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Harmonic oscillator A system in which a mass is subject to a linear restoring force, like an ideal spring. A harmonic oscillator vibrates at a fixed frequency, independent of amplitude.
Heat As defined in thermodynamics, heat is the energy that flows between two systems as a result of temperature differences (a system contains neither heat nor work, but can produce heat or do work). Heat thus differs from thermal energy.
Heat capacity The ratio of the heat input to the temperature increase in a system. Note that this definition does not imply that a system contains heat, despite the name heat capacity.
Helmholtz free energy The internal energy of a system minus the product of its entropy and temperature; see free energy.
Hydrocarbon A molecule consisting only of H and C.
Hydrogen bond A hydrogen atom covalently bound to an electronegative atom (e.g., nitrogen, oxygen) has a significant positive charge and can form a weak bond to another electronegative atom; this is termed a hydrogen bond.
Hydrophobic force Water molecules are linked by a network of hydrogen bonds. A nonpolar, nonwetting, surface (e.g., wax) cannot form hydrogen bonds. To form their full complement of hydrogen bonds, the nearby water molecules must form a more orderly (hence lower entropy) network. This both increases free energy and causes forces that tend to draw hydrophobic surfaces together across distances of several nanometers.


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