jeheron

I recently attended a popular science lecture at the University of Sydney. The speaker said that if, given enough time, you travelled in a straight line out into space, you would eventually arrive back where you started from as if space was circular and you had circumnavigated it. Is this remotely correct?

Two parallel wires are separated by a distance of 6 cm. Wire 1, on the left, carries current into the page. An equal current flows in wire 2 out of the page. What direction is the net magnetic field at point A, half between the wires? In our Physics course we are given the equation F=Eq, and Ed=V. I have tried for 20 minutres using the information we have been given to work out the answer. How do you do this question?

How do NPN and PNP transistors work. I understand n-type materials and p-type materials as well as the workings of a p-n junction in terms of doping and electrons and electron holes working as charge carriers. Jack

In terms of the proerties of the materials themselves. I am asking the question to better understand each individual kind of semiconductor matieral before moving onto p-n junctions and npn and pnp interfaces.

Is there any observable difference between the physical and electrical properties of n-type semiconductor materials and p-type semiconductor materials?

How does the behaviour of semiconductors vary with different densities of electrons or electron holes?

In terms of semiconductors, and electrons moving from the valence to conduction bands, how do the remaining electron holes contribute to electrical conductivity? Also, can anyone provide a short qualitative description of the properties of an electron hole as I am not totally familiar with the concept?

1) I am completing my last year of secondary school in Australia, and energy bands are included in the course syllabus. This however is not my only motivation for asking the question I have. My passion is for physics, and to me, this is very interesting. 2) I have read a lot of superficial material that discusses QM, but nothing to deep. So my knowledge is limited. I am always looking to learn more though. In regards to this topic, I have read into the Pauli exclusion principle, and energy levels, in an attempt to understand energy bands.

In what materials/states of matter do energy bands form? What determines whether an energy band will exist? For example, do energy bands ever form in gases? Cheers!

I have recently started studying the photoelectric effect. I currently have very little understanding of the theory. As i understand it retarding voltage is a voltage applied to the anode and the cathode of a photovoltaic cell such that any photoelectrons emitted are decelerated just enough such that they don’t reach the anode. This is a very superficial understanding, and could be totally wrong. Can anyone help me qualitatively refine my understanding? Also, why is a retarding voltage ever used? Jack.

Ok, so in a crystalline substance there are billions of atoms and molecules. Their energy levels, due to the Pauli exclusion principle, cannot all have the same value, as a result they each vary slightly. This creates bands of electron (or moledule?) energy. Is that right?

What is the difference between an energy band and an energy level (how, if at all, are they related)? (In summary) my current understanding is that an energy level is "any of the possible discrete energies of an atom, molecule, or nucleus." and that energy bands have something to do with semiconductors. Hopefully very soon my understand will be much greater.

I am a high school student in Australia, we briefly discussed Maxwell’s equations in class however what you have said here does not helpfully answer my question (from me at least). It seems to me as unhelpful as saying that the fundamental quality of the universe that causes gravity is an equation Newton published in 1679. I would like to believe Maxwell’s equations, rather than being the fundamental origin of electromagnetic radiation, merely (although amazingly elegantly) describe this radiation. The words "accelerating charges" however begin spark thought in my mind. An accelerating charge induces a magnetic field, which in turn induces an electric field, ad infinitum. Is this perhaps the origin of EM radiation (although not the fundamental origin of course)? Thank you very much Klaynos, for your reply, Jack

What is the fundamental origin of electromagnetic radiation?

Thanks for your help. I just found the source the question was taken from. In the copy of the question we were given the photocopier had cut off "The plates are 0.1m lin length." That changes the method of calculation, making it alot easier. Thanks again.

I was given this question. I have worked through it and get the answers below. I am unsure as to if they are correct or not. The method I have used was never discussed in lectures. An electron with v= 3 x 10^6 m/s enters a region of uniform electric field between two charged plates. The plates are 0.1 meters apart. The diagram included showed the electron entering the field between the plates from the side such that its direction was perpendicular to the field lines which flowed from the bottom plate (drawn horizontally) upward to the top plate. The field is uniform (E=200 N/C) a) Find acceleration: MY WORKING: F= qE = (1.6*10^-19)(200) = 3.2*10^-17 N therefore using F= ma a= F/m a= (3.2*10^-17)/(9.11*10^-31) a= 3.513*10^13 m/s b) Find time electron in field: Assuming the electron enters the field halfway between the plates (from diagram): distance travelled by electron (d) = (ut) + (1/2at^2) t= sqrt((2d)/a) = 1.7*10^-7 s Is my reasoning sound? I am high school student studying in Australia. Cheers in adv.

This may be an inappropriate place to post this question. I will ask anyway. How do you get the small number images to appear when you write an equation?

With root 4 its way to easy.

That equals 18 (I may be wrong), I think your order of operation is wrong. Try: 4!x(4/4+4/4)-((4!)/4))

Its so reassuring to know that I can calculate amounts of energy in terms of stones and furlongs. Thank you.

Why, when calculating values with this equation do we use the speed of light. And why do we use the value: 300000 and not the speed of light in meters per seconds or cms per second? There may be an obvious answer, but I can find it.

The magnetic field does not protect the earth from solar flares. It simply guides the ions of the solar wind to the polar regions, where their energetic collisions with the upper atmosphere produce the luminous plasma known as the Northern and Southern Lights (Aurora Borealis and Aurora Australis). It is the earth's atmosphere that actually prevents the solar wind from reaching the surface, and that is independent of the magnetic field. written by: Richard E. Barrans Jr. CHM/200/M019 Argonne National Laboratory 9700 South Cass Avenue Argonne, IL 60439 found at: http://www.newton.dep.anl.gov/askasci/env99/env034.htm

The Higgs boson is a hypothesised particle which, if it exists, would give the mechanism by which particles acquire mass. Matter is made of molecules; molecules of atoms; atoms of a cloud of electrons about one-hundred-millionth of a centimetre and a nucleus about one-hundred-thousandth the size of the electron cloud. The nucleus is made of protons and neutrons. Each proton (or neutron) has about two thousand times the mass of an electron. We know a good deal about why the nucleus is so small. We do not know, however, how the particles get their masses. Why are the masses what they are? Why are the ratios of masses what they are? We can't be said to understand the constituents of matter if we don't have a satisfactory answer to this question. Peter Higgs has a model in which particle masses arise in a beautiful, but complex, progression. He starts with a particle that has only mass, and no other characteristics, such as charge, that distinguish particles from empty space. We can call his particle H. H interacts with other particles; for example if H is near an electron, there is a force between the two. H is of a class of particles called "bosons". We first attempt a more precise, but non-mathematical statement of the point of the model; then we give explanatory pictures. In the mathematics of quantum mechanics describing creation and annihilation of elementary particles, as observed at accelerators, particles at particular points arise from "fields" spread over space and time. Higgs found that parameters in the equations for the field associated with the particle H can be chosen in such a way that the lowest energy state of that field (empty space) is one with the field not zero. It is surprising that the field is not zero in empty space, but the result, not an obvious one, is: all particles that can interact with H gain mass from the interaction. Thus mathematics links the existence of H to a contribution to the mass of all particles with which H interacts. A picture that corresponds to the mathematics is of the lowest energy state, "empty" space, having a crown of H particles with no energy of their own. Other particles get their masses by interacting with this collection of zero-energy H particles. The mass (or inertia or resistance to change in motion) of a particle comes from its being "grabbed at" by Higgs particles when we try and move it. If particles no get their masses from interacting with the empty space Higgs field, then the Higgs particle must exist; but we can't be certain without finding the Higgs. We have other hints about the Higgs; for example, if it exists, it plays a role in "unifying" different forces. However, we believe that nature could contrive to get the results that would flow from the Higgs in other ways. In fact, proving the Higgs particle does not exist would be scientifically every bit as valuable as proving it does. These questions, the mechanisms by which particles get their masses, and the relationship amongs different forces of nature, are major ones and so basic to having an understanding of the constituents of matter and the forces among them, that it is hard to see how we can make significant progress in our understanding of the stuff of which the earth is made without answering them. By Mary and Ian Butterworth, Imperial College London, and Doris and Vigdor Teplitz, Southern Methodist University, Dallas, Texas, USA. http://www.phy.uct.ac.za/courses/phy400w/particle/higgs1.htm

What are the consiquences of a magnetic pole shift?

In reply to explaining the force of gravity... we are still waiting for the Higgs Boson