### F.A.Q's :Condensed Matter and Nonlinear Dynamics

1. What causes sonoluminescence?  Sonoluminescence is the generation of small light bursts in liquids caused by sound.  Bubbles form in the liquid at low pressure points of the sound wave, then collapse again as a high pressure wave passes.  At the point of collapse a small flash of light is produced.  The exact cause has been the subject of intense speculation and research.
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2. What causes high temperature superconductivity?  Is it possible to make a material that is a superconductor at room temperature?  Superconductivity at very low temperatures has been understood since 1957 in terms of the BCS theory, but high temperature superconductors discovered in 1986 are still unexplained.
3. How can turbulence be understood and its effects calculated?  One of the oldest problems of them all.  A vast amount is known about turbulence, and we can simulate it on a computer, but much about it remains mysterious.
4. The Navier-Stokes equations are the basic equations describing fluid flow.  Do these equations have solutions that last for all time, given arbitrary sufficiently nice initial data?  Or do singularities develop in the fluid flow, which prevent the solution from continuing?
This is more of a question of mathematical physics than physics per se—but it's related to the previous question, since (one might argue) how can we deeply understand turbulence if we don't even know that the equations for fluid motion have solutions?  At the turn of the millennium, the Clay Mathematics Institute offered a \$1,000,000 prize for solving this problem.  For details, see:

### COMMON COLLECTOR CONFIGURATION OF A TRANSISTOR

COMMON COLLECTOR CONNECTION

In  this  configuration  the  input  is  applied  between the  base  and  the  collector and  the  output  is  taken  from  the  collector  and  the  emitter.  Here  the  collector  is common to both the input and the output circuits as shown in Fig.

Common Collector Transistor Circuit

In  common  collector  configuration  the  input  current  is  the  base current  IB  and  the output current is the emitter current IE. The ratio of change in emitter current to the  change in the base current is called current amplification factor.

It is represented by

COMMON COLLECTOR CIRCUIT

A test  circuit  for determining the  static characteristic  of an NPN transistor is shown in Fig. In this circuit the collector is common to both the input and the output circuits.   To   measure   the   base   and   the   emitter   currents,   milli   ammeters   are connected in series with the base and the emitter circuits. Voltmeters are connected   across the input an…