Recent content by Welshy

Homework Statement By shining and intense laser beam on to a semiconductor, one can create a collection of electrons (charge -e, and effective mass me) and holes (charge +e, and effective mass mh) in the bulk. The oppositely charged particle may pair up (as in a hydrogen atom) to form a gas of...

Duh! I couldn't see the wood for the trees! :P Thanks!

Ok. My explanation of the mass deficit due to energy conservation was oversimplified. But the problem lies in the momentum conservation. Any idea how I would explain that?

Was stuck at this too! I get it now. Thanks!

The mass should be of fermions in general - not just electrons. Same with the number density. And since protons and neutrons are fermions, you're equation should work.

If you take the log then E - TS pops out. And that's the free energy.

Homework Statement An atom initially at rest emits a photon. Explain why the mass of the atom decreases, taking into account energy and momentum conservation. The Attempt at a Solution I get the energy conservation part. E=delta(m)c^2=hf. But how would momentum conservation cause an...

More Edinburgh people! It's only 2 marks. But yeah it's that easy.

Hmm. What I get is: B = (u0IR2cos(O)/((d2+e2+R2)(3/2)) where O is the angle between the (e to centre of coils) vector and the z axis and the B vector is in the z axis. I'm not sure if that's right because I don't see how that can be expanded to powers of e in any way.

Homework Statement Two co-axial, parallel, circular wires of radius R each carrying current I are placed a distance 2d apart. Find an expression for magnetic field B on the axis at distance e from the midpoint between the two loops up to and including terms of 0(e3). Homework Equations...

Edit: Found it! Thanks.

Homework Statement Past exam question: Briefly explain the roles of symmetry, uniqueness and superposition in solving electrostatics problems. Homework Equations N/A The Attempt at a Solution I know the definition of these words and could cobble together an answer from that - but...

The black holes would be attracted to each other. But because the distance between them may be so great the force between them will be less than the dark energy expansion "forces" - just the same way that galaxies behave just now. The density is average but the fate of the universe depends...

Any more hints that anyone can give? Is tiny-tim's method valid for A not equal to B? And, if so, where will I pull my speed equations from? The sheer algebra of this just has me stumped. :/

You're thinking of entropy in the wrong way. Entropy is not a system tending to disorder - disorder is merely a metaphor for entropy. A better way of thinking of it is as a degradation of energy. Amounts of energy are becoming unusable due to irreversible processes. As an aside - the entropy...