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THE LAST WORD
Dangerous current
In a bin full of old batteries awaiting
recycling, what is the probability
that a closed circuit will form,
allowing electrical current to flow
and potentially cause a fire?QIt depends on the batteries.
If they are cylindrical ones with
insulation around the outside,
then I don’t think it would be
possible in a normal-sized bin.
There would have to be a circle
of batteries with their ends
touching. The circle would have
to be quite big so that the angle
between any two batteries isn’t
too large for contact to occur.
Eric Kvaalen
Les Essarts-le-Roi, FranceQThe main danger is from
batteries with terminals on the
same end, such as the rectangular
9-volt battery. The terminals of
two batteries could come into
contact, or the circuit could be
completed by another conducting
object. For example, a few years
ago I felt something getting very
hot in my trouser pocket. It was
a 9-volt battery shorted out by
a 2-penny piece.
Derrick Grover
Haywards Heath, West Sussex, UKQI replace the PP9 batteries,
which have both electrodes at
one end, in my home fire alarms
annually regardless of how near
they are to the end of their life.
I once put a handful of them in
my pocket to take to work, where
there is a recycling facility. By thetime I arrived, they were too hot to
handle. Left any longer, my jacket
may well have ignited.
David Drew
Sheffield, UKQIt is easy for the terminals of
lead-acid automobile batteries to
make contact with the opposite
polarity terminals of another and
cause a high current to flow. This
is the case even if the batteries
are weak enough to be sent for
recycling. But the odds of a fire
are still rather low, depending,
of course, on what else may be in
that bin. Acid leakage would be
a greater concern.
Nickel-cadmium and lithium-
ion batteries of the sort generally
used in mobile phones and power
tools have terminals designed to
avoid unintended contact, so the
risk there is quite low. There have
been instances of lithium-ionbattery fires, but these were
caused by internal flaws.
Howard Bobry
Nehalem, Oregon, USQI used to keep steel wool in the
same cupboard as some spare
batteries. I was about to accuse
family members of leaving a torch
on in the cupboard when I realised
that the iron fibre was on fire,
set alight by the batteries. This is
unlikely to happen with a bin of
cylindrical batteries on their own,however, because you need a
conductor, like my iron fibre,
to make a circuit.
Keith Ross
Villembits, FranceIcy grip
When water freezes on the surface
of rock, metal, plastic and many other
materials, it can be very difficult to
remove. What makes the bonds so
strong? What materials bond most
strongly with ice?QThe strength of the bond
depends on the roughness of
the material. Fingers of ice grow
into the cracks and irregularities
on the surface of the contact
material. The more cracks, the
stronger the bond.
For example, glass is smooth,
so although bonding is strong
enough for ice to remain on
vertical panes, it is easily lifted
off in fairly large sheets. On wood,
the ice bonds are strong enough
to break the wood itself.
Brian King
Barton on Sea, Hampshire, UKQThe strength of ice adhesion
depends on the chemical
composition of the surface.
Water molecules are electrically
polarised so adhere more readily
to materials that are attractive to
polar substances, such as metal
and rock, through hydrogen
bonding, and electrostatic and
van der Waals forces. Non-polar
materials, such as hydrocarbon
plastics, are less attractive so iceshears more easily from them.
Ice can cause problems for
aircraft, shipping and wind
turbines. The aviation industry
spends a fortune spraying planes
with de-icing fluid in wintry
weather, so there is a demand for
a permanent solution. For
example, researchers led by
Joanna Aizenberg of Harvard
University are working to create
porous, liquid-infused surfaces
that effectively flatten the surface
at the nanoscale level and make
it very difficult for ice to stick.
David Muir
Edinburgh, UKQWe often attribute stickiness
to glue, but it can also be achieved
by maximising the surface area
of a material. Many insects rely on
the same mechanism to cling to a
vertical surface. Minuscule hairs
on their feet create the largest
area possible and friction between
the hairs and the surface does the
rest of the work.
Emily Fox
Fortrose, Highland, UKThis week’s question
CLEARING THE AIR
The odour of a selection of cheeses
in our fridge was so overpowering
that I looked online for a solution.
As suggested, I left a small plate
of ground coffee in the fridge.
It worked like magic, absorbing
all the odours in a matter of hours.
How does this work?
Trevor Jones
Sheringham, Norfolk, UKWe pay £25 for every answer
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newscientist.com/lastword/“ The batteries in my pocket
were too hot to handle.
Left any longer, my jacket
may well have ignited”