2
votes
1answer
49 views

Can a lone black hole in a closed Universe evaporate?

If there is a closed Universe which only has a black hole in it, can that black hole evaporate? As the black hole evaporates, it gives off energy, which will eventually come back and be re-absorbed ...
2
votes
3answers
130 views

When is momentum not conserved?

What are some common examples where momentum is not conserved? This question arose in my mind when I read that a ball dropped from a height penetrates into a bed of sand and that momentum is ...
0
votes
0answers
36 views

How to make scattering matrix? [on hold]

I need to make a scattering matrix. Potential is unknown. I have the list of scattering data: time, mass and coordinate of the particle. How to make scattering matrix by the given data?
0
votes
1answer
15 views

N-type semiconductors and the number of holes?

Let us say we have Germanium which has been doped with antimony. Antinomy has a a fifth valence electron not involved in bonding. Is this electron in the valence band? I ask because all texts on the ...
0
votes
0answers
33 views

Electric Flux. Integral of $E$ with respect to $A$ is equal to TOTAL Electric FLUX? [on hold]

The first equality is simple to me but I don't understand the second one,$\oint {\bf E}\cdot d{\bf A}$. And the second second last line is much hard to understand. Could you please give some lecture ...
2
votes
0answers
37 views

Is there a Fermi estimations toolbox?

I am a theoretical physics student and am a little ashamed at my inability to estimate any measurable quantity. I would like to develop my skills at Fermi estimations. Although it is hopeless to ...
0
votes
1answer
33 views

Why part of debroglie wave is negative?

I mean de Broglie wave for a particle represents the probability of the particle existing at a point, then, how can it be negative as well as positive?
0
votes
1answer
32 views

Why do roads appear wet on hot days? I do not understand [duplicate]

Why do roads appear wet on hot days? I don't understand the bending of light rays in relation to the density fluctuations. How are they related? Why do we see mirror images of cars on road in hot ...
0
votes
1answer
53 views

Is antimatter power theoretically possible?

There is a quote and citation on the "Antimatter" page of the obvious encyclopedic site (https://en.wikipedia.org/wiki/Antimatter), saying that positrons could be produced (in the future) at a rate of ...
0
votes
4answers
32 views

How greatly can heat increase resistance? [on hold]

If a copper/aluminum wire has a resistance of 0.001Ohms at room temperature, how greatly would the resistance increase if it we're increase it by a 100 degrees?
4
votes
1answer
674 views

Why do we only talk about three options when it comes to the Shape of Spacetime?

The shape of spacetime is a hotly debated topic which can determine how we understand the ultimate fate of the universe and many more things. Despite this I only ever see factions supporting three ...
1
vote
1answer
49 views

Path of wheels of a bicycle

Why are the wheels of a bicycle moving in concentric circles with the center O? I know that the velocity of the back wheel is parallel to the frame of the bicycle and the velocity of the front wheel ...
1
vote
1answer
19 views

What theory unifies the electroweak and the strong forces? [on hold]

As the title itself suggests, here's my question: What theory unifies the electroweak and the strong forces?
0
votes
0answers
11 views

Which frictional force is the least [on hold]

Which is the least frictional force?This was the question in my paper today.What could be the answer??I wrote it is on surface of ice .is it something on static and limiting friction?
1
vote
1answer
43 views

Using Ampere's Law without Right-Hand-Rule to derive an expression for the magnetic field around a current

I'm a little confused over the textbook example of applying Amperians to get the magnetic field around a current. I understand we take a loop which shares the rotational symmetry of the wire (a ...
2
votes
3answers
157 views

Armageddon prevention

I was watching the movie Armageddon and it made me think of a few things. We would probably not need to send a crew to blow the asteroid in two, I think we could still send a warhead with enough power ...
1
vote
3answers
118 views

Can a bullet really fly through space forever?

Some people says that if it would be possible to shoot a bullet so high that it would get over the Earth gravitational pull, the bullet could fly through space forever, because of no deceleration of ...
2
votes
2answers
146 views

“Magic” Speed to Drive Over a Speed Bump: Myth or Reality?

A somewhat controversial aspect of speed bumps (sleeping policemen for those in the UK) is that they arguably cause more loss of life than they prevent by acting as a hindrance to emergency response ...
0
votes
0answers
17 views

Will charges redistribute on the surface of an insulator?

When I shot an electron beam onto an insulator, electrons will be knocked out of the material, leaving an immobile positive charge at their place. Some of the knocked out electrons might manage to ...
2
votes
2answers
62 views

How to define pseudovector mathematically?

The textbook describes pseudovector like this: Let $a,b$ be vectors and $c=a\times b$, $P$ be the parity operator. Then $P(a)=-a,P(b)=-b$ by definition. But $P(c)=c$ since both $a$ and $b$ reverse ...
0
votes
0answers
17 views

How is it shown that the composition of two real operators is generally not real? [duplicate]

Dirac on page 28 of his QM book writes: Thus the conjugate complex of the product of two linear operators equals the product of the conjugate complexes of the factors in the reverse order. As ...
0
votes
0answers
28 views

Optimum speed for the highest possible safe transportation of a straight road? [on hold]

By saying optimum speed I mean "Safe Speed". Because in a ice road, the speed has to be low. Whereas the speed in a normal road speed is a reasonable speed. So my question is how do you find the ...
0
votes
0answers
20 views

Kinematic related question [on hold]

Question: A baseball is hit with a velocity of of 30 m/s at an angle of 42.6 degrees to the horizontal. An outfielder who is good at 2D kinematics knows he is exactly 8 meters from where the ball will ...
0
votes
0answers
25 views

Maxwell equations in cylindrical coordinates [on hold]

Give a coil of radius $R$ and $N$ turns crossed by a mean current of $I_0$ ampere at $10 MHz$ frequency (sinusoidal), I need to calculate the behaviour of the electrical and magnetic field inside the ...
1
vote
1answer
26 views

Finding the velocity of a ball thrown upwards [on hold]

A ball is thrown upwards with a speed of 16.1 m/s. What is the ball's velocity 1.6 seconds after it is released? I tried to solve this using the equation: $v = u+at$, substituting u for 16.1, a for ...
5
votes
1answer
50 views

Thermodynamics: heat transfer

I hope this question isn't too simplistic but I've been in a discussion with someone who claims that no energy is transferred between a cool object and a warm one (either by radiation or conduction) ...
3
votes
0answers
32 views

Entanglement g-2-experiment - Which components do I need?

I would like to measure whether my source emits entangled photon pairs. To that order I want to build a g-2-experiment, which measures photon coincidence counts as a function of time delay between ...
1
vote
2answers
24 views

Possiblity of predicting behavior of system when properties described as functions over space?

Suppose I am given a system that consists of a distribution of charged particles(which are all over space and are point-charges). They are described by a set of functions instead of variables. These ...
0
votes
1answer
34 views

How to construct this oracle quantum gate?

I was reading the paper Quantum Computational Complexity in the Presence of Closed Timelike Curves. In this the author mentions that following quantum oracle gate which operates on $n+1$ qubits, can ...
1
vote
2answers
69 views

Can you punch a hole in an A4 sheet of paper with your hand?

The Heading says it all. But let me explain it a little further. Suppose you hang a sheet of paper (A4 size), from one end leaving the other end free. Now practically it seems impossible to punch a ...
0
votes
1answer
50 views

Does a normal torch emit entangled photons?

I was reading a sciencenews.org post about three photons being entangled. My question here is, why is the chance of producing an entangled pair once in a billion times? Isn't every particle produced ...
3
votes
1answer
32 views

Oscillation of air particles and speed of sound wave

A sound wave is essentially air particles oscillating parallel to the direction of travel of the wave. We learnt that $v = f\lambda$, where $v$ is the speed of the wave, $f$ is the frequency of the ...
1
vote
1answer
42 views

Convenient coordinate systems and symmetries

I recall in my basic electromagnetism and quantum mechanics lectures that choosing one coordinate system over another may greatly simplify the equations involved in solving a problem (think about ...
0
votes
0answers
14 views

How to derive the time period equation for a spring mass system taking into account the mass of the spring without involving energy analysis?

I want to know the way to derive the time period equation of a spring mass system accounting for the mass of the spring but not using the energy analysis method but by proceeding in the same way as we ...
5
votes
1answer
178 views

How does the Hubble parameter change with the age of the universe?

How does the Hubble parameter change with the age of the universe? This question was posted recently, and I had almost finished writing an answer when the question was deleted. Since it's a shame to ...
1
vote
1answer
36 views

Why are speakers louder in a corner?

You can test this very easily using an iPhone's speaker pointed towards a wall. When the speaker is more than an inch or two away, it is normal volume. However, when the speaker is nearly touching the ...
0
votes
1answer
41 views

Why a hole in the Dirac sea can be seen as having opposite energy?

In the Dirac's old attempt of relativistic quantum mechanics, a hole in the $E'$ level of the Dirac sea was interpreted as a particle of opposite charge and positive energy. I do get why it can be ...
0
votes
2answers
21 views

Weight of solid partially immersed in water

Suppose an object(spherical) is kept in water and 9/10 of the object is inside the water 1/10 floats. Find the weight of solid inside liquid. By Archimedes principle, if an object is immersed in a ...
1
vote
2answers
55 views

What do they mean by a pendulum losing seconds?

In many pendulum related question, a pendulum is taken do a different place where it loses seconds. For example: A second's pendulum is taken to a mountain and it loses 20 seconds per day. What ...
6
votes
2answers
92 views

Why are these two definitions for symmetries in the Lagrangian equivalent?

I have heard the following two definitions for a symmetry of the Lagrangian: If under a coordinate transformation the form of the Lagrangian remains unchanged then there is a symmetry. If $\delta ...
1
vote
0answers
22 views

Optimal size of a windmill for a given windspeed

Here is the problem: Assume that you have some constant wind speed. I want to run a windmill but I need to decide how big a windmill I want. The size is characterized by the length of the blades, $r$. ...
1
vote
0answers
14 views

Kinetic Damping Behavior [on hold]

For a problem I am given a block attached to a spring attached to a wall with mass m and coefficient of friction u. The magnitude when it is sliding is friction=$mgu$ opposite motion. It won't move ...
3
votes
2answers
60 views

Did Newton argue that particles speed up when entering a more dense medium?

Statement: Newton argued that particles speed up as they travel from air into a dense, transparent object, such as glass. From this source, I gather that he did argue that the light particles ...
2
votes
1answer
19 views

Specular or Diffuse Reflection - Overhead Projection

Question: What type of reflection is exhibited as light waves from an overhead projector reflect off a white projection screen and are scattered throughout a room? I said specular reflection, ...
1
vote
2answers
51 views

Relationship between two viewpoints on Thermodynamics

I've always seem the standard viewpoint on Thermodynamics that it is all about studying phenomena related to a property of systems called temperature. Then we have the zeroth law which allows us to ...
1
vote
2answers
31 views

What defines camera's size?

Smartphone camera is small while others are big. I know some functions like zoom can change physical size and multi lens,.. etc So, assume that a without zoom, what define camera lens? I want to get ...
4
votes
2answers
62 views

Can a bullet shot through a glass make a clean hole?

One can often see (hopefully only in the movies) that a bullet shot through a pane of window glass leaves a hole in it, but overall the glass is left largely intact. However, a low-speed projectile, ...
0
votes
0answers
31 views
+100

What methods exist to calculate the density of states in the continuum of a molecule?

Say I have an arbitrary molecule in the Born-Oppenheimer approximation, and furthermore say that I can approximate the molecule as having only one active electron. What methods exist to calculate the ...
1
vote
1answer
35 views

Can someone please explain impedance matching and wave reflection?

I'm reading Wikipedia's article on impedance matching (http://en.wikipedia.org/wiki/Reflections_of_signals_on_conducting_lines) and this just seems so foreign to me. 1) What does it mean for a ...
3
votes
1answer
44 views

Calculating time to reach certain velocity with drag force

I was given a problem for homework where we needed to calculate the time for a falling object to reach a certain velocity when accounting for drag force. I did it by setting up acceleration as a ...

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