Engineering

Getting the inside track on street design

Large-scale analysis of pedestrian data from three European cities examines how building density and street design influence pedestrian behavior.

Engineering

Big breakthrough for 'massless' energy storage

Researchers from Chalmers University of Technology have produced a structural battery that performs ten times better than all previous versions. It contains carbon fiber that serves simultaneously as an electrode, conductor, ...

Engineering

Novel cathode designed for high-performance sodium ion batteries

Due to their large-scale energy storage, sodium ion batteries (SIBs) are a promising alternative to lithium-ion batteries (LIBs). However, it's challenging to develop high-energy and high-power SIBs due to the greater atomic ...

Energy & Green Tech

How to prevent short-circuiting in next-gen lithium batteries

As researchers push the boundaries of battery design, seeking to pack ever greater amounts of power and energy into a given amount of space or weight, one of the more promising technologies being studied is lithium-ion batteries ...

Energy & Green Tech

Simple tools reveal high-fidelity truth in lithium-ion batteries

Acceleration due to gravity here on Earth is about 9.8m/s2, but if you're trying to build a rocket that will escape Earth's pull, you're going to have to do better than that. You would need to factor in wind resistance, heat ...

Energy & Green Tech

Tuning electrode surfaces to optimize solar fuel production

Scientists have demonstrated that modifying the topmost layer of atoms on the surface of electrodes can have a remarkable impact on the activity of solar water splitting. As they reported in Nature Energy on Feb. 18, bismuth ...

Energy & Green Tech

Scientists develop novel high-energy-density lithium metal battery

Prof. Liu Zhaoping's team at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences (CAS) has developed an electrolyte engineering strategy for lithium (Li) metal batteries ...

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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