DENSITY

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DENSITY

Important property of any material is its density, defined as its mass. A homogeneous material such as ice or iron has the same throughout. We use the Greek letter p (“rho”) for density. of material has volume V, the density p is is useful. Densities of several common substances at ordinary temperatures are gi en in Table 14-1. Note the wide range of magnitudes. The densest material found on earth is the metal osmium (p = 22,500 kg/m’), but this pales by comparison to the densities of exotic astronomical objects such as white dwarf stars and neutron stars. The specific gravity of a material is the ratio of its density to the density of water at 4.0°C, 1000 kg/m’; it is a pure number without units. For figure

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example, the specific gravity of aluminum is 2.7. “Specific gravity” is a poor term, since it has nothing to do with gravity; “relative density” would have been better. Measuring density is an important analytically technique. For example, we can determine the charge condition of a storage battery by measuring the density of its electrolyte, a sulfuric acid solution. As the battery discharges, the sulfuric acid (H2 S0, J combines with lead in the battery plates to form insoluble lead sulfate (Pb SO,J, decreasing the concentration of the solution. The density decreases from about 1.30 x 10′ kg/m’ for a fully charged battery to 1.15 x 10′ kg/m’ for a discharged battery. Another automotive example is permanent-type antifreeze, which is usually a solution of ethylene glycerol (p = 1.12 X 10′ kg/m’) and water. The freezing point of the solution depends on the glycerol concentration, which can be determined by measuring the density. Such measurements of fluid density are performed routinely in service stations with a device called a hydrometer, which we’ll discuss in Section 14-4.

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DENSITY