Number density is an intensive quantity used to describe the degree of concentration of countable objects (particles, molecules, stars, etc.) in physical space.
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Pro Tips
Standard SI unit is objects per cubic meter (m⁻³).
For gases, you can calculate number density using the Ideal Gas Law: n = P / (kT).
Avogadro's number (6.022 × 10²³) is often used when dealing with moles of particles.
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Fun Facts
"The number density of the interstellar medium is only about 1 atom per cubic centimeter."
"The number density of air at sea level is approximately 2.5 × 10²⁵ molecules per cubic meter."
"In a neutron star, the number density is so high that a teaspoon of its material would weigh billions of tons."
