Questions and Answers

Some of the questions that people have submitted and their answers:

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?

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:

  1. dropping a bean bag (it doesn't bounce since all its kinetic energy is absorbed by movement of the beans)
  2. evolution of weather systems (due to chaotic influences on their development)
  3. a supernova exploding!

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.

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

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.

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.

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).

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

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'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.

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.

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!

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.

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