ÐÓ°ÉÔ­´´

Q: When I take a vacuum flask full of a watery beverage such as tea
or coffee on a picnic, it is still hot hours later. However, if I take a
more viscous liquid such as soup, it cools far more quickly. Why?

* * *

A: The link between ‘heat’ (thermal energy in transfer) and temperature
is one of the most tricky concepts that students encounter when studying
physics for the first time.

The soup in the vacuum flask may start off at the same temperature as
the tea, but the soup’s temperature falls faster for the following reasons.

When thermal energy (heat), measured in joules, is transferred from
the soup to the surroundings outside the flask, the temperature of the soup
falls according to the following equation: thermal energy trans-ferred (joules)
= mass of soup (kilograms) X specific heat capacity of soup (joules per
kilogram degree C or J/kg degree C) X change in temperature of soup (degree
C).

The important factor here is the specific heat capacity of the soup,
which is the number of joules that are needed to change the temperature
of 1 kilogram of soup by 1 degree C. Water (and hence tea) has a very large
specific heat capacity: 4200 J/kg degree C. Fats (and the solid matter in
the viscous soup) have much lower values and hence their temperatures drop
far more for the same thermal energy flow through the flask. Such foods
also heat up and cool faster in all situations. A good example is full-fat
milk, which boils much more rapidly on a stove than water.

The potato’s high water content makes it an ideal reservoir of thermal
energy, hence the hot potato, so useful for warming hands on 5 November.

Richard Horton Harrogate, North Yorkshire

* * *

A: Thanks to the hydrogen bond’s unique characteristics, water has an
anomalously high specific heat capacity. You need to extract more energy
from a kilogram of water than from a kilogram of any other material to
cool it by 1 degree C.

Thick soups contain a substantial proportion of starch, fat and fibre,
all of which have much lower specific heat capacities than water, so they
cool more rapidly than tea kept in the same container. Wise outdoor types
carry flasks of hot water and packets of dry additives: less tasty than
home cooking, but hotter on arrival.

Beware of French onion soup. The heat capacity of the clear, watery
base which is protected by a floating, anti-evaporation oil film, can still
blister the unwary diner while companions are eating their third helping
of viscous bouillabaisse.

* * *

Alan Calverd Bishop’s Stortford Hertfordshire

In the green

Q: The benefits of camouflage would suggest that there should be green
mammals. Are there any – and if not, why not?

* * *

A: There is only one green mammal, the three-toed sloth. This is because
a coat of algae covers the sloth’s fur. Because of the sloth’s tardiness
and lack of personal hygiene, this is never cleaned off. No known mammal
is capable of producing its own green epidermal pigment. The main reason
for the absence of green mammals seems to be an ecological one. In general,
mammals are simply too big to use a single colour for camouflage as there
are no blocks of green large enough to conceal them. Most mammals have an
environment that is made up of patches of light and dark and composed of
many different colours. This means that those mammals which are camouflaged
tend to be dappled or striped. Animals that do use green coloration for
camouflage, such as frogs and lizards, are small enough to use solid blocks
of green – leaves and foliage – for cover.

Paul Barrett Department of Earth Sciences University of Cambridge

* * *

A: The main predators of most mammals are other mammals, especially
the carnivores, such as the cat, dog and weasel families. Carnivores are
all colour-blind or, at best, have very limited colour vision. Hence effective
camouflage against them is not a matter of coloration but of a combination
of factors such as brightness, texture, pattern and movement.

Graeme Ruxton Scottish Agricultural Statistics Service Edinburgh

Head cold

Q: Whenever I eat something cold, such as ice cream, too quickly, I
get a headache which lasts for a few seconds. This doesn’t happen when
I eat something hot too quickly. Why?

* * *

A: Blood vessels in your mouth and nose contract when you eat something
very cold, in the same way as they would contract near your skin when the
temperature outside the body drops. This is to prevent large heat losses
from the body via the blood.

The contraction of vessels in the nose and near the forehead gives you
a headache as local blood pressure rises. However, hot food dilates the
vessels and you do not feel anything.

Ann Kotomin Billericay, Essex

This week’s questions – Shady past

Shady past: Did Antoine Lavoisier invent sunglasses? Paul Board describes
how Lavoisier was helped in his work by ‘a large burning lens that belonged
to the French Academy of Sciences’ (‘The aristocrat who revolutionised chemistry’,
New ÐÓ°ÉÔ­´´, 7 May). I recall seeing a copy of a contemporary illustration
of the scientist standing alongside the giant lens. He appears to be wearing
sunglasses to protect his eyes. If he was not the first person to use them,
who did invent them?

Peter Barnes Oxford

This week’s questions

Round water: Occasionally, when running water is falling into a container
with a large surface area, such as tap water into a washing-up bowl, separate
droplets form, which run around on the surface of the water. Anyone who
has noticed this phenomenon will be aware that it is not a misinterpretation
of bubbles – the characteristics of the droplets are totally different.
They have virtually no friction and run around very readily. They even bounce
off the side of the bowl. They are also relatively short-lived; it seems
that they can only exist while moving reasonably quickly, as they seem
to disappear by absorption into the main body of water as they slow down,
or shortly after they become stationary. Can anyone explain this?

D. W. Evans Allens Nek, South Africa

This week’s questions – Spins doctored

Spins doctored: Does anyone know why 78 revolutions per minute was chosen
as the standard rotation speed of old-fashioned gramophone records, rather
than a round number, such as 75 or 80 rpm? And are there convincing explanations
for the choice of speeds for later EPs and LPs of 33 1/3 rpm and 45 rpm?

James Lee Oisterwijk, The Netherlands

This week’s questions – Primary school

Primary school: I was taught at school that the three primary colours
are red, blue and yellow and that by mixing these, all the other colours
could be made. If you look closely at a television screen you can see that
the picture is made up of red, blue and green dots. Why and how do tele-
visions use these colours and how, for instance, do they make yellow from
them?

John Hawkins University of Canterbury Christchurch, New Zealand