CHEMISTRY GLOSSARY

angular momentum    →   moment količine gibanja

Angular momentum is a physical quantity defined for rotating motion (in analogy to momentum that is defined for linear motion). If a body rotates around a specified axis, its angular momentum equals

Where I is the rotational inertia concerning that axis and ω is the angular velocity of the body.

Angular momentum can also be defined for a point-like body concerning a specified origin (in that case, it is not necessary that the point-like body undergoes circular motion). Rotational inertia of the point-like body, concerning that origin equals:

Where m is the mass of the body and r is its distance from the origin.

angular velocity    →   kutna brzina

A point-like object that undergoes circular motion changes its angular position from initial Θ_i to final Θ_f, relative to a fixed axis, specified in a coordinate system with an origin that coincides the centre of the circular path of object. The change in its angular position is called angular displacement ΔΘ = Θ_f - Θ_i. Also, a rigid body that rotates about a specified rotation axis, changing its angular position from initial Θ_i to final Θ_f, undergoes an angular displacement ΔΘ.

The average angular velocity, ω_av, is the ratio of the angular displacement and the time interval Δt=t_f-t_i, in which that displacement occurs.

Θ_f and Θ_i are the initial and final angular position, respectively.

The instantaneous angular velocity ω is the limit of the average angular velocity, as Δt is made to approach zero.

ω_av and ω are positive for the counterclockwise rotation (in direction of increasing Θ) and negative for the clockwise rotation (in direction of decreasing Θ).

SI unit for angular velocity is s^-1.The measure for the angle Θ is radian. The relationship between radians and degrees is:

For example, the angular velocity of the minute hand of a clock is:

centrifugal force    →   centrifugalna sila

Centrifugal force is a force which moves a body away from the centre of motion.

centripetal force    →   centripetalna sila

Centripetal force is a force which makes body move through a curvature. Its orientation is always in path forwards the centre of rotation.

chemiluminescence    →   kemiluminiscencija

Chemiluminescence is energy release in form of electromagnetic radiation during a chemical reaction.

coal    →   ugljen

Coal is a black or brownish-black, combustible sedimentary rock, with 30 % (lignite) to 98 % (anthracite) carbon by weight, mixed with various amounts of water and small amounts of sulfur and nitrogen compounds. It is formed from plant matter that decayed in swamps and bogs that has been compressed and altered by geological processes over millions of years. Coal is primarily used as a fuel.

conduction    →   kondukcija

This process occurs most significantly in solids. The atoms or molecules in a solid state do not leave their mean positions, but continue to vibrate about their mean positions. They transfer heat energy from one atom to another. This happens because of the coupling between them due to mutually attractive forces.

balance    →   vaga

Balance is an instrument to measure the mass (or weight) of a body. Balance beam type scales are the oldest type and measure weight using a fulcrum or pivot and a lever with the unknown weight placed on one end of the lever, and a counterweight applied to the other end. When the lever is balanced, the unknown weight and the counterweight are equal. The equal-arm balance consists of two identical pans hung from either end of a centrally suspended beam. The unequal-arm balance is made with one arm of the balance much longer than the other.

More modern substitution balances use the substitution principle. In this calibrated weights are removed from the single lever arm to bring the single pan suspended from it into equilibrium with a fixed counter weight. The substitution balance is more accurate than the two-pan device and enables weighing to be carried out more rapidly.

Electromagnetic force restoration balances also use a lever system but a magnetic field is used to generate the force on the opposite end of the lever and balance out the unknown mass. The current used to drive the magnetic coil is proportional to the mass of the object placed on the platform.

candela    →   kandela

Candela (cd) is the SI base unit of luminous intensity.

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10¹² Hz and that has a radiant intensity in that direction of 1/683 watt per steradian.

cobalt    →   kobalt

Cobalt was discovered by Georg Brandt (Germany) in 1735. The origin of the name comes from the German word kobald meaning goblin or evil spirit. It is hard, ductile, lustrous bluish-grey metal. Surfaces stable in air. Reacts over time with dilute acids. It has remarkable magnetic properties. Cobalt occurs in compounds with arsenic and sulfur as in cobaltine (CoAsS) and linneite (Co₃S₄). Pure cobalt is obtained as a by-product of refining nickel, copper and iron. Used in many hard alloys; for magnets, ceramics and special glasses. Radioactive cobalt-60 is used in cancer therapy.