Why do tubelights make noise?

If you ever wondered why does your tube light or other fluorescent lights make noise (often noticeable in a quiet room and sometimes even in normal background noise in case of old lights). The reason magnetostriction. When current moves through coils or wires, magnetic field is produced with it. This magnetic field causes the material to magnetic and demagnetize (and in case of alternating current (AC) at the frequency of ~50Hz). During the process of magnetization and its reversal, the magnetic domains change their orientation and hence the material changes dimensions to some extent.

This change due to rotation of magnetic domains to align with the magnetic field causes the material to vibrate at hence causing the noise. This is generally the noise that we hear from the tube lights, but one man's limitation is another man's sensor. This effect can be used to create magnetostrictive materials that act as sensors or actuators.

Note (from Wikipedia under creative commons share alike license): Most magnetic materials are polycrystalline, composed of microscopic crystalline grains. These grains are not the same as domains. Each grain is a little crystal, with the crystal lattices of separate grains oriented in random directions. In most materials, each grain is big enough to contain several domains. Each crystal has an "easy" axis of magnetization, and is divided into domains with the axis of magnetization parallel to this axis, in alternate directions.

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.