TVs on strike
Question: A lightning bolt recently struck the ground near my home. I could
accept the fact that local telephones were damaged, fuses were blown and
computers burnt out, but other strange things happened.
Some television sets switched on spontaneously and showed perfect pictures,
yet those that were switched on normally after the strike had an odd colour
shift. Red became green, white became yellow and green became blue. If you
switched them off for a minute, then on again, the problem was cured. Why did
this happen?
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It might not be significant, but at the time of the strike, burglar alarms
sounded spontaneously.
Answer: The peculiar results of the lightning strike can be explained by a
mixture of electromagnetism and economics.
A TV picture is drawn on the screen by an electron beam. The beam is steered
across the glass by two magnetic fields for horizontal and vertical positioning.
It is translated into a visible glow by phosphors coated on the inside of the
screen. The more intense the beam, the brighter the glow from the selected
pixel.
Lightning strikes involve peak currents of approximately 100,000 amps. This
abrupt current makes a strong magnetic field, which apparently magnetised the
steel in your TV. The effect of having a magnetised TV is to add a constant
offset to the steering of the electron beam. This might cause the beam to miss
the spot intended and land on an adjacent site.
Each pixel on the TV is divided into three, one third for each primary
colour, so along each horizontal line you could have blue-red-green,
blue-red-green. In this case a shift of one place to the right would send red to
green, and green to blue. White to yellow could be explained because the blue
phosphors are relatively inefficient compared with the green and red phosphors.
An electron beam capable of generating a bright green would only produce a dim
blue, and the overall white would be tainted yellow.
The reason that your TV contains steel is economic because steel is
comparatively cheap and easy to manufacture compared with non-magnetic
counterparts. Because many things can magnetise your TV, manufacturers include a
鈥渄egaussing鈥� coil that demagnetises the bodywork each time the TV is turned on
by producing a transient oscillating magnetic field. The 鈥渢hunk鈥� noise you hear
when you turn on a TV is this degaussing. The TVs鈥� colour returned to normal if
they were switched off and switched on again after the strike because they
automatically demagnetise themselves at power-on, wiping away the lightning鈥檚
effect.
Richard George
Cambridge
Answer: The TVs that switched on were probably in standby mode. This means
they are not actually switched off, but running in a very low-power state,
waiting for a signal from the remote control to switch them fully on. A local
lightning strike could easily induce a strong enough false signal into the
circuitry to be mistaken for the 鈥渟witch on鈥� message.
Mike Harrison
Loughton, Essex
Marine boy
Question: Many science-fiction books and films feature humans that have been
altered so they can breathe underwater. However, none of the mammals that have
returned to the sea have evolved gills. Why is this? Do warm-blooded creatures
such as dolphins burn so much energy that they couldn鈥檛 meet their oxygen
requirements from water using gills?
Answer: Warm-blooded mammals do need more oxygen than equivalent-sized
cold-blooded fish, and the oxygen concentration in water is much lower than in
air.
Given that a mammal might need 10 times as much oxygen as a fish of the same
size, and that water can contain 20 times less oxygen than air, a mammal able to
extract dissolved oxygen from water would have to process this water at a very
high rate. This would require very energetically expensive pumping, or
continuous forward motion to drive water across the gills.
However, the real restriction on an aquatic mammal using water as an oxygen
source is the problem of heat loss. In gills and lungs, oxygen diffuses into the
blood, but at the same time heat will diffuse from the blood into the water (if
the blood is warmer than the water). Heat diffuses much more readily than oxygen
does, and so in order to take sufficient oxygen from water, an aquatic mammal
with gills would lose a great amount of heat. This heat loss could not be
compensated for by metabolically generated heat, and so the animal could not
sustain the high and stable body temperatures characteristic of mammals.
This is why fish cannot sustain a body temperature greater than the
temperature of the water around them. The only exception to this are some tuna
that, by a complicated system of heat exchangers, cool blood on its way to the
gills and reheat it on the way back as a means of reducing heat loss. This
allows part of their body to be a few degrees higher than the water around them.
Such heat exchangers could not provide a temperature differential like that
between marine mammals and seawater, even in tropical oceans.
Graeme Ruxton
and Stuart Humphries
University of Glasgow
This week鈥檚 questions
Rough read: Some time after applying a transparent, adhesive black plastic
sheet to a book cover, a raised, convoluted pattern appeared. I have
seen it on other surfaces covered in adhesive plastic. What causes it?
W. H. Kennedy
Wolverhampton, West Midlands
On silent wings: How can paper aeroplanes fly? They seem to have no
aerodynamic features to produce lift, and the paper plane with the world record
for distance has the smallest wing surface I have seen on any plane.
Benidikt Walker
Shepperton, Middlesex
What goes in: Is there a formula for working out how much excrement is
produced from a certain amount of food? For example, if you ate 1 kilogram of
food, how much excrement would it produce? And how much faeces will the average
adult human produce each day and what is its composition?
Nigel Watkins
Epsom, Surrey
and David Baxter
Salford, Lancashire