# Search results

1. ### PN Junction at Equilibrium

phyzguy, A sketch would include four currents, two from the field (electrons move against the field, holes with the field) and two due to random thermal motions (electrons from P to N, holes from N to P). In equilibrium these effects cancel.
2. ### PN Junction at Equilibrium

Consider a PN junction doped with say phosphorous on the N side, and Boron on the P side. Initially, there is an opportunity for the electrons just below the N conduction band to drop to the lower available energy states just above the P valence band. This leaves the N side positively charged...
3. ### I Phase Speed of Wave in non-relativistic Doppler Shift Derivation

To answer my own question, we are comparing the frequency measured by the ground observer -- who is at rest relative to the medium air -- with that measured by an observer moving with the siren and at rest relative to the air. Since they are both at rest relative to the air, they will measure...
4. ### I Phase Speed of Wave in non-relativistic Doppler Shift Derivation

Consider the situation where an observer at rest on the ground measures the frequency of a siren which is moving away from the observer at speed ##v_{Ex}##. Let ##v_w## be the speed of the sound wave. Let ##\lambda_0##, ##f_0##, ##\lambda_D##, and ##f_D## be the wavelengths and frequencies...
5. ### Generalized Lorentz Transformation

If what I have is a valid transformation, then I'm confused about what is calculating. Given the coordinates in S, does it give the coordinates in S(bar)?
6. ### Generalized Lorentz Transformation

I noticed that of course. But, isn't it true that the matrix gives S(bar) in terms of S? I don't see how the logic could be wrong. We give S(bar) in terms of S' and S' in terms of S, thus S(bar) in terms of S. I agree the formula is not a boost, but isn't it still correct?

12. ### A simple Proper Time Calculation

Integral updated to include a dt term.
13. ### A simple Proper Time Calculation

Thanks for taking a look at it. The dimensions are correct. I'm using relativistic units, so |a| has units of 1/s. I'll email the professor and see if he is in agreement with our answers.
14. ### A simple Proper Time Calculation

Homework Statement Suppose a spaceship starts from rest from a space station floating in deep space and accelerates at a rate of |a| relative to the space station for 1.0 Ms. It then decelerates for the same amount of time at the same constant rate |a| to arrive at rest at another space station...
15. ### Tension on a string attached to two objects

OK, I see your point. The friction argument shouldn't matter anyways, because the result still holds in a friction free environment. Of course what I meant was the net forces acting on the person, so friction from the ground plus the two Tensions all sum to 0. But, I like the way you are...
16. ### Tension on a string attached to two objects

On second thought, I don't think I should ignore the frictional force with the ground. The frictional force with the ground is acting say downward, and for static equilibrium, the upward forces from the chains have to cancel, and I think I get the right answer this way. Is that how you thought...
17. ### Tension on a string attached to two objects

Of course I've done that. If we ignore the frictional forces with the ground as the book usually does in a situation like this, and cancel the gravitational and normal forces, there are two forces acting on the person along the two chains.
18. ### A simple static equilibrium problem

I created the thread. I can't see how I'm missing a factor of 2. You should like my thread, it's all in easily to follow algebraic symbols with the calculation at the end.
19. ### Tension on a string attached to two objects

Homework Statement A person would like to pull a car out of a ditch. This person ties one end of a chain to the car's bumper and wraps the other end around a tree so that the chain is taut. The person then pulls on the chain perpendicular to its length. If the distance between the car and tree...
20. ### A simple static equilibrium problem

Thanks for helping me with that problem. I'm doing self study just for the fun of it. I don't want to start another thread, but I'm confused about another problem on the same topic, and was wondering if you have any insight about it. The problem reads: A person would like to pull a car out of a...
21. ### A simple static equilibrium problem

You're right. I missed that. I reworked it as you suggested and got 57N. Thanks.
22. ### A simple static equilibrium problem

Let d_b be the distance from the floor to the bucket along the slope which is given to be 2.5m. d_s be the distance from the floor to the support along the slope of the ladder. I found this value to be about 1.3462m by using similar triangles. Let theta be the angle between the left leg and...
23. ### A simple static equilibrium problem

Using my method essentially of calculating, you didn't change anything else?
24. ### A simple static equilibrium problem

It's measured up the ladder slope. I considered the division of weight between the two halves, but that would still be off. I don't think you're supposed to make that assumption. I believe the weight rests and the top of the triangle so that they both receive the 20 kg.
25. ### A simple static equilibrium problem

Of course I have. The diagram is an isosceles triangle. That's how I calculated the distance from ground to the support along the ladder and the other calculations.
26. ### A simple static equilibrium problem

Homework Statement Suppose we have a folding ladder, so that when its legs are spread it makes an isosceles triangle. Suppose it has a support that runs parallel to the ground. There is a bucket of paint that rests on top of the ladder, and the mass of the ladder is negligible. Data: The legs...
27. ### Temperature change of Water after dropping a ball into it

If the Iron ball and water have different initial temperatures (and the book certainly didn't say they have the same initial temperature) then delta temperature iron = T_f - T_i(Iron) which is not equal to T_f - T_i(water). They would certainly share the same final temperature after an...
28. ### Temperature change of Water after dropping a ball into it

No, the problem does not indicate that. I already thought about that before. I just redid the problem where I assume that the "Iron" ball does not undergo any temperature change. I almost get the exact same answer. I think we're supposed to assume that the "Iron" ball does not undergo any...
29. ### Temperature change of Water after dropping a ball into it

I don't understand your comment. For example, we worked a similar problem where a lead ball crashes into the ground, and we assumed the ground doesn't have any temperature change at all. Here, we are assuming the lead ball and water have the same temperature change, and I don't see why.
30. ### Temperature change of Water after dropping a ball into it

Homework Statement Suppose we drop a lead ball of mass M into water of mass m from a height h and allow everything to settle down. What is the temperature change of the water? Assume that the container is well insulated. Homework Equations Potential Energy = mgh dU = mcdT where dU is the...
31. ### A basic conservation of energy question

I literally just figured that out as you wrote it. It all makes sense now. Thanks a lot for the discussion!
32. ### A basic conservation of energy question

the potential energy equation is kq1q2/r. Positive terms for both q1 and q2 would result in a positive term and would result in nonsense. v would have to be 0 in that case.
33. ### A basic conservation of energy question

Look closer, it's a squared term. Right? They have equal but opposite charges?
34. ### A basic conservation of energy question

The equation is -4kq_p^2/r + (1/2)*m2*(v2^2) = 0 right? where q_p is the charge of a proton, and m2 is 2(m_p + m_n). It's just plug and chug from there to solve for the kinetic energy which is the second term on the left hand side. I get .12pj
35. ### A basic conservation of energy question

Even when I follow escape velocity derivation where the potential is an electric potential, I still get the same answer I have written above. The only thing I can imagine is that I'm making a charge calculation error, but I don't believe it.
36. ### A basic conservation of energy question

The question asks: Uranium nuclei are unstable and decay by emitting a helium nucleus, which contains two protons and two neutrons. Suppose the original nucleus has 92 protons and 146 neutrons before it emits the helium nucleus, and that the helium nucleus has an knknown speed as it departs from...
37. ### Why does quantum entanglement not allow ftl communication

Thanks for the welcome! I've actually been reading your posts for quite some time now. I thought we could have some good talks. I'm not so interested in alternative theories to quantum mechanics, with the single exception of Bohmian Mechanics. I sometimes wonder why there are so few Bohmians...
38. ### Why does quantum entanglement not allow ftl communication

Agreed. But this leaves the question of how the entangled particles "know" what to do. If the correlation can't be explained in terms of a past interaction, I don't see how you can ever escape from "what I do over hear influences what happens over there". I think that's the whole point of Bell's...