In every wire, electrons flow backwards against the official current direction
Circuit diagrams show current running from positive to negative. In a copper wire, the particles actually carrying the charge, electrons, travel the opposite way. The convention was fixed before anyone knew what moved, and it survives because a flow of positive charge one way behaves exactly like negative charge flowing the other.
Electric current is the net rate at which charge crosses a surface. What carries it varies: electrons in metal wires, electrons or positively charged holes in semiconductors, ions in electrolytes, and both ions and electrons in plasma. The unit is the ampere, one coulomb per second, an SI base unit, and current is measured with an ammeter. Its symbol, I, comes from the French intensité du courant; André-Marie Ampère used it in formulating his force law in 1820, and it spread to Britain, though at least one journal clung to C until 1896.
In metals the positively charged nuclei stay locked in place while free electrons move, so electrons drift against conventional current. Semiconductors can carry either sign depending on how they are doped, and an electrochemical cell carries both at once. When engineers analyse a circuit they often pick a reference direction arbitrarily; if the answer comes out negative, the current simply flows the other way. Ohm's law ties it together: current between two points is proportional to the voltage across them, with resistance as the constant.
Current comes in two broad forms. Alternating current reverses direction periodically, usually as a sine wave, and is what homes and businesses receive. Direct current flows one way and comes from batteries, solar cells, thermocouples and dynamos, or from alternating current passed through a rectifier. Nature supplies its own examples in lightning, static sparks and the solar wind behind the auroras, and in the ion flows through nerves that underlie sensation and thought.
Its effects run modern life. Current produces a magnetic field circling the wire, the basis of motors, generators, transformers and electromagnets, which lose their magnetism the moment the current stops. A changing magnetic field, in turn, induces current. Passing current through a conductor heats it, as James Prescott Joule showed in 1841 by measuring how a current-carrying wire warmed water; the unit of energy bears his name. Currents oscillating at radio frequencies radiate waves that travel at light speed.
Source: Electric current
