spin very fast as the liquid inside boils and the resulting gas
escapes. Refer back to the original demonstration with volunteers. Another Ping-Pong ball
demonstration is to drop two Ping-Pong balls on the floor, one that has been cooled in liquid
nitrogen, the other at room temperature. The room temperature one bounces back, the cold
one either shatters or doesn't bounce. You can also explain this by reference back to the
original demo with volunteers.
Next, tell them there's a serious side to this and demonstrate the circuit. By plunging the
resistor into the liquid nitrogen, you can make the light bulb light up more brightly. That's
because the resistance of the platinum resistor falls from about 100 ohms at room
temperature to about 20 ohms at liquid nitrogen temperature. Explain that some materials,
usually at temperatures even lower than liquid nitrogen, lose all their resistance allowing
electric current to flow freely. Show them the bundles of copper and superconducting cable
needed to carry the same (very high) current of 12 500 Amps.
Explain that the bending magnets at CERN's next accelerator, the LHC, will use
superconducting cable to achieve the high magnetic fields needed to keep LHC beams on
track.
Optional finale (ice-cream): mix roughly 8 parts heavy cream, 3 parts light cream, 3 parts
sugar and add coulis of strawberry or raspberry. Pour some into a metal bowl, pour liquid
nitrogen on top and stir like mad. Alternatively you can use yoghurt or Danette. When the
clouds evaporate, you are left with instant ice cream, serve in plastic cups with plastic
spoons to the audience.
2
.
Rutherford Scattering - What to do
This is a scaled up version of Ernest Rutherford's famous 1909 experiment (performed by
Geiger and Marsden) in which he discovered the atomic nucleus.
Get another volunteer to roll marbles down the ramp, varying the energy by varying the
height at which they are launched, and varying the angle. The goal is to show that you
can learn a lot about something without actually seeing it.
Rutherford fired alpha-particles, the nuclei of helium atoms, at a gold foil. At the time,
people thought that an atomic nucleus was like a plum pudding, a mass of positive charge
with negative charges stuck to it like raisins on the outside. What Rutherford saw suggested
something rather different. Most of the time, the alpha-particles passed straight through,
but sometimes they scattered through wide angles, and from time to time they bounced
straight back the way they came. Rutherford used this to deduce the real atomic structure
of electrons orbiting a nucleus. Moreover, he calculated that the size of the nucleus most be
about ten thousand times smaller than the atom, with the space in between being largely
empty (atom 10^-10 metres, nucleus 10^-15 metres). That means that if one of these
marbles were a nucleus, a whole atom would be around 100 metres across and mostly
empty space. Rutherford was able to learn a lot about the structure of matter, even though
he couldn't see it, by using energetic particles. Today physicists at CERN use similar
techniques, with much more energetic particles to probe ever more deeply into the structure
of matter.
3
.
Vortex Generator - What to do
The vortex generator shows the power of air if it's concentrated in the right way. It is
just a box with a hole in the front.
Get a volunteer to hold a candle and take it several metres away, ask them if they think you
can blow out the candle from that distance. Point the vortex generator at the candle and
blow it out by hitting the rubber sheet on the back . Point it at people and let them feel the
force of the air. What's happening is that as the air escapes, it shears around the edges of
the round hole forming a vortex.
A vortex is what happens when air or liquid swirls around itself, like a whirlpool, a tornado,
or even water swirling down a plug-hole. The power in a vortex can be devastating. This
demo has nothing to do with CERN! It's there just for fun.
4
.
Human Gyroscope - What to do
Demonstrate the gyroscope then ask if anyone in the audience would like to become a
gyroscope. Sit your volunteer on a swivel chair or stool, set the appropriately-sized bicycle
wheel spinning and hand it to them. Get them to tip it from one side to the other. They'll
feel a force turning them around on the chair.
Explain that spinning things like to stay in the same plane. That's why gyroscopes are used
in aircraft to keep them level, and it's also why bicycles don't fall over. Again, nothing to do
with CERN, it's just there for fun.
5
.
TV set: the particle accelerator in your living room - What to do
Ask how many people have a particle accelerator at home, tell them that many of them
probably do.
Tell them that there are two on the desk, similar to the ones we have at CERN but smaller.
Particle beams start in both at the back where a device like a light bulb boils off electrons.
These are accelerated by an electric field. In the simplest of the two accelerators, the
oscilloscope, the electron beam just makes a green dot on the screen. In the more complex
one, the television, the beam sweeps across the screen too fast to see building up a picture
as it goes. In the television, electric fields are used to deflect the beam across the screen.
(OPTIONAL: In the oscilloscope, there are electric fields to bend the beam too, you can use
them to make the beam trace out a line, or to move it about.)
In CERN's accelerators, magnetic fields are used to bend particle beams, and we can show
how that works here. Start with the 'scope, ask for a volunteer and get them to bring the
magnet up to the side of the 'scope. See what happens when you change the polarity by
turning the magnet over. Now get a volunteer (same or new) to do the same thing with the
television. Tell them not to do this at home because it destroys the television. You
can also see the field lines of the magnet by putting the poles of the magnet flat against the
screen.
After the demonstration, explain that at CERN, electric fields accelerate the particles, just as
in the television, but that magnetic fields are used to guide the beam in a circular orbit.
Each time the beam passes an 'accelerating cavity', it picks up a bit more energy. The
energy of LEP's beams was about 10 million times higher than the beam in the television. In
other words, to build a particle accelerator out of television sets, you'd need the
accelerating power of 10 million of them placed one after the other.
6
.
Lead Glass - What to do
Ask people what they think it's made of. Get a volunteer to try and pick it up. Explain that it
is 55% lead glass (ordinary crystal is less than 30% lead).
As well as accelerators to boost particles up to high energy, physicists need detectors to see
what happens when those particles collide. This lead glass block is part of a previous CERN
detector (there will be a picture of it on the wall) called OPAL. OPAL used some 12000
blocks of glass like this to measure particle energies.
Invite people to look through the glass at the picture below. The picture appears raised up
because light is slowed down in the dense glass. It is this density (4.06 gcm-3) that
makes lead glass attractive to physicists. What happens when a fast-moving charged
particle hits a lead glass block is that it quickly slows down and stops, but for just a
moment, it travels through the glass faster than the speed of light in the glass.
Explain that it is the speed of light in a vacuum that can never be exceeded. What happens
then is rather like a sonic boom but with light instead. The particle slows down by shedding
energy in the form of blue light, so the block glows blue. This light bounces around inside
the glass until it reaches the end where it's collected and converted into an electrical signal
by a special device stuck on the end. The size of the electrical signal tells physicists how
much energy the particle was carrying.
Extra info: The refractive index of the glass is 1.708 at 400nm (violet light), meaning
that light travels in the glass at about 58% its normal speed.