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!
QUESTION:
ANSWER:
If you remove an electron, which has a negative charge, from an atom you
produce an ion, which has a positive charge. A plasma is a gas which has a
substantial percentage of ions and electrons (charged particles) present
(that percentage can be anything from ~5-100%). Typically plasmas are
produced by adding energy to a gas until it "breaks down" - that is a
sufficient number of electrons and ions are produced. Plasmas are
sometimes referred to as the "4th state of matter" - since adding energy
(heat) to a solid gives you a liquid, if you continue to add energy you
get a gas, and finally a plasma. It's not *quite* as straightforward as
that, but the principle is right. In order to keep the ions and electrons
separate and stop them coming back together some form of energy must be
continually supplied. Typically plasmas glow, because some of the energy
they absorb is turned into light by collisions between electrons and
neutral atoms.
Now, because a plasma has both ions (positive charge) and electrons
(negative charge) it has no overall charge, BUT the presence of charged
particles means that a plasma behaves very differently from a gas, where
the particles don't interact very much. In a plasma the particles interact
strongly because of their charges (opposite charges attract and similar
charges repel). A plasma can conduct a current, which a gas can't do, and
plasmas also react strongly to electric and magnetic fields. There aren't
many plasmas which occur naturally on earth, since the air we breath is at
a very high pressure which means that a *LOT* of energy is needed to break
it down. However, in the very, very low pressures occuring in space
plasmas are readily formed and in fact > 99% of visible matter in the
universe is in the form of a plasma.
Examples of naturally occuring plasmas include stars, the solar wind,
aurora and lightning. I'm not certain whether fire is technically
a plasma, although it certainly has charged particles present, and it
converts the chemical energy in wood and other flammable substances
into heat and light. Man-made plasmas include arc-welders, fluorescent
lights, and a plethora of special-purpose industrial and research
devices for producing plasmas.
As for Bose-Einstein condensation, I'm afraid that's way, WAY out of my
field. Bose-Einstein statistics relate to energy distributions of atoms
occuring at very high gas densities, calculated using quantum theory.
Bose-Einstein condensation occurs when high density gases are cooled to
very low temperatures, producing very, very cold atoms. In this regime all
sorts of strange quantum effects come into play. I'm afraid I don't know
any more than that. If you're interested in very, very low temperature
atoms have a look at the
Atomic manipulation project at the Research
School of Physical Sciences
QUESTION:
ANSWER:
The reason that electrons and ions don't recombine in the plasma (at
least not to a significant extent) is that when you supply energy to the
plasma you heat up the electrons to tens of thousands of degrees. This
does two things:
(1) these very hot electrons move very quickly ... about 10^6 m/s, which
means they could travel from the earth to the moon in 2 minutes. Because
the electrons are moving so quickly (thousands of times faster than the
ions) it's difficult for the ions to trap them to form neutral atoms
again.
(2) when electrons hit neutral atoms, which are moving very, very slowly
in comparison, they can sometimes have enough energy to knock out one of the
electrons in orbit around the atom, forming a free electron and a
positive ion. The new ion/electron pair help maintain the number of
charged particles in the plasma, replacing particles which are lost by
various processes.
In the type of plasmas that I work on the ions remain roughly at "room
temperature" and don't get hot like the electrons. However, if the right
conditions are supplied then you can also get very hot ions - similar to
conditions which occur in the sun. Because these plasmas are so hot, you
can't contain them in a metal or glass box, you have to use magnetic
fields to create a kind of "bottle". People use these plasmas to
research the possibility of fusion - fusing two hydrogen atoms to create
a helium atom, which releases lots of energy, exactly the process
which fuels the sun. Fusion reactors may, in the future, provide a new
source of electricity, which is cleaner than coal and oil and doesn't
produce the long-lived radioactive waste you get from fission reactors
(which use uranium).
As for naturally occurring plasmas on earth, I've listed all the ones I
could think of - lightning, aurora and possibly fire. Ball lightning -
free-moving balls of glowing gas - occurs very, very rarely during
electrical storms, although it isn't well studied due to the
difficulty of catching it. Aurora occur when streams of plasma emitted
by the sun, known as the solar wind, spiral down the earth's magnetic
fields to the poles (which is why aurora are only seen near the Arctic
and Antarctic) interacting with the atmosphere to produce glowing sheets
of plasma.
ANSWER:
2nd question:
3rd question:
QUESTION:
ANSWER:
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:
ANSWER:
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:
ANSWER:
I was wondering if you could explain what plasma
is? I am aware of the general differences between
solids, liquids and gases but I have always
been interested in what makes plasma different.
Once a teacher told me that fire or flame is plasma,
is this true? Also I am equally interested in the
research that is being done into bose-einstein
condensation so if you have any info on this
I would greatly appreciate it.
Please excuse me in advance if my explaination is too simplified
(or too complex) - it's difficult to determine in advance how much
background knowledge I can assume.
You have certainly enabled me to understand what a plasma is, but
unfortunately you have now made me more curious and wanting to ask more
questions. :). Firstly I was wondering what process is involved that
stops the electrons from rejoing the atoms. And the other question I
have is that you said that there are not many natrually occuring
plasma's on earth, well I was wondering if you could tell me of any that
do exist.
I'm glad you find science interesing! Ask as many questions
as you like - after all science is about asking questions and
looking for the answers.
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?
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).
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.
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
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?
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 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?
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.
Is plasma a conductor?
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.