The bulk modulus of a substance is the ratio of a small decrease in pressure to the resulting fractional increase in volume:
B=−VdVdP.
Show that B=(5/3)P, in the free electron gas model, and use your result in Problem 5.21(d) to estimate the bulk modulus of copper. Comment: The observed value is 13.4×1010 N/m2, but don't expect perfect agreement—after all, we're neglecting all electron–nucleus and electron–electron forces! Actually, it is rather surprising that this calculation comes as close as it does.