Why do two parts of a broken magnet repel each other?

When you break a magnet, it often seems that the polarities have flipped.

This doesn't actually happen.

No polarity reversal occurs when you think the poles have flipped

- you are dealing with a magnet that has an axial field 

(pointing out through the flat face.) When you break 
it, each half has similar field, pointing in the same direction, which 
is unstable. One piece will want to flip so that the fields line up 
anti-parallel (lower energy situation).  

Whether the broken magnet attracts or repels each other depends

on how the poles were previously present.

If the original magnet looked like
  +-----------------------------+
N |                             | S
  +-----------------------------+ 

After it's broken, it becomes
  +-----------+     +---------------+
N |           |S   N|               | S
  +-----------+     +---------------+ 

The two broken parts will attract each other.

However, if the original magnet looked like:

(poles on the flat faces of the magnet)

                N
  +--------------------------------+
  |                                |
  +--------------------------------+
                S

After it is broken, it becomes


         N                    N
  +---------------+   +-----------------+
  |               |   |                 |
  +---------------+   +-----------------+
         S                    S

Therefore the two parts will repel each other and will try to invert.

These are only two simple cases. In reality, the poles
of a magnet can be more complicated. This just illustrates how the poles
affect behavior of magnets.

Tensile testing a single molecule

Nanotechnology has enabled things like Atomic Forced Microscopy (AFM) to benefit materials scientists and engineers but there's a use of the same equipment a materials engineer would find extremely amazing and would probably not have thought of using AFM for; tensile testing a single molecule.

This can be done by combining AFM with a piezoelectric positioner. under the cantilever probe of the microscope. The molecule once attached / bound (chemically or otherwise), at one side to the probe and the other side to the positioner, can be stressed by the piezoelectric material which will change height with application of electricity and the tensile testing data will be recorded by AFM's sensors. Now how much information can be acquired by tensile testing a single molecule alone puts materials engineering to a nano scale perspective against the bulk and the macro properties which are often completely different.

Basic operation of the AFM. As the AFM cantilever probes the surface by moving its tip along its contours, or when it pulls on a protein, the movement of the cantilever is detected with a laser beam that is focused to beam to the head of the cantilever and refracts into a photodetecter. The movement, or deflection, of the cantilever deflects the laser correspondingly, and this creates an image or produces other data about the surface or the sample (which may be a molecule). In single molecule force spectroscopy, the cantilever is pressed against a layer of proteins attached to a substrate, and the tip adsorbs a single protein molecule, which is then extended. Extension of the molecule by retraction of the piezoelectric positioner results in deflection of the cantilever. [Oberhauser et al. PNAS (January 2001), Vol. 98 (2): 468-472]

 

Earth sniped

A satirical depiction of how it would look to an astronaut on the Moon if earth gets hit by an ultra hard asteroid at an ultra high speed. A better satirical caption would be, "...all my stuff was there!"

What happens when a Tank rolls over a car?