Questions and Answers
Some of the questions that people have submitted and their answers:
QUESTION:
All things, to my knowledge, in physics, if "filmed" and the film run backwards remain credible. Most things in biology if filmed and the film run backwards are not credible. Are there any examples in physics of NON reversible causation?
ANSWER:
Reversability in physics ... that's a very interesting question.
It is true that many of the problems given in high-school and first year
university are completely reversable. This is because they are mainly
linear problems (which have simple solutions)... however in "real" life
very few situations are completely linear and non-linear or chaotic effects
introduce irreversability. For instance, you might be familiar with the
typical "conservation of energy" problem of an elastic collision -
eg a ball bouncing on a hard surface, where all the kinetic energy of
the ball is converted to potential energy stored in the deformation of
the ball and then re-converted to kinetic energy as the ball
"un-deforms". However, in reality, energy is dissipated as heat when the
ball deforms, and in friction with air and the surface and so the ball loses
energy at each bounce and gradually stops bouncing. This gradual loss of energy
is described by laws of thermodynamics, and particularly in the law
which says that the entropy "or dis-orderness" of a system always has
a net increase in time.
Some examples of irreversable actions include:
- dropping a bean bag (it doesn't bounce since all its kinetic energy is absorbed by movement of the beans)
- evolution of weather systems (due to chaotic influences on their development)
- a supernova exploding!
1st - Is it possible to knock a proton out of an atom with a fast moving electron? 2nd - Do you know where I could find some info on those moving balls of glowing gas - ball lightning? 3rd - Do you know anything about those new plasma screens and TV's that are comming onto the market these days and how they work?
ANSWER:
1st question:
As far as I'm aware it isn't. Protons and neutrons are held together in the
nucleus of an atom very, very tightly by two forces, known as the weak and the
strong nuclear force. Even a *very* fast moving electron doesn't have
enough "energy" to counter-act these forces. Nuclei can only by split
by hitting them with other, very fast-moving, nuclei (or protons).
2nd question:
I'm afraid that very little is known about ball lightning
(even whether it is a real phenomena, or only a "mis-observation"
due to other effects, such as visual phantoms caused by looking at a
lightning strike).
There are some interesting web sites on
lightning, for example:
The Lightning Page
(if you can cope with the religious commentary that goes with it);
Chuck Doswell's
Lightning Photography
page and
Storm Stalker
have some great photos.
3rd question:
Not much, but there's heaps of info on the web
about Flat Panel Displays (of which Plasma Display Panels are a subset)
for tv screens etc. See f'rinstance DTM
or Plasmavision
QUESTION:
Our Grade 9 girls are investigating the effect of 50
Hz electromagnetic radiation on the growth of grass seeds.
We are using about 50 coils of copper wire carrying a current of 1.00 A
wrapped around a wooden dowel. Thsi is suspended about 5 cm above the
soil surface.
Would you be able to explain to us how to calculate the intensity of the
radiation at any distance from the coil?
ANSWER:
Electromagnetic radiation consists of
both electric and magnetic fields - when you say you want to know the
radiation intensity do you mean you want to know the electric or (and) the
magnetic field strength? As you're using a solenoid (a series of coils)
I'll assume that
you're primarily interested in the magnetic field, since solenoids only
produce weak electric fields. (nb if you want strong electric fields you
need to use a pair of plates, like a capacitor, with high voltages ...
which might not be entirely suitable for Year 9 students!)
I've written a web page which explains how to calculate the magnetic field on the axis of the solenoid, both inside and and outside the solenoid. HOWEVER, it's very difficult to calculate the field off-axis (since you lose the advantage of symmetry). If you would like me to numerically calculate these fields for you (and send you a plot of the fields as a function of position) let me know and I'll see what I can do.
Best of luck with your experiment! By the way, have you seen the web site Visual Physics - it has some nifty simulations of physics experiments.
QUESTION:
I am a year 10 student and we have a science research assignment which we
have to conduct. For my assignment i chose to do the absorption of sound
by different materials. I tried to do this by setting up a shoe box with
the material and decibel meter on one side and the speaker and signal
generator on the other. By finding the background sound then the
transmitted sound behind the material i intended to find out how much
sound was actually absorbed or really absorbed and reflected.
My problem is that i am unable to get a steady reading, the signal is
always jumping. Do you have any suggestions for how i could solve this
problem, or any other experiments that could be conducted to find out the
same things?
ANSWER:
Okay, for a start you obviously want to get the "background" noise down as
low as possible. HOWEVER, sound reflects well from smooth surfaces so you
probably have lots of echos bouncing around inside your shoe-box. You could
try lining your box with soft, fuzzy material or crumpled up toilet
paper (for a bigger box cardboard egg containers are good) Try and do
the experiment in a quiet room to reduce background noise. Maybe try a
cardboard shield around the experiment if you can't go somewhere quiet.
Then try and keep everything as close together as possible to maximise your signal (don't forget to leave enough space to put your "material" in between the generator and the meter). Have you tried detecting the signal without anything between the generator and the decibel meter - can you get a good signal? Adjust it to get a maximum reading and then put your material in between (nb you should have already done a measurement of the background noise). Make sure the material you chose isn't too smooth and "reflective" so that you have a reasonable transmitted signal - some possibilities are a mesh bag (eg that onions or oranges come in), tissue paper, a sheet of writing paper (try a smooth piece first and then try crumpling it), various types of cloth.
If that works see what happens if you put a smooth surface in the way - can you pick up a reflected signal from the side? Make sure you aren't picking up the signal generator directly, by putting a shield between the generator and the meter.
I hope that helps. Good luck with your project!
QUESTION:
Is plasma a conductor?
ANSWER:
Yes a plasma is a very good conductor. Plasmas are typically
glowing gaseous bodies, which contain ions and electrons (and normally some
neutral
atoms). The ions and electrons are able to move freely in the plasma and
so they can carry currents in applied electric fields. One method of
understanding plasma behaviour, is to represent the plasma as a series of
circuit elements (resistors, capacitors, diodes) connected together.