Many U.S. counties lack infectious disease specialists

(HealthDay)—The distribution of infectious disease (ID) physicians in the United States is geographically skewed, with 90 percent of U.S. counties having below-average ID physician density or no ID physicians at all, according ...

Diseases, Conditions, Syndromes

Crowded homes, poor neighborhoods linked to COVID-19

A study of nearly 400 pregnant women in New York City is among the first to show that lower neighborhood socioeconomic status and greater household crowding increase the risk of becoming infected with SARS-CoV-2, the virus ...

Diseases, Conditions, Syndromes

Refugee camps vulnerable to COVID-19 outbreaks

A COVID-19 outbreak in a refugee settlement will likely overwhelm the available healthcare capacity and infrastructure and spread through nearly the entire settlement population if left unchecked, according to a new study ...


How exercise reduces cataract risk

Chinese and Australian researchers have combined studies of more than 170,000 people and found conclusive evidence that regular physical exercise reduces the risk of age-related cataracts, the cause of blindness in an estimated ...

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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