Question

Why does aligning the inner dipoles of a material decrease its heat capacity?

Answer 1

I am not so sure - can u show me the source of this assertion ?

Answer 2

http://en.wikipedia.org/wiki/Magnetic_refrigeration#Thermodynamic_cycle -Adiabatic magnetization

Answer 3

The heat capacity of a material is proportional to
\[ \frac{\partial S}{\partial T} \]
the rate of change of entropy with respect to temperature. Aligning the inner dipoles of a material reduces the amount by which the entropy changes corresponding to an increase in temperature, so the above derivative decreases in magnitude, and the heat capacity follows.

Answer 4

Is it to do with the fact that it has a lower absolute entropy, or that with alignment a 'push' by temperature has less effect at increasing entropy as the 'low-entropy structure' is self-supporting?

Answer 5

Though the first part is true, that's not what changes the heat capacity. The second part is, though for most materials the structure is not self-supporting. They maintain their alignment due to the presence of the external magnetic field. If that's removed, the arrangement breaks down and the heat capacity returns to normal.

Answer 6

Can heat capacity be negative or does increasing temperature always increase S?

Answer 7

I believe heat capacity must be positive.

Answer 8

I stand corrected. Though ordinary materials exhibit positive heat capacity, I did not consider some larger systems: http://en.wikipedia.org/wiki/Heat_capacity#Negative_heat_capacity_.28stars.29

Answer 9

Interesting- thanks