If you have no idea about what density means or how it's calculated, then this information will be helpful to clear your confusion.
Omkar Phatak
Density is a fundamental property of matter and one of the most important quantities in physics.
Definition
Density is simply the amount of mass, packed in a unit volume. On a broader level, you could roughly define it as the quantity packed in a unit volume or unit part of space.
That quantity may be mass, charge, or even energy. Its formula will be the total quantity of that substance, divided by the total volume contained in it.
Everything is made up of atoms and density is the quantity that tells us how closely these atoms are packed, in a unit volume of space.
The density of the same material may change and it may go through different 'phases'. The different phases of matter have different densities.
For example, ice, water, and water vapor are the same thing, but what sets them apart, is their density. When ice is heated, the molecules of water gain energy and they start separating from each other, leading to its melting.
As the ice melts, density decreases and it is converted into water. As water is further heated, the distance between the water molecules increases even more, its density further decreases, and it leads to the formation of water vapor. Thus, a change in this parameter is associated with a change in the phase of a substance.
It can be of three types, depending on the number of dimensions in which you are measuring the spread of matter. Accordingly, formulas for its calculation, change too. Here are the three main types:
★ Volume Density: It is a measure of the amount of matter, that has spread in three dimensions of space.
★ Surface Density: It is a measure of amount of matter, spread per unit surface area.
★ Line Density: It is a measure of charge, spread in one dimension.
The concept can be broadly applied, to include the spread of energy in a given space, in the form of energy density.
Formulas For Calculation
Here are the commonly used formulas:
Volume Density (Kg/m3)= Mass (in Kg)/Volume (in m3)
Here are the formulas that are frequently used in calculating charge densities in electrostatics (for a discrete charge distribution):
Volume Charge Density (Coulomb/m3) = Q (in Coulomb)/V (in m3)
Surface Charge Density (Coulomb/m2) = Q (in Coulomb)/S (in m2)
Line Charge Density (Coulomb/m) = Q (in Coulomb)/L (in meter)
Here, Q is the total charge, V is the volume, S is the surface area, and L is the length. Number density is simply the number of discrete objects, spread in a unit volume.