Liquid helium

Key takeaways

Liquid helium belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Liquid helium to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Liquid helium from memory before moving on to harder problems.

Reference excerpt

Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures. Liquid helium may show superfluidity. At standard pressure, the chemical element helium exists in a liquid form only at the extremely low temperature of −269 °C (−452.20 °F; 4.15 K). Its boiling point and critical point depend on the isotope of helium present: the common isotope helium-4 or the rare isotope helium-3. These are the only two stable isotopes of helium. See the table below for the values of these physical quantities. The density of liquid helium-4 at its boiling point and a pressure of one atmosphere (101.3 kilopascals) is about 125 g/L (0.125 g/ml), or about one-eighth the density of liquid water.

Liquefaction Helium was first liquefied on July 10, 1908, by the Dutch physicist Heike Kamerlingh Onnes at the University of Leiden in the Netherlands. At that time, helium-3 was unknown because the mass spectrometer had not yet been invented. In more recent decades, liquid helium has been used as a cryogenic refrigerant (which is used in cryocoolers), and liquid helium is produced commercially for use in superconducting magnets such as those used in magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), magnetoencephalography (MEG), and experiments in physics, such as low temperature Mössbauer spectroscopy. The Large Hadron Collider contains superconducting magnets that are cooled with 120 tonnes of liquid helium.

Liquified helium-3

A helium-3 atom is a fermion and at very low temperatures, they form two-atom Cooper pairs which are bosonic and condense into a superfluid. These Cooper pairs are substantially larger than the interatomic separation.

Characteristics

The temperature required to produce liquid helium is low because of the weakness of the attractions between the helium atoms. These interatomic forces in helium are weak to begin with because helium is a noble gas, but the interatomic attractions are reduced even more by the effects of quantum mechanics. These are significant in helium because of its low atomic mass of about four daltons. The zero-point energy of liquid helium is less if its atoms are less confined by their neighbors. Hence in liquid helium, its ground state energy can decrease by a naturally occurring increase in its average interatomic distance. However at greater distances, the effects of the interatomic forces in helium are even weaker. Because of the very weak interatomic forces in helium, the element remains a liquid at atmospheric pressure all the way from its liquefaction point down to absolute zero. At temperatures below their liquefaction points, both helium-4 and helium-3 undergo transitions to superfluids. (See the table below.) Liquid helium can be solidified only under very low temperatures and high pressures. Liquid helium-4 and the rare helium-3 are not completely miscible. Below 0.9 kelvin at their saturated vapor pressure, a mixture of the two isotopes undergoes a phase separation into a normal fluid (mostly helium-3) that floats on a denser superfluid consisting mostly of helium-4. This phase separation happens because the overall mass of liquid helium can reduce its thermodynamic enthalpy by separating. At extremely low temperatures, the superfluid phase, rich in helium-4, can contain up to 6% helium-3 in solution. This makes the small-scale use of the dilution refrigerator possible, which is capable of reaching temperatures of a few millikelvins. Superfluid helium-4 has substantially different properties from ordinary liquid helium.

History In 1908, Kamerlingh-Onnes succeeded in liquifying a small quantity of helium. In 1923, he provided advice to the Canadian physicist John Cunningham McLennan, who was the first to produce quantities of liquid helium almost on demand. Important early work on the characteristics of liquid helium was done by the Soviet physicist Lev Landau, later extended by the American physicist Richard Feynman. In 1961, Vignos and Fairbank reported the existence of a different phase of solid helium-4, designated the gamma-phase. It exists for a narrow range of pressure between 1.45 and 1.78 K.

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Illustrations

Worked examples

Example 1 — a first encounter with Liquid helium

Start with the simplest possible case. Write down what Liquid helium claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Liquid helium before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Liquid helium ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Liquid helium

In research: Liquid helium appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Liquid helium in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Liquid helium is common in secondary-school and first-year university syllabi. It links to neighbouring topics 1908 in science, Coolants, Cryogenics, so understanding it makes those chapters shorter.
In everyday life: Look for Liquid helium outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Liquid helium in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Liquid helium means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Liquid helium out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Liquid helium in simple terms?

Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures. Liquid helium may show superfluidity.

Why does Liquid helium matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Liquid helium?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Liquid helium.