# Search results

1. ### Why does perspective skew some lines and not others?

Ok so I think I've now convinced myself that if I were to shine a light at an opaque square (slanted so it casts a shadow), then the shadow it would cast would be narrower at the top than it is at the bottom. Is this diagram that I've drawn physically accurate? Is this the same principle...
2. ### Why does perspective skew some lines and not others?

Thanks for your help. So I've started to draw a ray diagram to try and help understand what you said. The green line is a side view of the square of paper on the table. Am I going in the right direction? From this diagram, how can I now get what the eye is seeing?
3. ### Why does perspective skew some lines and not others?

I'm still confused unfortunately. Lets take a very simple example. I know from life experience, that if I have a perfect square of green paper, lie it flat on a table, and put it a little distance in front of me, then look down on it - this is what my eyes will see...
4. ### Why does perspective skew some lines and not others?

In the above image, all the z-direction lines are converging at the vanishing point, as we would expect. Are you saying the x or y lines should be skewed too and converge too?
5. ### Why does perspective skew some lines and not others?

Imagine looking at a set of x-y-z axes, from directly along the z-axis. Say in this image, the road is the line z=0. Why is it, that only lines on or parallel to the z-axis get skewed? All the surfaces that are in or parallel to the x-y plane remain undistorted, and their angles are at 90...
6. ### LaTeX PS and Jpg in Latex Files

Hi everyone, I was wondering if someone could clarify how to put both .ps and .jpg files in the same latex file. At the minute I'm having to convert all my .jpgs to .ps using photoshop then compiling my latex file by going Latex->PS->PDF. However all my converted jpgs look all pixelated...
7. ### Net Flux from isotropic source

Hi everybody. Apologies for asking what may be a very simple question, but in every text book I've read, they say that the net flux through an area dA from an isotropic emitter is zero. But it also says that the sun is an isotropic emitter. Now hold on, the net flux from the sun is clearly...
8. ### Electron Transitions: Superposition of States

Thanks very much for that reply, it cleared a lot of things up. I'm just still a little confused about this time dependence In this video, it shows a transition from 2p to 1s. Let u2 = the wavefunction for the excited state and u1 = the ground state. At time 0, the total probability...
9. ### Electron Transitions: Superposition of States

It'd be great if you could help me clarify a few things in my head. Firstly I've got written in my notes "quantum mechanics forbids spontaneous transitions from one energy level to another because energy eigenfunctions are time independent". However this seems a bit of a circular...
10. ### Twin paradox without length contraction

Thanks for the replies, I'm understanding this much more. In response to JesseM, yes it is this return length that I'm having difficulty calculating. Here's my tentative method which gives an answer, is this the right way of doing it? Find the difference in speed between twin A and B by...
11. ### Twin paradox without length contraction

Hi all, apologies if this has been posted a million times before.... I'm trying to explain the twin paradox without getting involved with length contraction. One way to think of it is Twin A remains at rest on Earth then twin B goes off at 4c/5 to Alpha centauri 4 light years away, then...
12. ### Speed of light pulse if you have a velocity of c

Just to clarify, by light pulse, I mean for example a bulb is switched on and off sending out a sphere of light.
13. ### Speed of light pulse if you have a velocity of c

But how is it possible for the light pulse to catch up to you if you're 10m ahead of it and both travelling a c?
14. ### Speed of light pulse if you have a velocity of c

Homework Statement A light pulse is emitted in an inertial frame. You are moving at a constant velocity of c AWAY from the pulse, and initially 10m away from the pulse. Will the pulse reach you, and what velocity will you measure for c? Homework Equations The Attempt at a...
15. ### Perturbation in Infinite Square well

Homework Statement Calculate the 1st order probability an electron in the ground state of an infinite sqaure well (width 1) will be found in the first excited state t seconds after the pertubation H=sin(PI*x) is switched on. Homework Equations Transition frequency is omega_12 The Attempt...
16. ### Michelson Interferometer Pattern

Homework Statement Explain how a pattern of straight line fringes of equal thickness can be observed in a michelson interferometer. Homework Equations see diagram 1/3 of the way down page at http://www.holo.com/holo//book/book6&7.html [Broken] The Attempt at a Solution I really...
17. ### Microscopic Ohm's Law

Hmmm I guess I've spent so long doing electrostatics that I'm struggling a bit with the intuition of currents. So from the link above, am I right in thinking the E field is from within the wire caused by a potential difference from a power source?
18. ### Microscopic Ohm's Law

Homework Statement Explain the terms used in the microscopic version of Ohm's Law Homework Equations j=oE The Attempt at a Solution Well I know what we have here is the current density within a conductor is equal to its conductivity multiplied by an 'E-field'. But what is this...
19. ### Work: Sign Convention Problem

So using the convention work done BY the system is positive, is it correct to say if a piston is pushed out by a gas, the gas has done positive work on the surroundings. Does this mean work done by the surroundings on gas is negative? Are the two statements equivalent? BUT looking at it from...
20. ### Work: Sign Convention Problem

Ah yes but I guess the main thing that is confusing me is: do you do work ON the system when you PUSH the piston or is it when you PULL the piston and why? The only convention I've been told is "work done ON the system is negative".
21. ### Work: Sign Convention Problem

Homework Statement One mole of an ideal gas at 300K expands isothermally from a pressure of 2x10^6 Nm^-2 to a pressure of 2x10^5 Nm^-2. Calculate the work done ON the gas. Homework Equations dW = PdV The Attempt at a Solution Following through the integral, it becomes clear that work...
22. ### Magnetic Field Vector due to Infinitely long wire

Thanks for the quick response! What I was trying to get at, though, is there a way of putting that into some kind of notation with vectors? A cross product of the current vector and r-hat will give the direction of the B-field (I think!) but as I have never seen this anywhere, is it right?
23. ### Magnetic Field Vector due to Infinitely long wire

Hi everybody, I have quite a simple question here. I know that for an infinitely long current carrying wire, the magnitude of the magnetic field is given by (mu_0*I)/(2*Pi*R) where mu_0 is the magnetic permeability of free space. What I would like to know is what is the vector form of...
24. ### Electric Field due to a charged conducting finite cylindrical shell.

Ok then, but how can you calculate the field inbetween the two finite cylinders? You need the field so you can integrate it to get the potential, which you can then use to calculate the capacitance, which, I'm told, turns out to be C=(2*Pi*epsilon_0*Length)/log((second radius)/(first radius))
25. ### Electric Field due to a charged conducting finite cylindrical shell.

Hi everyone. I'm having a bit of trouble with finding an electric field. Basically, I'm trying to understand the formula for a cylindrical capacitor, so the method involves integrating the field between two conducting cylindrical shells. Firstly can Gauss's law be used in this case, because the...