A current is the rate of fluid flow, especially water or air. A current is any continuous, directed movement of a fluid. Naturally occurring currents can be explained as an increase in entropy. In fluid dynamics, the rate of fluid flow is the volume of fluid which passes through a given area per unit time. ... Water (from the Old English word wæter; c. ... Air is a name for the mixture of gases present in the Earths atmosphere. ...

In a river or stream, the water is influenced by gravity, and flows downhill to reduce its potential energy. The movement of water in a stream is called the current and this movement will vary from place to place and time to time dependent upon the volume of water, the slope, and the shape and other characteristics of the stream bed. A running stream. ... Gravity is the force of attraction between massive particles. ... Potential energy is stored energy. ... The bed of this stream is made up of rocks, some very rounded (having had a longer life in the stream) and some not. ...

Air currents may be caused by differences in temperature, pressure, or impurity concentration. Temperature differences can cause air currents because warmer air is less dense than cooler air, causing the warmer air to appear "lighter." Thus, if the warm air is under the cool air, air currents will form as they exchange places. Pressure differences also cause air currents as the air flows from areas of higher pressure to areas of lower pressure. Impurities cause air currents due to entropy, which is the natural tendency of a system to move to a disordered state. For example, if perfume is sprayed in the corner of a closed room, it will eventually disperse evenly to the entire room. Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... Atmospheric pressure is the pressure caused by the weight of air above any area in the Earths atmosphere. ... Impurities refer to any substances inside a confined amount of liquid, gas, or solid, which differs from the chemical composition of the material or compound. ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ... Perfume is a mixture of fragrant essential oils and aroma compounds, fixatives, and solvents used to give the human body, objects, and living spaces a lasting and pleasant smell. ...

See also

• Ocean current
• Volumetric flow rate