currents

Currents refer to directed flows of a substance, property, or quantity within a medium, driven by gradients in pressure, temperature, salinity, potential, or other forces. They appear in diverse systems—from electric circuits to the oceans and the atmosphere—and are characterized by a rate of transfer known as the current or flux.

In electricity, current is the rate at which electric charges move through a conductor. It is measured in amperes and follows Ohm's law in simple circuits, I = V/R. Direct current involves a constant direction of flow, while alternating current reverses periodically. Electrical currents power devices and enable signal transmission in communications.

In the oceans, currents are sustained horizontal movements of seawater caused by wind, the rotation of the Earth (Coriolis effect), differences in water density, and tides. Surface currents form gyres and transport heat across continents, shaping climate and weather. Deep currents, driven by density contrasts, contribute to the global thermohaline circulation.

In the atmosphere, air currents are wind patterns driven by solar heating, the Earth's rotation, and topography. They organize into features such as trade winds, westerlies, and jet streams, and play key roles in weather systems and climate.

Biological and other systems also exhibit currents. In physiology, ionic currents flow across membranes to enable nerve impulses and muscle activity. In industrial contexts, liquid or gas currents are controlled in processes such as mixing, cooling, and transport.

Currents influence navigation, climate, and energy systems, and are measured by devices such as ammeters for electrical currents, ship-mounted current meters for oceans, and meteorological observations for air currents.