Energy & Green Tech

Carbon-air battery as a next-generation energy storage system

One of the barriers to generating electricity from wind and solar energy is their intermittent nature. A promising alternative to accommodate the fluctuations in power output during unfavorable environmental conditions are ...

Energy & Green Tech

Improving plastic waste separation with magnetic fields

In 2018, 61.8 million metric tons of plastic waste was produced in the European Union with only 9.4 million metric ton recycled. This constitutes a huge amount of plastic waste, which rapidly needs to be addressed. One solution ...


A new solid-state battery surprises the researchers who created it

Engineers created a new type of battery that weaves two promising battery sub-fields into a single battery. The battery uses both a solid state electrolyte and an all-silicon anode, making it a silicon all-solid-state battery. ...

Energy & Green Tech

Scientist leads study on faster charging energy storage

An award-winning Tulane University researcher has led a team in discoveries that could result in significantly faster charging electric vehicles and portable devices such as cell phones and laptops. The team, led by Michael ...

Electronics & Semiconductors

New 3D-printing ink used to create tiny thermo-generators

A team of researchers working at the Ulsan National Institute of Science and Technology has created a new type of ink that can be used to print tiny 3D generators. In their paper published in the journal Nature Electronics, ...

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The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ (the Greek letter rho). In some cases (for instance, in the United States oil and gas industry), density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight. Different materials usually have different densities, so density is an important concept regarding buoyancy, purity and packaging. Osmium and iridium are the densest known metal elements at standard conditions for temperature and pressure but not the densest materials.

Less dense fluids float on more dense fluids if they do not mix. This concept can be extended, with some care, to less dense solids floating on more dense fluids. If the average density (including any air below the waterline) of an object is less than water (1000 kg/m3) it will float in water and if it is more than water's it will sink in water.

In some cases density is expressed as the dimensionless quantities specific gravity (SG) or relative density (RD), in which case it is expressed in multiples of the density of some other standard material, usually water or air/gas. (For example, a specific gravity less than one means that the substance floats in water.)

The mass density of a material varies with temperature and pressure. (The variance is typically small for solids and liquids and much greater for gasses.) Increasing the pressure on an object decreases the volume of the object and therefore increase its density. Increasing the temperature of a substance (with some exceptions) decreases its density by increasing the volume of that substance. In most materials, heating the bottom of a fluid results in convection of the heat from bottom to top of the fluid due to the decrease of the density of the heated fluid. This causes it to rise relative to more dense unheated material.

The reciprocal of the density of a substance is called its specific volume, a representation commonly used in thermodynamics. Density is an intensive property in that increasing the amount of a substance does not increase its density; rather it increases its mass.

This text uses material from Wikipedia, licensed under CC BY-SA