Page 81 - BBC Focus - August 2017

How is helium turned into a liquid and
a superfluid?
TOBY CARTER, BY EMAIL

At -269°C, helium gas condenses This is achieved by compressing
to become a liquid. Cool it even the gas, and then expelling it
further and it becomes a state of through a small nozzle. As the
matter called a superfluid. In gas expands, it rapidly cools
this state it has no measurable (you’ll have noticed this effect if
viscosity and so does some odd you’ve ever used an aerosol
things, such as climbing up the deodorant). The process is
walls of a dish, leaking through repeated until the gas that Could you throw a frisbee on Mars?
apparently solid materials and rushes out of the nozzle is cold
JONATHAN HINCHLIFFE, BIRMINGHAM
staying motionless while its enough to condense to a liquid,
container is spun. To create the then if you repeat the cycle a few
liquid and superfluid states, you more times the helium will Since the Martian atmosphere is about 100 times less
cool down helium gas to a few become cold enough to turn to a dense than Earth’s, the ‘lift’ a frisbee experiences
degrees above absolute zero. superfluid. ML would also be about 100 times less. But the
gravitational force on Mars is about a third of that on
Earth, so a frisbee on Mars would act as if it is about
33 times heavier (100/3). Since the lift depends on
the size of the frisbee, the angle of attack and the
velocity it is thrown (as well as the air density), it
would still be possible to make a frisbee glide, but it
would require much more effort on the part of the
thrower! AGu

THE THOUGHT EXPERIMENT

WHAT WOULD HAPPEN IF ALL EARTH’S INSECTS VANISHED?

PHOTOS: GETTY X5 ILLUSTRATIONS: RAJA LOCKEY insects. Almost all birds eat insects, and even tomato, coffee, cocoa and most fruits would crops every year. In the US, pesticide residues
2. NO POLLINATION
3. LESS INSECTICIDE
1. FOOD CHAIN COLLAPSE
Most non-marine food chains depend on
On the plus side, if there were no longer any
Of the world’s food crops, 75 per cent are
insects, we wouldn’t need the 430,000
pollinated by insects. Without insects, we
tonnes of insecticides that are sprayed onto
those that eat seeds as adults still feed
could still grow many foods, but onions,
insects to their young. It takes 200,000
cabbage, broccoli, chillies, most varieties of
insects to raise a swallow chick to adulthood.
cause between 4,000 and 20,000 cases of
Insects also break down plant matter and
be off the menu. So would sunflower and
cancer each year, according to the National
Academy of Sciences. But this is a small
rapeseed oil. Demand for synthetic fibres
help recycle nutrients into the soil. Without
compensation for total ecological collapse
would also surge because bees are needed to
any insects at all, most bird and amphibian
and global famine.
pollinate both cotton and flax for linen.
species would be extinct in two months.
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