Liquid Nitrogen Demos

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Liquid nitrogen Experiments

Liquid nitrogen is a great spectacle and lots of intresting physics can be shown.

Background

This document is for technician reference. Liquid nitrogen is used in a few physics experiments. Technician work is mostly setting up and delivery. The special Dewar storage flask, if available, 25litre peferably with its own stand should be located in a well ventilated store.
In response to queries about costs etc.:
Cost is about £125 for 25l but ask BOC they will tell you current prices (2008 price).
Delivery is usually 25l. But It is believed other quantities are available. You may need to hire/borrow a dewar. Contact BOC first. Our local deliver is very obliging but you can't guarantee the state of the dewar.
BOC supply their own safety sheets. Make sure you have suitable containers for delivering to the demonstration.

The web has some useful sites that may give ideas for practicals, but beware as the risks assessments need to be for school work...

Health and Safety

Please read the leaflet “care with cryogenics” which you can also obtain from the DOWNLOAD area of TecHKnow. (Visit the health and Safety section area).

See also;

  • CLEAPSS Laboratory Handbook (2005 or later), section 11.2 Cryogenics. On the 2007 CD
It is recommended that a copy be placed with any demonstration equipment specifically for 
this equipment.Teachers should have their own risk assessment. CLEAPSS etc. 
can help with this.
Demonstration is best done with an experienced user. If possible, someone who is used to liquid nitrogen should demonstrate the following before using.
  • Any local rules for liquid nitrogen should be followed.
  • When pouring liquid nitrogen wear safety spectacles.
  • Normally, no Laboratory or lecture room is small enough to present an anoxia risk if only 1 litre is dispensed at a time.
  • Avoid carrying up stairs when pupils are likely to be around.
  • Wear leather gauntlets for dispensing.
  • Do not wear open toed shoes. Do not use unmodified thermos flasks.
Thermos flasks can be simply modified for liquid nitrogen work. 
A hole should be drilled through the both container tops so as to ensure that they are not air tight. 
IF this is not done the flask could explode.

Standard practicals

Meisner effect - (more advanced groups only)

The super conductivity kit contains a ceramic slab that is superconducting at the temperature of liquid nitrogen.

This is best demonstrated with a "Flexicam" or similar for all but the smallest groups.


Apparatus:

Superconductor kit (Available through Leybold-didactic); plastic tweezers; laboratory jack; liquid nitrogen in thermos; Optional: TV; flexicam and anglepoise lamp.


Set up:

Adjust the laboratory jack and flexicam if used, so that the view is as close to the top of the foam cup as possible. Illuminate, if necessary, from behind the camera.

Place the large ceramic slab on the central support of the foam cup. If the demonstration is to include showing that the slab is not superconducting at room temperature place the magnet on the slab now.

Pour liquid nitrogen onto the slab and continue to fill so that the nitrogen is contact or just covering the slab.

Place the magnet onto the slab using the tweezers and as it cools the magnet will rise.


Clearing down:

The slab should be put into the plastic bag cold (use tweezers). This should keep it in a moisture poor environment. The magnet also goes into the box with the instructions, the tweezers and cup will go into the larger box.

The kit is old and has a limited life, depending on exposure to moisture.

Jumping ring (Thompson’s)

A solid aluminium ring is placed on top of a large induction (mains) coil (from the demountable transformer kit ). If an ac current is applied the ring will jump. If the coil is immersed in liquid nitrogen the ring will jump considerably higher!. Take care with placement to avoid light fittings etc.


Change in physical properties

Method1: Rubber Ball

A squash ball is inserted into liquid nitrogen and retrieved using tongs. If transferred to a glove hand it may be thrown at a wall away from bystanders (to avoid splinters) and smashed.


Method 2: Rubber tube

A short length of rubber tube is immersed in liquid nitrogen (BEWARE of nitrogen flowing up the tube- a discussion point in its own right!). Taken out with tongs, it can be smashed onto the table or hit with a hammer. Beware of splinters.


Method 3: Flower crushing

Use the clear dewar for this and any delicate seasonal flower. Immerse in liquid nitrogen. When cold remove and crush.

Method 4: Banana Hammer

Cut a banana into a short (ca 3 inch length) and insert a nail. Use a polystyrene foam bowl to immerse banana in liquid nitrogen. When frozen use to hammer a nail into a piece of soft wood.

A variation is to use a blu-tak nail.


Method 5 Lead Bell

The lead bell requires a large volume of liquid nitrogen. Using a plastic box and insulating with expanded polystyrene can achieve this. The bell is rung when warm a dull thud sounds. On cooling a feeble clang is heard. The lead bell is hard to source and the demonstration is not worth too much effort.

Linear expansitivity

Use standard expansion equipment to show this. The ball and ring or gauge and bi-metallic strip .


Cubic expansitivity

Cubic expansitivity1

A table tennis ball should have sections coloured alternatively light and dark. A pinhole is made at a slight angle at the centre of a section.. The ball is then held under liquid nitrogen for a few moments so that the ball fills. When removed the ball acts as a “Hero’s engine” and spins as the gas escapes.


Cubic expansitivity 2

A normal balloon is inflated and the end tied off. The balloon has liquid nitrogen poured over it. It collapses. As it heats up again normal size will be achieved.

Conductivity of a wire

Method1

There are two ways to show this the simplest (equipment wise) is with a coil of thin insulated copper wire and a multimeter. The thinner wire used the more significant the change.

The most awkward part of this experiment is the joining of the wire to the multimeter. It is suggested that 4mm plugs are attached to a short length of equipment cable. The thin wire is stripped of insulation (emery cloth or match, but be aware of toxic fumes). Tin the ends to be joined and solder together. The joint may be strengthened with a heat shrinkable sleeve.


Method 2

A large coil of insulated wire must be made several tens of meters of copper. This should have a resistance comparable to twice that of a 12V 48W bulb.

Use a 48W 12V lamp and laboratory power supply in series with the coil. When immersed in liquid nitrogen the bulb will get brighter as the coil resistance decreases


Chemical reactions

Rate of reaction

A battery (AAA) has 4mm leads soldered to it. If a replacement is required, rough

The top and base of the cell with emery paper and apply solder. Attach pre-soldered leads.

Attach a miniature 1.5V 0.3A M.E.S. lamp in holder to the ends and immerse in

Liquid nitrogen. The bulb will dim in a few seconds. Take out and it will begin to glow again after a few minutes indicating a reversible process.

A hair drier is useful.


Possible faults: solder fracture during cooling- re-solder.


Oxidation of a lightbulb

Use a standard SBC connector and a 36W 12V lamp. Remove lamp from connector and score with glass cutting tool near the base. Cover with cloth and gently break with a light hammer or similar.

With luck scoring will reduce the sharp edges formed. A pair of pliers can be used to tidy up any sharp parts. Still using cloth replace bulb. ENSURE FILAMENT IS IN TACT or redo.


The bulb is immersed in liquid nitrogen and a current passed. Rapid boiling will result. The filament will glow but not blow. Removing the filament from the nitrogen will cause the bulb to blow.

Safety: Care with sharp glass edges. Consider disposal methods.

Making Solid Nitrogen

Experiment performed after seeing: New Zealand's Victoria University Teaching Resource.
It may seem crazy at first, but the temperature difference between liquid and solid nitrogen is only about 15 degrees. By using a well insulated beaker in a vacuum chamber this can be achieved in a few minutes. Because you are using the vacuum pump at very low pressure, care should be taken with the evacuation of the chamber and selection of glassware. Do NOT use a thermos flask as it may not stand the low pressure and thermal shock.
It is suggested that only competent staff familiar with both use of the vacuum pump and handling cryogens perform this. As always it pays to try this out before demonstrating. The earlier NZ site's tip about using black card is useful to make the solid more visible. There are also other tips on that site.
A 100ml beaker (pre-cooled) containing about 20-25ml of liquid nitrogen is sufficient. Insulating the beaker (thermally) from the pump is essential.
When performing this success was achieved using, initially, a needle valve slightly open and ballast open. These were gradually closed until a solid formed.
Use of a video camera linked to projector will allow a larger class to see safely.

External Links



Back to Physics Experiments

--D.B.Ferguson 20:24, 30 October 2007 (GMT)

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