That鈥檚 the rub
Question: I am an old-fashioned designer and work in pencil. I have noticed
over the years that a line recently drawn on paper can easily be rubbed out with
a conventional eraser but that the same line if left for a few weeks becomes
virtually indelible and even the hardest rubbing still leaves a vestigial mark.
What is occurring here?
Answer: I suspect that there are two effects at work. Pencil graphite
contains wax and while this wax is still soft, minute particles of graphite
slowly get absorbed into hollows in or between the paper fibres. Next, the wax
slowly oxidises and hardens, sticking the dark particles to the paper. The
pigment is then both harder to rub off and harder to get at with the rubber.
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Jon Richfield
by e-mail, no address supplied
Tap dancing
Question: Why do we tap our feet in time to music?
Answer: One answer to this may be found in a recent theory of rhythm
perception. The theory suggests that the perception of rhythm involves the motor
system just as much as the sensory system.
It postulates that a 鈥渂eat鈥� is actually perceived as a movement (something
which dance fans may well agree with).
The theory then goes on to suggest that because a beat is perceived as a
movement, the activation of a stereotyped behaviour such as tapping your foot in
time with the beat is a natural extension of the way motor and sensory systems
often work together to produce a percept.
Duncan Brown
by e-mail, no address supplied
Answer: Speaking from a musician鈥檚 point of view, when I was learning to play
the clarinet (my first instrument) at the age of 10, my teacher taught us to tap
our foot to keep time. Now I do it even when I鈥檓 not the one performing. Some
habits are hard to break.
Lena
by e-mail, no address supplied
The heat is on
Question: What is fire made of? What is its atomic structure, what causes
things to burst into flame in the first place and why can鈥檛 all materials be
made to produce flame?
Answer: Fire involves a chemical reaction between fuel and atmospheric
oxygen. Once initiated it is self-sustaining, generates high temperatures and
releases a combination of heat, light, noxious gases and particulate matter.
The visible flame is the region in which this chemical process occurs and so
flame is essentially a gas phase phenomenon. For flaming combustion to occur,
solid and liquid fuels must be converted into gaseous form.
For liquid fuels this is achieved by evaporative boiling. For solid fuels,
the solid is chemically decomposed through the process of pyrolysis to generate
volatile gases.
Ed Galea
University of Greenwich
Answer: A flame is a region containing very hot atoms. At high enough
temperatures all atoms will emit energy in the form of light as their electrons,
which have been prompted to higher energy levels by absorbing heat energy, fall
to lower energy states.
Because this light is emitted in discrete quanta according to the
relationship E = hv (where E = energy, h = Planck鈥檚 constant and v = frequency),
flame colour is related to the magnitude of the energy quantum which is
transformed to light.
This can most easily be seen with a Bunsen burner. A Bunsen burner that has a
choked air supply burns cool, the light emissions from carbon atoms are
relatively low in energy and appear more red or orange. However, when the Bunsen
is allowed air so that combustion is complete, the flame is hotter and the light
emitted is of a higher energy and frequency and appears blue.
The luminescence of a flame is only half of the story. The structure of the
flame region is important to understand too. The flame area in a normal
combustion environment, such as an open-air bonfire, is structured by convection
currents which form as hotter, lighter air rises and allows cooler fresh air to
replace it. It is this channelling effect and movement of air that shapes the
dancing flames. It is interesting that in space, in zero gravity, the hotter and
cooler air cannot move by convection, so flames take on weird shapes and may be
stifled by their own combustion products.
Roger Doonan
London
Microlight
Question: When a plate with a gold decorative edge is placed in a microwave
oven, sparks appear at the gold surfaces. Why?
Answer: I must refute a previous answer to this question. Microwaves are not
a very energetic form of radiation. They are far less energetic than optical
photons or even infrared photons.
Free electrons in a metal are not buoyant and so do not 鈥渇loat鈥�. Metals
exclude radiation because the free electrons are able to screen out incident
fields.
The sparks are not 鈥渟mall pieces of metal that are heating and exploding鈥�.
They are actually caused by the ionisation of the gas (air) atoms in the high
field regions near to the metal asperities.
Finally, it is incorrect to say that water molecules are the same size as a
microwave wavelength. Water molecules are almost 0.2 nanometres while microwaves
are of the order of centimetres.
J. Samble
Crediton, Devon
Answer: Water molecules are not 鈥渋nduced to rotate back and forth, causing
frictional heating of your food鈥�. Frictional heating is what makes blocks of
wood hot when you rub them together as Scouts and Guides are taught to do to
make a camp fire.
At the molecular level we see increased motion when things get hot, not the
other way round. So the motion induced by the microwaves does not cause
frictional heating鈥攊t is the heating鈥攁nd the more motion
there is (rotation or vibration), the hotter the food becomes.
Clive Morton
by e-mail, no address supplied
This week鈥檚 questions
Shafted: If you find yourself in a free-falling lift is there any action that
you can take to reduce the effect of the collision? Would jumping just before
you hit the bottom of the lift shaft help?
Nigel Osborn
Amersham, Buckinghamshire
Aaaaaargh: People鈥檚 hair is sometimes reported to have turned white overnight
as a result of shock. Can white hair be the result of shock and, if this is the
case, how long might it take? What is the process involved?
Sydney Gilbert
York