CHEMISTRY GLOSSARY

thermal expansion    →   toplinsko rastezanje

Thermal expansion is a change in dimensions of a material resulting from a change in temperature. All objects change size with changes in temperature. The change ΔL in any linear dimension L is given by

in which α is the thermal coefficient of linear expansion, L_o is the initial or reference dimension at temperature T_o (reference temperature) and ΔT is change in temperature which causes the change in dimension.

The change ΔV in the volume of a sample of solid or liquid is

Here γ is coefficient of volume expansion, V_o is the volume of the sample at temperature T_o and ΔV is the change in volume over the temperature range ΔT. With isotropic substances, the coefficient of volume expansion can be calculated from the coefficient of linear expansion: γ = 3α.

thermal resistance    →   toplinski otpor

Heat always flows from a higher to a lower temperature level. The driving force for the heat flux lies in the temperature difference ΔT between two temperature levels. Analogous to Ohm’s law, the following holds:

where H = dQ/dt is heat flux, measured in watts, ΔT is temperature difference across the thermal resistance, measured in kelvin, and R_th is thermal resistance, measured in K/W.

For example, suppose there were two houses with walls of equal thickness; one is made of glass and the other of asbestos. On a cold day, heat would pass through the glass house much faster. The thermal restistance of asbestos is then higher than of glass.

If the thermal Ohm’s law is divided by the heat capacity C, Newton’s law of cooling is obtained:

where dT/dt is rate of cooling or heating, measured in K s^-1, and C is heat capacity, measured in J K^-1.

thermodynamic laws    →   termodinamički zakoni

Thermodynamic laws are the foundation of the science of thermodynamics:

First law: The internal energy of an isolated system is constant; if energy is supplied to the system in the form of heat dq and work dw, then the change in energy dU = dq + dw.

Second law: No process is possible in which the only result is the transfer of heat from a reservoir and its complete conversion to work.

Third law: The entropy of a perfect crystal approaches zero as the thermodynamic temperature approaches zero.

thermosetting plastic    →   termostabilna plastika

Thermosetting plastics (thermosets) refer to a range of polymer materials that cure, through the addition of energy, to a stronger form. The energy may be in the form of heat (rubber), through a chemical reaction (two part epoxy), or irradiation. Thermoset materials are usually liquid or malleable prior to curing, and designed to be molded into their final form, or used as adhesive.

Thermoset polymer resins transformed into plastics or rubbers by cross-linking into a rigid, 3-D structure. A thermoset material cannot be melted and re-molded after it is cured.

tyrosine    →   tirozin

Tyrosine is hydrophobic amino acids with aromatic side chain. Tyrosine is large aromatic residue that is normally found buried in the interior of a protein and is important for protein stability. Tyrosine has special properties since its hydroxyl side chain may function as a powerful nucleophile in an enzyme active site (when ionized) and is a common site for phosphorylation in cell signaling cascades. Tyrosine absorbs ultraviolet radiation and contributes to the absorbance spectra of proteins. It is not essential (or semi-essential) to the human diet, since it is synthesized in the body from other metabolites.

• Abbreviations: Tyr, Y
• IUPAC name: 2-amino-3-(4-hydroxyphenyl)propanoic acid
• Molecular formula: C₉H₁₁NO₃
• Molecular weight: 181.19 g/mol

ultraviolet light    →   ultraljubičasto svjetlo

Ultraviolet light (UV light or UV radiation) is an electromagnetic radiation with a wavelength longer than that of x-rays, but shorter than that of visible light. Ultraviolet light can break some chemical bonds and cause cell damage.

velocity    →   brzina

If a point-like object moves so that its position vector changes from being r_i to r_f, than the displacement Δr of object is

If a point-like object undergoes a displacement, Δr, in time Δt, its average velocity, v is defined as

The instantaneous velocity, v, is obtained from the average velocity by shrinking the time interval Δt towards zero. The average velocity approaches a limiting value, which is the velocity of a given instant:

Velocity is a vector quantity. If we plot the path of a moving particle as a curve in a coordinate system, the instantaneous velocity is always tangent to that curve.

SI unit for velocity is m s^-1.

Wilson’s chamber    →   Wilsonova komora

Wilson’s chamber is used for detection of radioactive radiation. Wilson’s chamber has a glass cylinder filled with air that has been saturated with water vapour. Radioactive radiation in its way ionises molecules of gas which then function as centres on which water vapour condenses into very small drops, thereupon showing Tyndall’s effect, i.e. is they are visible as a bright trail.

wire mesh    →   azbestna mrežica

Wire mesh (wire gauze) is used for uniform distribution of flame heat in base of a heated body. It is made of a iron wire strands with or without ceramic interior cores.

X-ray diffraction    →   rendgenska difrakcija

The regular array of atoms in a crystal is a three-dimensional diffraction grating for short-wavelength waves such as X-rays. The atoms are arranged in planes with interplanar spacing d. Diffraction maxima occur in the incident direction of the wave, measured from the surface of a plane of atoms, and the wavelength λ of the radiation satisfy Braggs’s law: