Current (fluid)

A current, in a river or stream, is the flow of water influenced by gravity as the water moves downhill to reduce its potential energy. The current varies spatially as well as temporally within the stream, dependent upon the flow volume of water, stream gradient, and channel geometrics. In tidal zones, the current in rivers and streams may reverse on the flood tide before resuming on the ebb tide.

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.