Engineering

Innovative batteries put flying cars on the horizon

Jet packs, robot maids and flying cars were all promises for the 21st century. We got mechanized, autonomous vacuum cleaners instead. Now a team of Penn State researchers are exploring the requirements for electric vertical ...

Electronics & Semiconductors

Making batteries live longer with ultrathin lithium

Our lives today are governed by electronics in all shapes and forms. Electronics, in turn, are governed by their batteries. However, the traditional lithium-ion batteries (LIBs), that are widely used in electronic devices, ...

Energy & Green Tech

Hybrid redox-flow battery with a long cycle life

Redox‑flow batteries store electrical energy in chemical compounds that are dissolved in an electrolyte. They are a particularly promising alternative to lithium‑ion batteries as stationary energy storage. A team headed ...

Energy & Green Tech

Ionophobic electrode boosts energy storage performance

Using renewable energy to replace fossil energy is now considered the best solution for greenhouse gas emission and air pollution problems. As a result, the demand for new and better energy storage technology is strong.

Engineering

World first concept for rechargeable cement-based batteries

Imagine an entire 20-storey concrete building that can store energy like a giant battery. Thanks to unique research from Chalmers University of Technology, Sweden, this vision could someday be a reality. Researchers from ...

Engineering

New design for isolated power supply chip

Recently, a research group led by Prof. Cheng Lin from School of Microelectronics, University of Science and Technology of China (USTC) of the Chinese Academy of Sciences has made achievements in the field of fully integrated ...

Engineering

A long-lasting, stable solid-state lithium battery

Long-lasting, quick-charging batteries are essential to the expansion of the electric vehicle market, but today's lithium-ion batteries fall short of what's needed—they're too heavy, too expensive and take too long to charge.

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Density

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